River.Space.Design: Planning Strategies, Methods and Projects for Urban Rivers 9783034611732

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River. Space. Design. Planning Strategies, Methods and Projects for Urban Rivers

River. Space. Sp Design. Des ign. Planning Strategies, Methods and Projects for Urban Rivers Martin Prominski Antje Stokman Susanne Zeller Daniel Stimberg Hinnerk Voermanek Birkhäuser · Basel

Is not every river quite extraordinary? Not one is identical in its morphology, limnology or atmosphere with another. Rivers, the veins of our landscape, are thrilling, living entities. One day a river can reflect the gentle dance of sunlight on the water; the next be a foaming torrent tearing at all that stands in its way and carrying it off. Rivers are far more than moving water – this would be an inadmissible simplification. It is the inimitable interplay of a body of flowing water with its bed, the shaping of its banks and its surroundings, which makes each river an inimitable personality with its own character, recounted in legends, songs and stories since time immemorial and still familiar to us today. Nearly all our cities and urban cultural spaces grew up on riverbanks and their development and the prosperity of their inhabitants also tell a story of their relationship with the water; trade, transport and industry flourished because of the navigability of these rivers and their significance as transport routes. For centuries, rivers were an important source of food for people settled close to their banks. Water, and the shaping of water landscapes by human hand, are the foundation of our cultures. However, a river can be both blessing and curse! It is no coincidence that humanity’s first engineering constructions were to regulate rivers; their purpose was always to protect settlements from the raging destructive forces of floodwater. Conversely, it was the taming and regulation of watercourses that, in many places, made it at all possible for cultural landscapes to evolve. Nowadays our rivers are to a large extent built up, straightened, transformed into feats of engineering – their original form and the way they shape the landscape are barely perceptible. However, it is not only since the catastrophic floods of recent years, the consequences of climate change and the decline of species diversity beside and in the water that the total control and one-sided technical perception of our straitjacketed rivers has been increasingly called into question. Both through the formulation and implementation of the EU Water Framework Directive, aiming to achieve a good condition of all European watercourses, and through steadily growing public awareness, rivers are increasingly prominent subjects of general attention. This is not solely a question of hydraulics and technical flood defences. The possibilities for recreational use are becoming more and more important as we rediscover our rivers as places for contemplation and recuperation. With considerable improvements in water quality through better wastewater treatment and rainwater management, an urban river is no longer the shunned stinking backwater of a city but its fairest face and first impression. And thus the aesthetics of a watercourse space, expressed in its morphology and the form of its banks, becomes ever more significant; the way we deal with a river is shifting from hard technical hydraulic engineering to semi-natural biological engineering for shaping watercourses as multifunctional places for all flora, fauna and people along and in the water. In accordance with contemporary expectations, we want rivers in good condition and well-designed that, as living organisms, are also a fount of vitality for city dwellers. How are such aims to be achieved, what examples are worthy of emulation, and what are the deciding factors of practical implementation? These are, today, the burning questions in watercourse design and water space revitalisation. It is for precisely these reasons that the appearance of such a book as River.Space. Design is long overdue. With its excellently prepared content, crystal-clear structure and methodology, this book is addressed to experts and interested laymen alike. For decision makers in politics and public administration the interdisciplinary connections are illuminated and planners, engineers and contractors will find valuable suggestions for their own work. Last not least, River.Space.Design is a rich source for everyone with a professional interest in water, a spring from which all may drink their fill! May we hope that our rivers retain, also in controlled form, the potency of their form and ancient power, to create and enliven a vital urban landscape for all town and city dwellers.

Foreword Herbert Dreiseitl

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Foreword

Fundamentals

Introduction Elbe, HafenCity Hamburg. Many cities are once again rediscovering their rivers. In these new waterside landscapes and waterfronts, responses to the multifarious demands of town planning, flood protection, ecology and amenity are interwoven in the most innovative ways.

Objectives Urban rivers and their environs have undergone a dramatic metamorphosis: having been long neglected, they are currently being developed into the most prestigious sites in town. This in its turn places a multitude of new requirements upon them, making their design disproportionately more demanding – urban riverscapes are to expected become attractive open spaces and a powerful locational factor in the economic competition between cities; the 2000 EU Water Framework Directive requires high ecological standards throughout, and at the same time urban development is exposed to extremes of weather and flooding as a consequence of climate change. All these requirements have to be met by urban watercourse systems – often within the most restricted of spaces.

Impetus for action from EU directives Seen in water management terms, predictions of climate change and isolated flood and low-flow emergencies have directed attention to the necessity of adapting urban river spaces. The prognosis of longer periods of drought, more frequent heavy downpours and rising sea levels has led to the critical examination of flood protection systems and of cities’ water supply and wastewater systems. The 2007 EU Flood Risk Management Directive committed member states to carry out precise evaluations of the dangers posed by flooding and to draw up management plans to improve flood protection. The resulting necessary mitigation works are bringing change to the urban environment, both above and below ground. In parallel, the EU Water Framework Directive prioritises ecological objectives such as better water quality and watercourse structure: The Directive requires member states to ‘protect, enhance and restore all bodies of surface water’ [WFD, Article 4]. Extensive surveys of the status quo are now being followed by many projects to fulfil the Directive’s requirements. Professional associations for water management such as the Deutsche Vereinigung für Wasserwirtschaft, Abwasser und Abfall e.V. [DWA 2009] are also devising their own regulatory frameworks for the design and enhancement of watercourses in the urban environment, and calling for holistic approaches to reconcile the sometimes contradictory claims upon them. In recent years, water in the city has been attracting increasing attention from an urban planning point of view. Cities are clearly turning their faces back towards their rivers and lakes: waterside living and work environments, city beaches, port regeneration and new riverside promenades are being developed to improve the quality of urban life. Responding to the relevance of this issue, for some time now many projects to redevelop the urban riparian landscape – both the watercourse itself and its banks – have emerged and been implemented. Taken as a whole, the performance requirements for these watercourses are complex, and demand close collaboration between the various stakeholders: water management professionals, town planners, architects, landscape architects, nature conservationists and representatives of other fields. Because of the multifarious nature of urban rivers, every project quickly becomes an interdisciplinary challenge; above all the conflicting interests of safety and the search for a new closeness to water places tremendous demands on designers’ capabilities. Today, many successful redevelopment measures have been undertaken around the world and are documented in diverse professional journals, books and databases. For those now faced with designing an urban river space, knowledge of good reference projects is important, but the search is laborious and time-consuming and the results tend to be unsatisfactory because each case study is too specific to be applicable to one’s own planning task. What has been lacking up to date is an overview that presents the wide diversity of design possibilities for urban river spaces in a systematic and transferable way. This book aims to fill this gap and serve as a primer and reference for designers of urban river spaces, and pursues the following primary objectives: 1. Create transferable knowledge From the built designs for urban river spaces that are exemplary for various reasons, the design tools were abstracted and sorted by typology. A catalogue of design strategies derived from this makes it easier for practitioners to

8 9

Fundamentals Introduction

transfer content to their own design tasks. Graphically clear presentations facilitate fast comprehension of the strategies and design tools and their spatial relevance. 2. Find an interdisciplinary language The typological order that has been devised integrates key aspects from all disciplines involved in the design of urban river spaces. This interdisciplinary presentation method and language promotes collaborative project work between landscape architects, ecologists, architects and urban planners – something of the utmost importance in the light of the complexity of designing urban river spaces. 3. Describe the processes of watercourses Rivers are constantly in motion – to say this is to make the obvious point that river spaces are subject to continuous transformation through the various water processes. Process orientation is therefore indispensable when designing river spaces, and should be reflected in the way a design is presented. Many presentations of designs for river spaces, however, concentrate on just one state or situation and thus fall short of their potential. To enable the principles of processual scenarios within watercourse systems to be understood, visual presentation forms and vivid descriptions of the water-related processes have been devised for this book. 4. Establish connections between ecology, flood protection and amenity The significance of process-oriented design for the three major thematic fields of river design in urban space – ecology, flood protection, and amenity – is made plain. Possible synergies (but also conflicts) between these three thematic foci in spatial design are revealed. The interdisciplinary composition of the team of authors, comprised of landscape architects and hydraulic engineers, made it possible to examine the projects selected for the book from various points of view. Interviews with local experts, literature research and the team’s own analyses were abstracted into design strategies, and a systematics of design tools and measures was devised. The basis of this systemisation was an analysis of watercourse processes, an understanding of which forms the basis for a conceptional categorisation and description of the various design possibilities.

D∂.3 Laid stone groynes

Two examples of transferable design tools from the projects examined: as part of the restoration of the River Birs in Basle, stone groynes were set across the current; in Wörth on the River Main spectacular folding floodgates were integrated in the old city wall when it was adapted as the flood protection barrier.

Selection of projects Preparation for this book began with selecting examples of good practice which fulfilled predefined criteria. The selected projects address at least two of the three objectives – ecology, flood protection, and amenity; and the projects pursue an integrative approach that combines at least two of the above mentioned requirements in the sense of multiple coding so that the restricted urban space is used in different ways and public funds can be effectively employed. Projects that pursue a single objective have been included only in exceptional cases. Projects with differing intentions and characters were deliberately juxtaposed. Ecological, hydraulic or architectural objectives may have provided the initial impulse for the project. Correspondingly, the composition of the editorial team and the design languages of the projects are diverse. Contrasting the various projects, especially according to the way they addressed river processes, engenders new interdisciplinary insights and synergies. Secondly, each project demonstrates a conscious design attempt to deal with river dynamics. The spectrum ranges from the smallest interventions in a riverside promenade to large-scale processes of altering the riverbed. Thirdly, at least one particularly innovative design tool should be present; each project addresses an aspect that distinguishes it from other projects, or exemplifies a specific aspect not found in other projects. The overall design quality and uniqueness of the projects were not the primary selection criteria. The rivers in the various reference projects are very diverse. Consequently, not all the design tools or measures are transferable to all other projects.

B5.3 Fold-out protection elements

∂0 ∂∂

Fundamentals Introduction

The book’s structure The book is divided into two volumes: The first, left-hand volume begins with a fundamental description of the essentials of high-quality design of urban river spaces and the various process typologies that shape rivers, their appearance and their transformations (Part 1, Fundamentals). These fundamentals lay the theoretical foundation for the heart of the book – a catalogue of systematically organised design strategies with their respective design tools and measures. The catalogue is divided into five different ‘Process Spaces’ (A to E) in which the water processes in the defined spatial area of the river are variously shaped by design measures (Part 2, Design Catalogue) The second, right-hand volume (Part 3, Project Catalogue) contains the projects researched as examples of best practice from which the design tools are derived. These references are fully described with illustrations and plans, and the contexts in which they came into being. The projects are grouped under the five Process Spaces, and then ordered alphabetically by the name of the river. Additionally, the appendix to the second volume contains an extensive specialist glossary and an overview of all projects, their design strategies and tools.

E2

Initiating channel dynamics

∫

All design tools in E2

can be combined with –––––––– C∂.4 Reprofiling the flood plain C3.5 Extensive natural areas E∂.∂ Removing riverbank and riverbed reinforcement E∂.2 Semi-natural riparian management E3.∂ Creating meanders E3.2 Incorporating a straightened channel E3.3 Creating multiple channels E4.∂ ‘Sleeping’ riverbank reinforcement E4.2 Bank reinforcement as needed E4.3 Selective bank reinforcement

Selective interventions in the river, in the form of specific shifting of substrate along the banks or on the riverbed, stimulate morphodynamic processes. This strategy is applied when the desired effect of channel development out of the river’s inherent dynamic would come about only after a very long time. Its aim is to optimise the framework conditions for the inherent dynamics to create a structurally rich watercourse more quickly. These redeployments, then, serve merely to prime a strong internal dynamic under which the initial condition of the river will quickly disappear. Within a short period, varied habitats can arise that facilitate speedy colonisation by flora and fauna. At the same time the appearance of the river can also alter quickly, making it easier to communicate the intervention to the public although the actual course of the river channel is not changed at first. Most design tools to introduce morphodynamic processes exert an influence on the currents in the river, setting erosion and sedimentation processes in motion through their specific variation. Broadening the river at certain points slows down the flow and thus creates aggradations, while disruptive elements placed in the current can deflect it towards one bank and initiate erosion. As design leitmotif, it is possible to strive for a semi-natural appearance, for instance by using dead wood, or conversely to make the deflection of the current obvious by using plainly artificial elements. Introducing disruptive elements also means expanding habitat diversity, and stepping stones can facilitate direct contact with the water. A further option is to influence the framework conditions for the discharge and sediment transport processes. Direct mechanical deposition of sediment on the riverbed makes it easier for sand- and gravel banks or natural beaches to arise, while stimulating the discharge dynamics reinforces the operative forces and thus accelerates developments.

E2.∂

E2.2

E2.3

Reprofiling the channel cross-section

Introducing disruptive elements

Adding bed load

Isar, Munich

Schunter, Braunschweig

Isar, Munich

Excavating the flood plain and flattening out the banks makes it possible for the river to develop beyond the former riverbank line. Selective excavations or shifting substrate within the riverbed creates sinks or shallow water zones; varying the crosssection raises the flow variation and a low water channel is created in the fasterflowing areas which serves to sustain the continuity of flow even during an extreme drought. Depending on the speed of the current, sediments of various particle sizes settle on the riverbed, as on the Isar in Munich, ranging from sand through gravel to larger stones – thus further enhancing the river’s structural diversity.

Disruptive elements are set at specifically chosen places in the riverbed to deflect the current towards the bank and catalyse erosion processes, or to encourage sedimentation on their sheltered downstream side. Disruptive elements can be just as well attached to the bank as placed in midstream. On the Schunter in Braunschweig fixed dead wood is used to divert the current and thus erode the opposite riverbank and create a steep slope. The disruptive elements enhance structural diversity and provide places to play and linger by on the waterside.

Many urban rivers lack natural sedimentation. Weirs or other cross-stream structures hinder the movement of bed load and the rivers suffer from sediment deficit. Deliberate deposition of sediments typical for the river provides material for riverbed enhancement. On the Isar in Munich, artificial sand or gravel banks were dumped, to be carried further downstream by the next high water event. If the lack of bed load is caused by weirs and ground sills, single openings in the structure can let the bed load overcome the barriers and contribute to the development of the next stretch of river.

–––––––– Isar, Munich ∆ 260

––––––––

–––––––– Emscher, Dortmund ∆ 256

Losse, Kassel ∆ 264

Isar, Munich ∆ 260

Isar, Munich ∆ 260

Werse, Beckum ∆ 270

Schunter, Braunschweig ∆ 266

Wahlebach, Kassel ∆ 268 Werse, Beckum ∆ 270

∂36 ∂37

Design Catalogue Dynamic River Landscapes

The design tools and strategies in the first volume, and the project examples in the second volume, are cross-referenced.

Connections

The two volumes are cross-referenced to be used in parallel. Several ways of reading the book are possible: in one direction, the abstracted design strategies in the left-hand volume can be put into tangible context by referring to the project examples in the right-hand volume, as all the design tools are linked by references to those projects that apply them. In the other direction, the projects in the right-hand volume make reference to the design tools in the left-hand volume. When the readers’ attention is caught by a tangible element of a project they can follow the reference to more closely examine the respective design tool and thus ascertain whether this element is transferable to their own project. The references are in the form of small arrows at the foot or the side of the page. An arrow to the left ∫ refers to the first, left-hand volume, an arrow to the right ∆ to the second, right-hand volume. This linking structure makes it possible to approach the book in several different ways; any of the three parts – Fundamentals (1), Design Catalogue (2) or Project Catalogue (3) – is an appropriate starting point.

2

A wild river with dynamic boundaries

1

Isar Isar-Plan, since 2000 Munich, Germany River data for project area Stream type: Large rivers in the alpine foothills Catchment area: 2814 km2 Mean discharge (MQ): 64 m3/s One-in-100-year flood discharge (HQ100): 1050 m3/s Width of riverbed: 50–60 m; Width of flood plain: 150 m Location: 48° 06’ 35’’ N – 11° 33’ 35’’ E ∫ Design tools –––––––– B3.∂ Invisible stabilisation C∂.4 Reprofiling the flood plain D∂.2 Dead wood D5.∂ Fish passes E∂.∂ Removing riverbank and riverbed reinforcement E∂.3 Regulating water extraction E2.∂ Reprofiling the channel cross-section E2.2 Introducing disruptive elements E2.3 Adding bed load E3.3 Creating multiple channels E4.∂ ‘Sleeping‘ riverbank reinforcement

260 26∂

As early as the middle of the 19th century the Isar was canalised and straightened. The ‘torrential water’, as the Celts called the river, was steadily altered as its riverbanks were reinforced, its forelands closely mown, and the flow rate restricted with low weirs, thus preventing fish from migrating and sediment from being transported. In addition, in the south of Munich nearly all of the water in the river was diverted into a parallel canal in order to generate electricity. Only about 5 m3/s continued to flow within the tight profile, little more than a rivulet. As negotiations about the establishment of a new residual flow in this section of the river began, a discussion about the conversion of the Isar was also set in motion. Today 15 m3/s flow through the actual riverbed. The Isar is a gravel-dominated river in the alpine foothills prone to violent and sometimes sudden floods. The project area of the so-called Isar-Plan begins upstream of the city proper and stretches over 8 km to the centrally located Museum Island. The plan is a joint project of the City of Munich and the Free State of Bavaria, represented by the Munich Water Authority. The intent of the Isar-Plan is to increase contact with nature, improve flood management and offer more activities for rest and relaxation. Broadening the average width of the river from 50 m to up to 90 m was ecologically prudent and also increased the discharge cross-section. Due to this broadening, the height of the existing dikes did not have to be increased and the existing trees on them could be preserved. The dikes were stabilised, however, by adding an underground wall construction to their cores.

In terms of improving the quality of the river, the concept will promote the development of large-scale morphodynamic processes within clearly defined boundaries. In this way the river can, within specific limits, develop its own course within the flood plain. In order to restore some of the Isar’s original momentum, it was important to free the river of its canal-like corset: The trapezoidal profile made of stones and concrete was broken up and other protective measures removed. In order to protect the dikes ‘sleeping’ bank reinforcements were put in place, i.e., underground layers of stones that prevent the areas behind them from eroding. The river’s gravelly banks are subject to continuous change and are used by Munich’s residents as a large urban beach in the summer. It’s the perfect place for bathing, barbecuing, sunbathing and for ballgames. It’s also a perfect place for small children, who can splash around in the shallow water, for dogs, and even for people on horseback. The long gravel beach is only interrupted near the various bridges that cross the river. In these locations the riverbank needs to be sealed and the gravel is replaced by stone steps or walls. At the steps it’s possible to see just how much the river’s level fluctuates. The steps also create an interesting contrast to the wild gravel banks and are used as waterside seating areas.

1 Steps secure critical bottlenecks and make the entire riverbank usable [E4.3]. 2 Schematic section: The location of the ‘sleeping’ reinforcement is clearly visible [E4.∂]. Dikes were stabilised by using a concrete core [B3.1]. 3 In the redesigned sections flat and constantly changing gravel beaches, like here at the Flaucher, have replaced steep grass-covered slopes. 4 Dead wood structures are held in place by foundations and initiate new processes of erosion and sedimentation. They are also very popular with playing children [D∂.2]. 5 The amount of water diverted into another branch of the river to produce energy has been reduced [E∂.3].

A learning process The floods in 2005 caused erosion damage beyond what had been planned for, and in doing so provided information about how the flood management strategy should be adapted. As there is no planned or paved network of paths near the edge of the riverbank, a rough trail running directly on top of the ‘sleeping’ bank reinforcement was created, which has destroyed the protective grass cover. Barriers have now been put in place to prevent people from using the trail. In some places the ‘sleeping’ reinforcement has even been washed out. And while in some areas this condition has been allowed to persist, the river will continue to be carefully monitored. In this way, the conversion of the river can be seen as a learning process.

4

3

5

Project Catalogue Dynamic River Landscapes

∂2 ∂3

Fundamentals Introduction

Prerequisites for Planning Urban River Spaces Reshaping river spaces in towns and cities can lead to simultaneous ecological improvement, regeneration of the urban habitat, and flood mitigation. The River Birs in Basle in 1987, and after revitalisation in 2005.

If the complex task of designing urban river spaces is to be undertaken successfully, in our view three fundamental prerequisites are required: firstly, the need to take into account the multiple demands made on urban river spaces – multifunctionality; secondly, constructive collaboration between professionals responsible for the design – interdisciplinarity; and thirdly, observing the principles and in-depth knowledge of the various water processes – process orientation.

Multifunctionality It is particularly in towns that the hybrid character of river spaces manifests itself: they are both artificial and natural at one and the same time. Urban rivers are spatially confined, artificially controlled hydraulic infrastructures. They are also important recreational spaces for city dwellers. Furthermore, they are linear ecosystems that link cities and regions to their entire catchment areas – the water from upstream regions flows through downstream regions and thus creates a feeling of community and a dependent relationship between riverside inhabitants, as changes in the upper reaches of a river always have consequences for the lower reaches. The question to be posed about the current restoration and restructuring of river systems in towns and cities is: How can the multifarious functional demands on the design of urban water spaces be combined? How can these demands be reconciled with the natural internal dynamics within the water itself? In the past, changing the internal dynamics of watercourses caused various problems; the attitude that urban spaces close to water could only be used to their full potential when they were protected from flooding and not subject to the river dynamics led to strict limitations on the space within the direct sphere of influence of the water or even to building over the water. Together with the frequently very poor water quality in the past, the result was that water spaces vanished almost completely from the awareness and daily life of city dwellers. At the same time, many aquatic plants and animals disappeared from the technically modified rivers; weirs and ground sills presented insuperable obstacles for many species and canalisation with its hard construction methods for riverbeds and banks destroyed natural habitats. A further cause of habitat degradation is the frequently radical clearance and dredging of watercourses with the sole aim of optimising drainage and flow rate with no regard for ecological and aesthetic interests. Despite such measures, a narrow watercourse crosssection is often too small to safely discharge the volumes of run-off water which result from increased surface sealing preventing infiltration and more extreme rainfall events. The objective of urban flood protection until recently was usually to discharge the flood wave downstream as fast as possible. It is only in the past few years that a new approach has asserted itself: of keeping precipitation where it falls whenever possible through infiltration, retention and storage of rainwater, and thus moderating the effects on downstream areas.

Interplay between ecology, flood protection and amenity

In this book an understanding of the internal dynamic processes of rivers serves as the starting point for all sustainable interdisciplinary projects with a view to contributing to the better integration of the many and various needs and challenges encountered in river restoration design. Three aspects dominate fulfilling this objective: more space for the water, more space for plants and animals, and more space for people. It is a matter of demonstrating the possibilities of a new synergy between what often appear to be incompatible demands. To this end, we will highlight aspects of the various design approaches and project examples which demonstrate the interplay of ecology, flood protection and amenity. When water has plenty of space, as it does in the Flaucher urban recreational landscape on the River Isar in Munich, reconciling these demands presents no great difficulties. The model of a natural river with a strong internal dynamic and riverside flood meadows that also serves, as does the Flaucher, as an important recreational space with a direct connection to the water, can, however, only be created within urban areas in the rarest of cases due to the otherwise more typical lack of space. Addressing this, one

∂4 ∂5

Fundamentals Prerequisites

particular focus of this book is on project examples and design approaches that combine responses to the various demands within very restricted spaces, aiming to stimulate creative and intelligent combinations, overlapping and interleaving of the various uses even when the possibilities appear limited – and even in places where the initial impetus for planning intervention was only one particular demand.

Interdisciplinarity The future design of water spaces presents a challenge that is not to be met by one discipline alone. In the light of this it makes sense to observe and reflect upon the mutual conditionality of hydraulic, ecological, urban planning and landscape architectural decisions.

Devising a common language

Often, collaboration is impeded by the lack of a common language, of basic technical knowledge of the other professional fields, and of interdisciplinary working structures. Projects are frequently prepared under the aegis of just one discipline, and the other disciplines, only brought in at a later date, are denied the opportunity to be involved in the fundamental conceptional planning decisions. In the course of revitalising urban rivers with the aim of improving flood protection and better integrating water in the townscape as a tangible presence, there have recently been numerous interdisciplinary design competitions and projects that have stipulated close cooperation between hydrologists, landscape architects and urban planners. These projects, however, lack a systematic foundation as they focus on single cases. Nevertheless, they offer the basis for creating new planning coalitions and innovative planning structures. The composition of and collaboration within planning teams has a crucial influence on the final quality of the projects.

System comprehension as a basis

This book seeks to contribute to a better understanding between professional disciplines. It is not intended to be read by just one profession but by representatives of all the professions involved in designing with water. By presenting the project examples and their design approaches from three specific points of view – ecological revitalisation, improving flood protection and integrating town planning and/or open space planning – we address the specific interests of different professional disciplines. Concurrently, presenting the cross-disciplinary nexuses stimulates better understanding of the complex demands on water space design; the basis of collaboration is served most strongly by a systematic understanding of water processes and a common language. In these respects, River.Space.Design can serve as a handbook for interdisciplinary teams and a basis for reaching mutual understanding.

The basis of interdisciplinary work on rivers is devising a shared language and an over-arching understanding of systems. This was the approach adopted for the joint creative work at the 2009 IBA laboratory on the consequences of climate change in Hamburg which also included fieldtrips.

Process orientation Can urban river spaces be designed without taking account of the various processes of water fluctuation and the power of water? Of course not – but glancing through publications on the design of river spaces will reveal hardly any presentations of various processual states; usually one set of water conditions (regarded by the designer as the ideal state) is shown, with no documentation or illustration of how the design responds to the changing rhythms of the water.

Designing with dynamic forces Reluctance to address processes can be found not only in the design of riverside areas; it appears to be a basic problem of all spatial design. Twenty years ago now, the American landscape architect George Hargreaves asked: ‘Why are static landscapes – frozen in space and time – the norm? Maybe it’s time to change that and the concept of beauty.’ [Hargreaves, 1993, p. 177]. To the present day, not much has changed to dispute this insight, and most clients (but also some designers) prefer glossy presentations of a project on a sunny day with normal water levels. We consider this less than productive, and wish this book to contribute to a processual understanding of design. Two main aspects are implicit in this objective: for one, we are concerned with a better comprehension of river dynamics and for this we have devised a new systematic, to be found in the following pages; for another, we use presentation methods that can illustrate the complex temporal-spatial interplay between water dynamics and design tools. Conveying the dynamic processes is essential if one is to put across ideas on designing urban river spaces effectively. Process-oriented work as task for the future

Urban river spaces are an excellent research subject for such process orientation for in them natural processes, civil engineering systems and designed landscapes are superimposed, constantly informing and reshaping one another in response to shifting conditions such as climatic change. The processual understanding and presentation methods devised for this book make it possible, because of their transferability, to consider every conceivable river space situation as a process and to design for it accordingly. Each project presents its own challenges, every body of water reacts differently, and every body of water has a different available space. Furthermore, it has to be accepted that the development of a project cannot be completely foreseen. Process-oriented designing means thinking and planning in terms of options, follow-up measures and responses to spontaneous developments. For many a local authority and planner, this ‘evolutionary’ way of designing is new; nevertheless, it is of great importance for the future. Process orientation is an important design principle for more than river spaces; it applies to all kinds of landscape, for they are shaped by a multiplicity of cultural and natural processes: settlement growth, transport access, the changing seasons, vegetation growth, geological processes and climatic change. Our hope is that the systematic typologies and presentation methods devised for this book will offer a model for future processoriented research and design, especially for urban landscapes.

∂6 ∂7

Fundamentals Prerequisites

Water Spaces and their Processes Historical map of the channel development of the Rivers Rhine and Neckar near Mannheim. The colours indicate how the river channels have shifted over time from the 6th century until 1850.

The word ‘process’ is derived from the Latin procedere, meaning ‘to advance, to proceed’; ‘process’ is a term for the directed course of an event – it is about movement, a dynamic, and an event that observes certain rules and regularities. ‘You could not step twice into the same river’ is Heraclitus’ famous dictum. This could also be understood to mean that water and processes can never be regarded as separate. The very currents and eddies of a river at any one moment show it to be a strongly dynamic element in the landscape, and observations over a longer period reveal that the entire river space exists in a constantly advancing, continuous process of change.

Rivers are dynamic

The entire scope of a river’s dynamic is hard to comprehend at first glance; nowadays it is to a large extent deliberately restricted and thereby mostly forgotten. Even so, the forces from which it is derived are ever-present and always potent. Mostly it is just the rise and fall of the water level which is clearly perceived by people, above all at times of extreme high or low water when changes are very noticeable. The extent to which bodies of water which are not subject to human influences are dynamic only becomes clear when observing the historical development of a watercourse over long periods. The constant shifting of the river’s course that can shape entire landscapes creates a complex, continually changing system – although the processes cover timescales that we cannot directly comprehend. The present course of a river is, seen in this light, no more than a snapshot in time of this ongoing process.

Processes and their driving forces The source of energy driving every dynamic process is the sun. It causes water to evaporate and the vapour rises to great heights where it condenses and turns into snow or rain. The potential energy stored in this way is transferred into kinetic energy when water falls as rain and flows down hills. The steeper the gradient, the more energy can be unleashed. In contact with the earth or rock, the kinetic energy of water can erode material and thus shape the terrain. The tractive force of flowing water carries the released material downstream; in principle, through erosion and sedimentation, rivers are continually wearing down higher-lying landscapes and raising the lower-lying river spaces. These processes are not constant and linear but occur in irregular phases: there are quieter and more dynamic phases but also sudden events such as cloudbursts and resulting flooding, through to natural disasters such as landslides or the avulsion of a loop in a river.

The energy that drives all the dynamic processes of a river and on which the natural water cycle is based is solar. When the energy-laden water meets the ground it creates a variety of river landscapes.

* * ** * * * * * ** * * * *

∂8 ∂9

Fundamentals Water Processes

Types of processes The processes that occur in flowing water are highly complex. Four spatially active processes are distinguished: vertical water level fluctuation and horizontal spread (bIue arrows), sedimentation shift (circular arrows), and the changing course of a river channel (grey arrows) through erosion (black arrows) and sedimentation (white arrows).

Rivers are highly complex systems within which interconnected processes occur simultaneously: physical, chemical and biological processes exert reciprocal influences. This book focuses on the spatially operant physical processes, as they are predominant for the shaping of river spaces. Essentially, it distinguishes between two types of dynamics, each with two sub-processes: 1. Temporary flow fluctuations Sub-process 1: vertical water level fluctuation Sub-process 2: lateral spread of the water 2. Morphodynamic processes Sub-process 1: sedimentation shift within the river Sub-process 2: self-dynamic river channel development

Temporary flow fluctuations

Lateral spread of water when discharge levels rise dramatically leads to regular flooding of the river plain, as here on the River Leine near Hanover.

The periodical spread and withdrawal of water, caused by the discharge dynamics of the river, occupies spaces temporarily. Water fluctuations are expressed both in rising water levels and in lateral spread across a flood plain. Fluctuations in water level are completely reversible; the watercourse returns to its original state. The volume of water that passes along a river varies dramatically through the year according to precipitation and snowmelt. The discharge volume differs in every river

system and also at every point within a river system, depending on the extent and characteristics of the catchment area and the local climate. Heavily built-up or steep catchment areas lead to more extreme discharge peaks in the river. Heavy rain causes high water conditions that flow downstream in a wave from the rainfall location. Sub-process 1: vertical water level fluctuations The discharge and resultant level of a river changes almost daily, although mostly it is only extreme high or low water events that are noticed. The water level in the river and during floods in the flood plain is in direct correlation to discharge from the catchment area. According to the space available and the roughness of the riverbed, the banks and the river foreland, a certain discharge rate causes a corresponding water level. This relationship can be described for single points along the watercourse as the ratio between water level and discharge. High water events are generally expressed in m3/s – the discharge volume and not the water level. Different water levels, both low and high water, have diverse consequences for the ecosystem and human use: while high water and floods present a danger for the riverside areas and can permanently alter the composition of species in an ecosystem through the power and depth of flooding, low water can cause serious problems for shipping and power station cooling systems. Should the water level sink very low or the watercourse dry up completely, this also places tremendous strain on the ecosystem.

cm

Leine

500

Main Isar

400 300 200 100 0 J

M

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A

M

J

J

A

O

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2008 cm 500 400 300 200 100 0 1

10

5

15

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cm 500 400 300 200 100 0 0h

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Each river has an individual flood pattern. The water level of a river changes continuously, even though generally only the extremes of flood and low water are noticed.

Sub-process 2: lateral spread of the water High water is especially conspicuous through flooding; minor rises in discharge levels can usually be contained within the river channel, but with larger high water events the river overflows its banks and covers the adjacent flood plain. This has a corrective effect: in flooding the foreland, which generally has a higher roughness, the water’s energy is dissipated and its height and speed reduced. Flooding is limited, when the river is not shaped by human measures, to the valley borders. Flood protection measures such as dikes cause an artificial limitation on the spread of the water and thus the flood area.

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Fundamentals Water Processes

The primary current carries water down the valley. The secondary current arises within the channel: along its central course, two contra-rotating spiral flows are created.

Reversible sediment shift processes: migrating gravel banks in the Isar River in Munich

Morphodynamic processes

The appearance of a river in the landscape represents the result of a manifold and complex morphodynamic development. The driving force is the river current, which on account of its numerous and complex sub-processes can barely be described comprehensively through scientific methodology. Exact predictions of how a river channel will develop are therefore not possible. The primary current carries the water down the valley. Secondary current is the rotation of water around the main flow direction; this is caused by the different flow rates near the banks, where the water is slowed by friction, and in the middle of the channel, where it flows faster. A secondary flow is created that pushes the water at the sides upwards and draws it down in the middle. Two contrary spiral flows are formed. In bends of the river the outer spiral flow is concentrated and accelerated, while in the inside curve the flow is slowed down because the distance covered is shorter. The flow of water causes erosion and sedimentation along the watercourse that subjects the river space to continuous morphological alteration. In these morphodynamic processes, that of sedimentation shift (sub-process 1) within the watercourse can be distinguished from shifts in the channel itself (sub-process 2). Sedimentation shifts in the watercourse are mainly expressed through the characteristics and structuring of the riverbed, and are to some extent reversible. With the inherent dynamic of the river channel, however, the river shifts its course and brings about irreversible restructuring across the whole river space.

Sub-process 1: sedimentation shift within the river The slower flow on the inner curve of a river leads to the deposit of sediment; a slip-off slope is created. On the outside curve, the cut bank, the fast cylindrical flowing current erodes the bank and deepens the bed. This means that the cross section of the river bends is asymmetric; the bank on the inner bend is flat, and on the outer cut bank there is a deeper channel (pool); the secondary flow cuts a channel in the riverbed that, when the water is low, carries most of the discharge and is thus called the low water channel. As a result of centrifugal forces created by the flow vortexes, the low water channel sinuously meanders from one side of the riverbed to the other, always on the outside edge of the outer bends. In straight sections of a river the riverbed is flat which is where riffles or a ford can form through sediment accretion. The state of the riverbed is constantly changing as a result of these dynamic processes. When discharge is low, water flow is slower and the deeper pools are filled with sediment, creating an almost level profile. When discharge is high the opposite happens: the tractive force is greater and the pools are further hollowed out. Where there are fords or riffles the flow rate is reduced, the sediment settles and the riverbed is raised. Thus the profile is further differentiated, the water is slowed by the irregularities and creates eddies. Through this alternating process of erosion and sedimentation the river as a system is self-regulating, and the longitudinal section of the riverbed varies around a relatively stable mean [Schaffernak 1950: p. 45]. Bumps and disruptive elements such as large boulders or dead wood create further variations in the flow profile along the river. These varying flow states cause small-scale erosion and sedimentation processes in which fine material settles in the quieter sections, while in sections with a faster current only coarse riverbed material can resist the flow. In this way temporary islands or sandbanks can emerge.

Low Water

Ford Pool

Pool

High Water

Ford Pool

Pool

Reversible sediment shift processes occur in rivers. At low water the pools fill with sediment while the fords are deepened. The low water channel remains. At high water the riverbed is deepened on the outside curve, the cross section becomes more irregular, and the rate of discharge is slowed.

Sub-process 2: self-dynamic river channel development An unrestrained river is continually shifting its channel, but this shifting occurs over such long periods that it is barely perceptible. With the help of historical and geological maps and soil analyses it is possible to reconstruct the old channel runs of a river and make this development visible. The images that emerge reveal the tremendous dynamics of natural rivers. All watercourses shift through erosion and sedimentation processes. The speed of this self-dynamic river channel development depends on the malleability of the local geology and the dynamics of the water. Rivers with a steep gradient and subject to extreme high discharge events can develop markedly faster than sluggish lowland rivers or spring fed streams. The meandering of a river is a self-reinforcing process, as the water flows faster on the cut bank on the outside of the bend and causes further erosion. The bank is literally ‘eaten away’ and steep edges are created. As the bank crumbles, the bend that thereby emerges shifts inexorably, both towards the edge of the valley and downstream.

22 23

Fundamentals Water Processes

Water landscapes as an expression of spatiotemporal processes Even though in principle the same processes occur in every river, no two rivers are identical. As exactly the same conditions can never be found in two places, each river is essentially unique. It follows that the design measures for each project must be precisely attuned to the individual river in question. River systems cover the surface of the Earth like a dense tracery of veins. Determined by topography, they comprise a branched system in which all the water of an area conjoins in ever-larger water channels. One speaks of catchment areas, between which ridges in the landscape act as watershed divides. According to whether they divide springs or areas with heavy rainfall, built-up areas or woodland, the volume and rhythms of the discharged water vary enormously.

Interplay between water and landscape

Each river forms the surrounding landscape in diverse ways, and conversely the surrounding environment exerts influence on the shape of the river through many factors. The land forming power of water arises from the close interplay of topography, geology, climatic conditions and the above mentioned erosive and accumulative activity of the current. Every river changes over various timescales and to various spatial extents; water landscapes are thus expressions of complex spatiotemporal processes. These formative processes arise from water as a transport medium, which through the power of its current, shifts soil and stone within the catchment area. The eroded material is ground down smaller as it travels downstream; depending on the gradient and the resulting flow rate, the transported material becomes finer on its journey from the steep headwaters in upland and mountain regions to the slow-flowing lowland reaches. Varying dynamics mean that different water landscapes are created along the upper, middle and lower reaches of a river. In the upper reaches, constant abrasion of material creates steep river valleys that cut deep into the ground. The degree of erosion is in direct Meanders of a river on the North German Plain – the River Leine near Hanover

24 25

Fundamentals Water Processes

River channels pervade the landscape like a dense tracery of veins.

relation to the geology of the local substratum and the discharge dynamics; because of the steep gradient no significant meanders are created. In areas where large amounts of sediment settle at once, such as plains at the foot of hills, the river can divide into several parallel branches. Material from upstream is transported to the middle reaches and some of it, along with locally eroded material, is carried on further downstream. In this way rivers are created that develop a relatively stable depth through their bed-load balance. Here, and in the lowlands of the lower reaches, one can observe marked meanderings of a river caused by slow discharge and heavy sediment deposits.

River sedimentation shapes the landscape In the lowland and delta areas of rivers, one can expect ongoing elevation of the land by deposited alluvial sediment. Varying flow conditions cause uneven rises in the flood plain areas: when the river overflows its banks the heavy sediments are deposited close to the river first – gravel and sands. These accumulate directly adjacent to the water as elevated rises, which are seldom submerged and on which the oldest human settlements along larger rivers are often to be found. During flooding the river current behind these rises is much calmer and finer sediments – clay and silts – settle and create clay soils on which the historically long uninhabited marshland landscapes developed. These were lower-lying than the sandy rises and could only be settled and farmed through major drainage construction and dike construction. A good example of such zoning is the Altes Land on the River Elbe near Hamburg. Its oldest settlements lie on the narrow riparian hill rises along the Elbe, from whence the lower-lying marshland could only be made productive by drainage and construction of ‘Marschhufendörfer’ villages, while the lowest marshland, furthest from the Elbe, has not been settled to this day and is used only for extensive farming. The elevation of land levels is particularly pronounced when the sea level rises and backs up the river discharge. The river water comes to a virtual standstill, sediments are deposited, and the riverbed and banks are built up. In delta areas the bed load carried downstream can create new land out to sea. As large quantities of bed load are deposited

when the flow is backed up, and impede and divert the main channels of the river, areas of new and divergent branching of the main channel occurs creating extensive delta landscapes with several parallel and interlacing water channels. One special case is that of inland deltas which occur when a river is influenced by the tidal flow and ebb of the nearby sea. Back from the coast, sediment is deposited where the river discharge is brought to a temporary halt by the flood tide and the current divides. The cities of Bremen and Hamburg were founded on these distinctive branchings of a river channel. This brief outline of the spatiotemporal dynamic of river processes makes it plain that the processes are in principle the same but the diverse characteristics of each location lead to a situation whereby no two river landscapes are alike.

Diverse conditions influence water processes, leading to different landscape and river types in the upper, middle and lower reaches.

Upper reaches

Middle reaches

Lower reaches Sediment deposits

Average discharge

River width

River depth

26 27

Particle size of riverbed material Gradient Catchment area

Fundamentals Water Processes

Designing Water Spaces The River Isar in Munich was given a new appearance through creative handling of the transitions from water to land. The river’s limits were shifted back at some points to allow natural beaches to reinstate themselves. Necessary bank reinforcements were shaped as steps to sit on.

The primary purpose of this book is to develop, by systematising of the innumerable existent water spaces, a readily comprehensible catalogue from which transferable knowledge can be extracted for future design tasks. The determinant factor of such a catalogue is the way it reveals common principles that apply to all conceivable water spaces. Through intense study, particularly of the processes that occur in bodies of water, we came to the conclusion that the determination of the river’s limits, the way in which those limits are set, is the decisive factor. The spatial character of a river emerges from delineating and restricting the two types of processes described above: variations in discharge and morphodynamic processes. Every anthropomorphically influenced river in an urban space has these two types of limits! It follows that designing for urban river spaces always addresses the limits of these two river processes.

Water spaces and their limits The spatiotemporal processes that unfold in a natural river present a daunting challenge for using the water spaces as human habitat. Uncontrolled shifting of the riverbed and the amount of space appropriated by high-discharge rivers endanger the settled and cultivated cultural landscape and have always challenged human beings to test their shaping powers as ‘Lords and Masters of Nature’ [Blackbourn, 2007, p. 37] against the dynamics of rivers and to set limits on rivers’ processes. We describe these limits in this chapter as ‘process limits’. The importance of setting limits on naturally occurring processes was described by Schaffernak in 1950 in his book on fluvial engineering: ‘Rivers run wild when left to their own devices. […] Agriculture is damaged by the destruction of cultivated land when a river breaks its banks, from flooding and changes to the groundwater level; shipping suffers from awkward shifts in the main channel and variations in water depth, and constructing hydroelectric plants is more expensive because their successful use is only possible on regulated watercourses.’ [Schaffernak, 1950, p. 5) The kinds of process limits that people have placed on river dynamics have, however, changed considerably in the course of time. In the pre-industrial era, coping with rivers was typically through small-scale measures to set the process limits, strongly oriented on the various dynamics of a river in the various landscape spaces and based on close familiarity with the habits of the river in question. As far back as the early middle ages, there were minor diversions and impoundments of watercourses to feed a mill or create defensive structures. Dikes and ditches were also constructed and the first breakthroughs in meander zones were made to divert water from endangered locations [Strobl, Zunic, 2006, p. 81]. On the River Rhine from the 12th century onwards, a whole range of terms appeared to describe the various types of main and subsidiary river branches, islands, narrow streams, oxbows and the different forms of flood meadows along the riparian corridor with a differentiated vocabulary [Blackbourn, 2007, p. 75]. The dynamics of the river were not markedly influenced by small-scale, uncoordinated measures, but attempts to redefine the process limits at a few points shifted the dynamics of the river further downstream so dramatically that the danger here was exacerbated – a situation that Blackbourn calls ‘hydrological leapfrog’ [Blackbourn, 2007, p. 77].

Large-scale canalisation in the 19th century

With industrialisation, rivers gained in importance as transportation routes and river valleys were correspondingly more densely settled. In the early 19th century, improved technical capabilities led to large-scale canalising of rivers and regularisation of the major river valleys: fundamental interventions such as straightening meanders to improve navigability, land reclamation and flood protection, along with the construction of large weirs. One very well-known engineer from this era was Johann Gottfried Tulla, revered as the ‘Tamer of the Rhine’ for his straightening of the river, whose first plans were submitted in 1809. These engineering works, which he called the ‘Rheinrektifikation’ (Rhine correction), were founded on the conviction that ‘no current, no river, not even the Rhine, needs more than one riverbed.’ [Tümmers, 1999, p. 145]. The straightening of the Rhine, involving breakthroughs of meanders and the removal of over 2000 islands, was the

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Fundamentals Designing Water Spaces

largest engineering project of the age, shortening the river between Basle and Worms by almost a quarter of its length, from 345 to 273 km and improving navigability immensely. Additionally, groynes and dredging made the river navigable all year round. In the course of industrialisation, setting process limits attained a completely new dimension in terms of scale and of durability. Not only at isolated points but along their entire length, rivers were forced into strong, continuous containments by sealing and securing the banks between dikes and reinforcements and the riverbed with groynes, weirs and ground sills. These changes to the process limits led to dramatic shifts in rivers’ dynamics: because of the faster water flow, erosion of the banks and bed increased markedly. As a result, the river cut ever deeper into the land, leading to sinking groundwater levels in the surrounding areas. Technical interventions made further sealing measures necessary; for instance, the entire riverbed on some sections of smaller branches was completely sealed. Dikes were constructed ever closer to the river. Although Tulla assumed that the Rhine would find a new equilibrium in its new bed and did not foresee the need for a comprehensive system of dikes, they became indispensible in the course of his ‘corrections’. Through the modern age, then, rivers became technically shaped civil engineering works. Today, only about one third of the original flood meadows along the rivers of Germany are flooded by major high water events. Along the Rivers Rhine, Elbe, Danube and Oder, the proportion is no more than 10 to 20 per cent [Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit, 2009, p. 4]. Ever more stable and higher dikes improve flood protection at particular locations but also create barriers in the landscape. Straightening rivers and cutting off backwaters stimulates faster water discharge. Simultaneously, however, the water retention capacity is reduced and the danger of flooding thereby aggravated. These lateral limitations on rivers are exacerbated by technical alterations to the river channel profile itself. Cross-current constructions such as barrages, falls and canalisation through pipes impair ecological permeability, that is, they present insuperable obstacles to the movement of most species. Sealed riverbanks and smooth riverbeds offer no habitats, and the loss of flood meadows means a scarcity of space for the metamorphic stages of water and riverbank inhabitants.

New objectives The photograph shows the River Wiese near Basle. Ground sills and the hard-surface trapezoid cross section have put the river in a straightjacket.

The findings of the survey carried out in the course of devising the EU Water Framework Directive show that 21 per cent of all the rivers and streams in Germany are still close to their natural state. The overwhelming majority of them lie outside conurbations [Umweltbundesamt, 2010]. Rivers in urban areas have been subject to the most radical alterations and represent the focus of this book, being dramatically

The classic trapezoidal cross section of a canalised river

—— —— ---

Flood limit Limit of the self-dynamic river channel development Riverbed seal

reshaped by the human hand both in their spatial structures and in their dynamics. Restoration of a river to its natural state is, because of tight spatial restrictions and the danger of flooding, usually neither feasible nor sensible. So what room for manoeuvre do we have for the shaping of process limits to achieve multifunctionality with regard to ecology, flood protection and amenity? What constructions or spatial solutions are suitable for the various types of rivers and the various local conditions? It is only when one is thoroughly familiar with the process limits that one can also modify them and sound out the corresponding scope for design interventions.

Types of limits In order to systematically collate and present the design possibilities for urban river spaces, this book defines two different process limits, corresponding to the two types of process dynamics explained in the ‘Types of processes’ section: temporary fluctuations in discharge, which affect the lateral spread of the water, and the morphodynamic processes that alter a river channel’s course. Accordingly, we define these two process limits as: 1. the flood limit 2. the limit of the self-dynamic river channel development

Flood limit The lateral spread of water from a river is possible up to a defined flood limit; the illustrations in this book mark this process limit as a green line, which could represent a dike or a flood wall. Within this limit, vertical fluctuations in the water level and lateral spread of the water surface happen. Beyond this line such occurrences are no longer taken into account; flooding would be a catastrophe. The limit is marked in the landscape as a dike, embankment wall or the natural edge of the river valley. The limit is always a relative limit, as it relates to a defined high water level; as high water events are never completely predictable, theoretically a level can occur that overflows all defence systems and thus the flood limit. The height of the green line for a defined protection level is calculated based on flood probability statistics or recurrence intervals. The risk that flooding exceeds this limit can, for example, lie at the height of a statistically defined one-in-100-year flood (HQ100) – a flood that could theoretically occur only once every 100 years. In densely built-up areas and places particularly vulnerable to flooding, such as Rotterdam in the Netherlands, there are ring dikes that are built to meet the possibility of a one-in-10 000-year flood. In terms of design and construction, this line represents a decisive interface as in front of and behind it fundamentally different preconditions for potential uses prevail and thus planning, construction and life patterns are also completely different. The limit of the flood area delineated by the green line, however, does not represent a particular protected status but the respectively defined protection level. It runs a little below the crest of the dike or flood wall, as a margin of safety for waves and wind pressure is calculated into the height of this crest, known as freeboard. Limit of self-dynamic river channel development

The second process limit introduced in this book is that of the self-dynamic river channel development, marked as a red line in the illustrations. Unlike in the space defined by the first process limit, erosion

30 3∂

Fundamentals Designing Water Spaces

Unlimited river dynamics

Limitation of flood area

Limitation of channel

Overflow no longer possible

+

Natural limit

On most rivers natural processes have been either restricted or are completely controlled. The flood limits (green line), set for instance by a dike, delineate the flood area. The limitation of the self-dynamic river channel development (red line) through, for example, embankment walls or low weirs curtails any shifting of the channel.

=

+

Flood limit

=

+

Channel development limit

=

Superimposition of the limits

and sedimentation processes occur within the area it delineates – the form of the river can change through its own dynamic and channel migration is possible. Often, this limit lies directly on the riverbank, so that the river can appropriate no more space through its own dynamic; the river is fixed in its course and the limit of a self-dynamic channel development is identical with that of the river itself. Frequently, the entire riverbed is also sealed and the river is thus physically constrained within a corset. If the limits of the channel development lie in the river flood plain, however, visible erosion and sedimentation processes are possible.

Riparian landscapes between control and dynamism The diagram of the four river spaces illustrates how the two limits and the process development of a river are interrelated. If a flood limit (green line) is set, erosion and sedimentation processes occur but can be influenced. The discharged volume of water is constrained into a narrower channel, whereby the water level and usually the flow rate increase along with erosive forces. The red limit hinders erosion of the banks and deepening of the channel; the self-dynamic development of the river channel is curtailed. If the green flood limit is brought to the water’s edge the two lines are superimposed and no flooding can occur, only fluctuations in water level. In such watercourses, which have usually also been straightened, in periods of high water the level rises fast and the current is very strong. This requires robust reinforcement of the riverbanks and bed if they are not to be breached. The problems increase, then, as the systems to control a river become increasingly constricting: the smaller the available flood space, the higher flood protection structures must be; the more the river’s inherent channel dynamics are restrained, the stronger the forces acting on the bank and bed reinforcements. Erosion or major sedimentation processes do not occur and cannot absorb the current’s energy.

The limits of controllability Today, around 200 years after large-scale river engineering began, the negative consequences of these interventions are apparent in our river systems. Both in the spirit of sustainable, long-term water resources management and in the light of ecological and amenity considerations, reorientation is needed. The policy of discharging water as fast and efficiently as possible, implemented through canalisation and dikes, may lead to better flood protection at specific locations but causes even greater problems at other points along a river’s course. The narrow flood limits accelerate and raise the high water surge. Moreover, precious retention areas in the landscape which delay flood waters are destroyed and can only rarely be reinstated. Structures with a retentive effect such as riparian woodland are removed, and retention areas such as backwaters are cut off from the main current. Maintenance measures to keep the river and the drainage forelands clear of obstructions have the same accelerative effect. Dead wood, sand and gravel banks, and riverbank vegetation that could slow down the current are removed from the river corridor. The ever increasing area of impermeable hard surfaces in new settlements intensifies the problem, as heavy rainfall runs off directly via storm drains into rivers. Urban river systems, which as a rule have little buffering capacity, create extreme and sudden discharge peaks. When floods occur DYNA

STATIC

←––––––––––––––––––––––––––––––––––––→ Temporary fluctuations Natural spread

Limit of lateral spread

More control concentrates more energy which needs to be restrained. The narrower the flood space the higher the water rises; the smaller the space for self-dynamic channel development the stronger the pressure on the bank and riverbed reinforcements.

No spread

Lateral spread Flood protection height

Morphodynamic processes River development without human intervention

Controlled flow processes

No processes

Channel development and sedimentation shift processes

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Forces acting upon riverbank reinforcements

Fundamentals Designing Water Spaces

the consequences are grave, as the apparently ‘safe’ hinterland is in no way prepared for flooding. As a result of climate change, we can probably expect more frequent and heavier rainfall events. The systems today often reveal themselves to be not flexible enough to meet these new demands. The highest priority, according to the EU Flood Risk Management Directive, should be the preservation and creation of retention areas along watercourses, and planning water retention measures in areas of human settlement [FRMD §14]. This would take pressure off river systems and create greater flexibility to absorb extreme rainfall events.

Structural diversity as an objective

Important impetus for the future design of rivers and waterways is provided by the EU Water Framework Directive. In that it sets important assessment criteria for the ecological quality of a watercourse not just based on the water quality itself but also on structural characteristics such as the form of the river channel, biological permeability, shape of the banks and the structure of the riparian corridor. The interventions of the last 200 years have led to a dramatic decline in biotope diversity and in the number of plant and animal species; important aquatic or marginal habitats such as areas of shallow water, reed beds, riparian woodland, tall herbaceous bank vegetation and flood meadows have given way along great stretches of rivers to a uniform trapezoidal section and restricted flood areas. This symmetrical, monoform canalisation of the river has flattened out variations in flow rates dramatically, and thus habitats such as gravel and sandbanks or steep, unreinforced riverbanks have almost completely disappeared. Weirs and ground sills obstruct the migration and procreation of many fish and amphibian species. The aim of implementing the EU Water Framework Directive is to reinstate a diversity of natural structures for which the potential natural state of the river serves as the model. Within urban spaces this is seldom feasible although there are some interesting approaches towards this aim, especially in the project examples of Process Space E in the third part of the book, the Project Catalogue. From an open space planning point of view, too, straightening and strict regulation of urban rivers is seen as lifeless and dull. Steep banks, a lack of shallow water areas, fords, sand or gravel banks, and strong currents make it difficult to access the water. Gently sloping, beach-like places have become rare. As water quality improves, playing and swimming beside and in rivers is theoretically possible again, but strong currents and steep banks make this unadvisable or even dangerous.

New flood limits

Improved flood protection with higher dikes and walls often separates urban areas from their rivers, blocking the view and making access difficult. On the other hand, raising the bank offers safe recreational spaces and a good view of the river, and charming rambling and cycling paths have been created along linear flood protection structures. Recent years have seen cities turning to face the water once more: In many communities there are new projects for living and working at the waterside, and in this connection much thought has been given to riverbank enhancement and new ways of coping with flood risks. The problems remain, however, of climate change and the attendant heavy rainfall events, and an increase in impermeable built surfaces in the catchment areas, from which new high water levels can be expected to make raising existing protection systems necessary. Urban riversides and dikes lying within dense development are difficult to make safer without permanently damaging the relationship between a city and its river. Responding to these planning challenges, there has recently been a re-evaluation of the rigid limits of earlier interventions in river systems. Limits have been drawn back from the water’s edge and river spaces thus expanded. At the same time urban regeneration or new-build measures have offered new opportunities to rethink spatial design addressing the process limits and find new approaches. All the projects examined here demonstrate this innovative approach to flood limits, to limits on the shifts in a river’s channel, or to the space between these limits. And so the objective of this book is to systematise the design strategies, tools and measures abstracted from the project examples. The proposed ordering into Process

Spaces is oriented on the interplay of water processes and spatial conditions – these are the fundamental categories with which all stakeholders in the design of river spaces must concern themselves. The resulting insight is intended to reveal possible areas of action and promote an interdisciplinary understanding that can overcome sector-specific, onedimensional approaches.

References Blackbourn, David, 2007. The Conquest of Nature: Water, Landscape and the Making of Modern Germany. London: Pimlico. Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit – BMU (Federal Ministry for the Environment, Nature Conservation and Nuclear Safety), department for public relations, 2009. Auenzustandsbericht, Flussauen in Deutschland. Berlin. http://www.bmu.de/files/pdfs/allgemein/application/pdf/auenzustandsbericht_bf.pdf, accessed March 3, 2010 Deutsche Vereinigung für Wasserwirtschaft, Abwasser und Abfall e. V. (DWA), 2009. Entwicklung urbaner Fließgewässer, part 1: Grundlagen, Planung und Umsetzung, DWA-M 609-1. European Flood Risk Management Directive (FRMD), 2007. Directive 2007/60/EC of the European Parliament and of the Council of 23 October 2007 on the assessment and management of flood risks. http://eur-lex.europa.eu/LexUriServ/LexUriServ.d o?uri=OJ:L:2007:288:0027:0034:EN:PDF, accessed July 25, 2011 European Water Framewok Directive (WFD), 2000. Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000 establishing a framework for community action in the field of water policy, Article 4. http://eur-lex.europa.eu/LexUriServ/LexUriServ. do?uri=CELEX:32000L0060:EN:HTML, accessed July 25, 2011 Hargreaves, George, 1993. Most Influential Landscapes. In: Landscape Journal, vol. 12 (2), p. 177. Schaffernak, Friedrich, 1950. Grundriss der Flussmorphologie und des Flussbaues. Wien: Springer. Strobl, Theodor und Zunic, Franz, 2006. Wasserbau: Aktuelle Grundlagen – Neue Entwicklungen. Berlin: Springer. Tümmers, Horst Johannes, 1999. Der Rhein – Ein europäischer Fluss und seine Geschichte. München: C. H. Beck. Umweltbundesamt (Federal Environment Agency), 2010. Daten zur Umwelt – Umweltzustand in Deutschland, Gewässerstruktur. http://www. umweltbundesamt-daten-zur-umwelt.de/umwelt daten/public/theme.do?nodeIdent=2393, accessed November 1, 2010 New flood limits in Wörth on the River Main: the dike park and flood protection walls, its gaps closed with temporary elements

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Design Catalogue

PROCESS SPACE

A

Embankment Walls and Promenades

→

DESIGN STRATEGY

A6 Adapting

Introduction

→

DESIGN TOOL

A6.2 Floating islands

The following Design Catalogue is the heart of this book – a collection that abstracts and identifies the ideas and design approaches from the projects we examined and presents them in the form of design tools and measures that are transferable to future projects. Thus it becomes easier for designers to discover appropriate measures for their specific contexts. To this end, the catalogue groups the design tools into five ‘Process Spaces’ in which they are applied. Within each Process Space the tools are subdivided into groups of design strategies.

Process spaces One of the greatest challenges of compiling this catalogue was to distil the common features from the multifarious urban river spaces we examined, and to summarise them in the form of a reasonable number of spatial types – it is only by means of such abstraction that it becomes possible to transfer and apply the many and various design tools to the most diverse design tasks. As a basis for the categorisation, specific urban river space situations were examined where the spatial conditions and river processes (from fluctuations in the water level to morphodynamic processes, see Part 1) are readily identifiable in clearly defined relationships to one another that vary according to the Process Space. We call these areas of the riparian space Process Spaces, and make a fundamental distinction between five types. In Process Space A, ‘Embankment Walls and Promenades’, the banks are very steep and there is hardly any flood area available. For this reason fluctuations in watercourse conditions are mainly vertical and morphodynamic processes are consequently excluded. In Process Space B, ‘Dikes and Flood Walls’, large vertical elements limit the flood area at some distance from the normal watercourse. Both horizontal and vertical fluctuations in the watercourse conditions take place, whereby the borders of this Process Space only permit very small-scale morphodynamic processes. Process Space C, ‘Flood Areas’, comprises spaces near the watercourse that are regularly submerged under its horizontal expansion and in which spatial design has to work with these processes. In these three Process Spaces A–C no alterations to the water space itself is intended; water flow fluctuations alone bring about their constantly changing appearance. In Process Spaces D and E, by contrast, morphodynamic processes dominate, such as the shifting of sediment or changes to the river’s course; the fluvial dynamics can be read not only in the changing water level but also in changes to the river itself. In Process Space D, ‘Riverbeds and Currents’, when the river is not sealed in places, reversible aggradation and erosion processes can happen along the riverbed, with consequences for the form of the riverbed and also the banks. Process Space E, ‘Dynamic River Landscapes’, is shaped by processes that are to be found in natural watercourses. By including the flood areas in the erosion and aggradation processes, the river can shift its entire course. In the graphic presentation of each Process Space, the processes that occur within the space and their limits are indicated in the same way as in Part 1: the flood limit is marked by a green line and the limits of a river’s self-dynamic development by a red line, while the location and extent of the Process Space is delineated by a grey rectangle. Most of the projects presented in this book can be categorised within one Process Space type, but very extensive projects can encompass several Process Spaces. For example, the project on the River Isar in Munich focussed on the revitalisation of the watercourse, and design measures can be assigned to Process Space E, ‘Dynamic River Landscapes’. However, the project also involved reinforcing the dikes, and this spatial situation comes under Process Space B, ‘Dikes and Flood Walls’ and employs its own particular design repertoire. Within a single project, then, design tools and measures from various Process Spaces can appear. As a rule, however, the design tools applied correspond with the Process Space to which the project is assigned in Part 3 of the book.

Design strategies The design strategies illustrate ways of responding to river processes in the design of waterside spaces. They describe an approach or an attitude that the designer adopts towards the water: for instance, to tolerate it, go with it, divert it, or do many other

38 39

Design Catalogue Introduction

PROCESS SPACES

A

Embankment Walls and Promenades

Limits Limits process space Flood limits Limits of self-dynamic river channel development Riverbed reinforcement

B

Limits of vertical water level fluctuation

Dikes and Flood Walls Processes

Limit of vertical water level fluctuation Horizontal spread Sedimentation shift

C

Flood Areas

Sedimentation Erosion Undercut bank Sediments

D E

Riverbeds and Currents

Dynamic River Landscapes

things. Each design strategy combines several practical design tools or measures that have all been influenced by this attitude. In Process Space A, for example, all the designs primarily address vertical fluctuations in the watercourse. One design strategy is to shape elements in such a way that they can be submerged when the water level rises without suffering damage. They are capable of ‘tolerating’ the rising water. Another strategy is to design elements to ‘adapt’ to rising water levels, as houseboats or floating jetties do. The spectrum of various design strategies makes it clear how many different ways there are within each Process Space of dealing with the respective water dynamics through design. Analysing the case studies made it possible to identify between four and six discrete strategies for each Process Space.

Design tools and measures

The individual design measures employed on site were identified using plans, literature, discussions and visits, subsequently abstracted in the form of transferable design tools and depicted in schematic sections or plans. Design tools can range from the smallest of measures such as individual seating areas by the riverside through to large-scale interventions such as the construction of retention areas. Two significant criteria had to be met before a design tool was included in the catalogue: constructive examination of and involvement with the watercourse dynamics, and multifunctional intervention. Preference was given to tools that responded creatively to the complex demands of urban water spaces and that could serve as a source of inspiration for future projects. The catalogue makes no claim to be a comprehensive list of all the possible design measures for watercourses, but is intended to offer many and varied suggestions for use in other designers’ work on water projects through its transferable design approaches and practical examples. The principle of each design tool is presented with a sectional or plan drawing and illustrated with a photograph of a project example. Links are indicated from each design tool to the projects in Part 3, while the Project Catalogue includes references back to explanations in the Design Catalogue of the design tools used in each project.

Combinations of design tools Hardly any design task for urban water spaces can be resolved using a single design tool; frequently, several design tools are combined within a Process Space. Proceeding from the experience gathered through our analysis of the case studies on combinations that often occur in practice or complement each other well, suggestions for combining design tools are made in the Design Catalogue. Each design strategy has a list of recommended combinations with design tools from other strategies: for example, flood protection walls (B2.1) from the list of B2 (Vertical resistance) strategies can be easily combined with a dike park concept (B1.1 Dike parks) by integrating the wall as a seating element or spatial organisation feature. The wall could just as easily be enhanced with mobile flood protection elements (B5.1-5.3) that make openings and windows in the wall possible.

40 4∂

Design Catalogue Introduction

List of process spaces and design strategies

A

Embankment Walls and Promenades

B

Dikes and Flood Walls

A∂

B∂

Linear spatial expansion

Differentiating resistance ∫ 72

∫ 52

A2

B2

Selective spatial expansion

Vertical resistance

∫ 54

∫ 76

A3

B3

Temporary resistance

Reinforcing resistance

∫ 56

∫ 78

A4

B4

Placing over the water

Integrating resistance

∫ 58

∫ 80

A5

B5

Tolerating

Temporary resistance

∫ 60

∫ 82

A6

B6

Adapting

Making river dynamics evident

∫ 64

∫ 84

C

D

Flood Areas

Riverbeds and Currents

E

Dynamic River Landscapes

C∂

D∂

E∂

Extending the space

Deflecting the current

∫ 92

∫ ∂∂4

Allowing channel migration ∫ ∂34

C2

D2

E2

Placing over the water

Grading the channel

∫ 96

∫ ∂∂8

Initiating channel dynamics ∫ ∂36

C3

D3

E3

Tolerating

Varying the riverbed

Creating new channels

∫ ∂20

∫ ∂38

C4

D4

E4

Evading

Varying the bank reinforcement

Restricting channel dynamics

∫ ∂22

∫ ∂40

∫ ∂00

∫ ∂04

C5

D5

Adapting

Varying the riverbed reinforcement

∫ ∂06

∫ ∂26

42 43

Design Catalogue Introduction

List of design tools and design measures

A

Embankment Walls and Promenades

Linear spatial expansion ∫ 52 A∂.∂ Intermediate levels ∫ 53 A∂.2 Terraces ∫ 53 A∂.3 Broad riverbank steps ∫ 53 A∂

A2

Selective spatial expansion ∫ 54

A2.∂ River access parallel to the bank ∫ 55 A2.2 River access perpendicular to the bank ∫ 55 A3

Temporary resistance ∫ 56

A3.∂ Closable access ∫ 57 A3.2 Retaining sightlines ∫ 57

B B∂

Dikes and Flood Walls

Differentiating resistance ∫ 72

B∂.∂ Dike parks ∫ 73 B∂.2 Trees on dikes ∫ 73 B∂.3 Reprofiling the dike section ∫ 74 B∂.4 Dikes as path networks ∫ 74 B∂.5 Dike steps and promenades ∫ 74 B∂.6 Superdikes ∫ 75 B2

Vertical resistance ∫ 76

B2.∂ Integrating flood

protection walls ∫ 77 B2.2 Influencing perceptions ∫ 77

A4

Placing over the water ∫ 58

A4.∂ Balconies ∫ 59 A4.2 Overhangs ∫ 59 A4.3 Suspended pathways ∫ 59 A5 A5.∂ A5.2 A5.3 A5.4 A5.5 A5.6 A5.7 A5.8 A5.9

Tolerating ∫ 60 Underwater steps ∫ 6∂ Boulders and stepping stones ∫ 6∂ Foreshores ∫ 6∂ Submergible riverside paths ∫ 62 Submergible boardwalks ∫ 62 Surmounting the embankment wall ∫ 62 Submergible furniture ∫ 63 Submergible planting ∫ 63 New embankment walls ∫ 63

Adapting ∫ 64 A6.∂ Floating jetties ∫ 65 A6.2 Floating islands ∫ 65 A6.3 Moored ships ∫ 65 A6

of the wall height Reinforcing resistance ∫ 78 B3.∂ Invisible stabilisation ∫ 79 B3.2 Glass walls ∫ 79 B3

Integrating resistance ∫ 80 B4.∂ Using the historical city wall ∫ 87 B4.2 Watertight facades ∫ 8∂ B4

Temporary resistance ∫ 82 B5.∂ Portable protection elements ∫ 83 B5.2 Attachable protection elements ∫ 83 B5.3 Fold-out protection elements ∫ 83 B5

Making river dynamics evident ∫ 84 B6.∂ High water marks ∫ 85 B6.2 Art objects and furniture ∫ 85 B6

C

Flood Areas

D

Riverbeds and Currents

C∂

Extending the space ∫ 92 C∂.∂ Setting back the dike ∫ 93 C∂.2 Branches ∫ 93 C∂.3 Flood channels ∫ 93

D∂

C∂.4 Reprofiling the flood plain ∫ 94 C∂.5 Backwaters ∫ 94

D∂.4 Piled stone groynes ∫ ∂∂6 D∂.5 Bioengineered groynes ∫ ∂∂6

C∂.6 Polder systems ∫ 94 C∂.7 Retention basins ∫ 95

D∂.6 Submerged groynes ∫ ∂∂6 D∂.7 Riverbed sills ∫ ∂∂7

C2 C2.∂ C2.2 C2.3 C2.4 C2.5

Placing over the water ∫ 96 Mounds ∫ 97 Mound principle with buildings ∫ 97 Buildings on piles ∫ 98 Escape routes ∫ 98 Cableways ∫ 99

Deflecting the current ∫ ∂∂4 D∂.∂ Large single rocks ∫ ∂∂5 D∂.2 Dead wood ∫ ∂∂5 D∂.3 Laid stone groynes ∫ ∂∂5

Grading the channel ∫ ∂∂8 D2.∂ Widening the channel ∫ ∂∂9 D2.2 Extending the flow length ∫ ∂∂9 Varying the riverbed ∫ ∂20 D3.∂ Sand and gravel beaches on inner bends ∫ ∂2∂

E3

D3

C3.4 Parks within the flood plain ∫ ∂02 C3.5 Extensive natural areas ∫ ∂02

D4

C3.6 Agriculture ∫ ∂03

D4.∂

C3.7 Camping and caravan sites ∫ ∂03 C3.8 Events grounds ∫ ∂03

D4.2

D3.3 Creating scour holes ∫ ∂2∂

D4.3 D4.4

Evading ∫ ∂04

C4.∂ Warning signs and barriers ∫ ∂05

Initiating channel dynamics ∫ ∂36 E2.∂ Reprofiling the channel cross-section ∫ ∂37 E2.2 Introducing disruptive elements ∫ ∂37 E2.3 Adding bed load ∫ ∂37 E2

D3.2 Sand and gravel beaches in bays ∫ ∂2∂

C4

Allowing channel migration ∫ ∂34 E∂.∂ Removing riverbank and riverbed reinforcement ∫ ∂35 E∂.2 Semi-natural riparian management ∫ ∂35 E∂.3 Regulating water extraction ∫ ∂35 E∂

D2

Tolerating ∫ ∂00 C3.∂ Paths within the flood plain ∫ ∂0∂ C3.2 Sports facilities and playgrounds ∫ ∂0∂ C3.3 Flood-tolerant buildings ∫ ∂0∂ C3

E

D4.5 D4.6

Varying the bank reinforcement ∫ ∂22 Partially naturalising the riverbank ∫ ∂23 Living revetment ∫ ∂23 Stone revetment ∫ ∂24 Terraced stone revetment ∫ ∂24 Masonry riverbank revetment ∫ ∂24 Building over the existing reinforcement ∫ ∂25

Dynamic River Landscapes

Creating new channels ∫ ∂38 E3.∂ Creating meanders ∫ ∂39 E3.2 Incorporating a straightened channel ∫ ∂39 E3.3 Creating multiple channels ∫ ∂39 Restricting channel dynamics ∫ ∂40 E4.∂ ‘Sleeping’ riverbank reinforcement ∫ ∂4∂ E4.2 Bank reinforcement as needed ∫ ∂4∂ E4.3 Selective bank reinforcement ∫ ∂4∂ E4

C4.2 Electronic warning systems ∫ ∂05

Varying the riverbed reinforcement ∫ ∂26 D5.∂ Fish passes ∫ ∂27 D5.2 Varying the riverbed and transverse structures ∫ ∂27 D5.3 Ramps and slides ∫ ∂27 D5

C5 Adapting ∫ ∂06 C5.∂ Floating and amphibious houses ∫ ∂07 C5.2 Marinas ∫ ∂07

44 45

Design Catalogue Introduction

A

Embankment Walls and Promenades

Leine, Hanover

From a hard embankment edge to a differentiated riverside area. Through the transformation, the boundary lines lose their separating character and a usable transitional area between water and land emerges. The scope for action is frequently limited to the steep embankment wall itself. 46 47

Design Catalogue Embankment Walls and Promenades

A

Embankment Walls and Promenades

Spatial situation

Process Space A comprises the vertical, artificially formed embankments often found in inner-city areas. Embankment walls serve both as flood protection and as riverbank reinforcement. Most of them were constructed centuries ago and thus exist in the context of a historical town centre or former industrial and harbour areas. They were the embryonic cells of the town’s development, the location of the earliest settlement where goods were loaded and unloaded from boats. They are to be found not only on former quays but also along old millraces where hydroelectric power was and sometimes still is harnessed. One special situation is that of rivers that, after being completely banished to underground culverts, are now being daylighted. The effect of these high, vertical banks, between which the water is constrained and runs far below ground level, is that rivers have effectively disappeared from the townscape. The water level at the mean or low water is so low that it is barely noticeable. Nevertheless, it is precisely these river settings that are crucially important for urban transformation and the development of high-quality inner-city open space. Additional physical space for all these watercourses and their banks is usually limited, and most of the vertical edges must therefore be retained during restructuring.

Operative processes

In stretches of a river located in Process Space A, characterised by vertical banks and no flood plain areas, variations in the discharge flow rate are seen only in vertical fluctuation; any horizontal spreading of the water is prevented. The flood limit (green line) is thus congruent with the limit of self-dynamic river channel development (red line) and defined by a single built element, as the embankment wall serves both as flood protection and as a riverbank retaining wall. Permitting morphodynamic channel development is, in these spaces, virtually excluded as a possibility. Small-scale current variations and sedimentation zones are, however, achievable through installations on the edge of the channel and by piercing the boxed profile at specific points.

Design approaches

The appropriate design tools and interventions for this Process Space transform its outer boundaries in sections or at periodic points, turning the narrow boundary line into an interface or a border zone. Restructuring this border area leads both to a stronger awareness of the river with its fluctuations in water level and to more differentiated usability. Rising water submerges the zoned border area successively and thus makes the spread of the river apparent.

A∂ Linear spatial expansion

A1.1 Intermediate levels A1.2 Terraces A1.3 Broad riverbank steps

A2 Selective spatial expansion

A2.1 River access parallel to the bank A2.2 River access perpendicular to the bank

A3 Temporary resistance

A3.1 Closable access A3.2 Retaining sightlines

A4 Placing over the water

A4.1 Balconies A4.2 Overhangs A4.3 Suspended pathways

A5 Tolerating

A5.1 A5.2 A5.3 A5.4 A5.5 A5.6 A5.7 A5.8 A5.9

Underwater steps Boulders and stepping stones Foreshores Submergible riverside paths Submergible boardwalks Surmounting the embankment wall Submergible furniture Submergible planting New embankment walls

A6 Adapting

A6.1 Floating jetties A6.2 Floating islands A6.3 Moored ships

48 49

Design Catalogue Embankment Walls and Promenades

A

Embankment Walls and Promenades

The design can encompass an entire urban riverbank or be just an installation of distinctive elements; water fluctuations can be taken into account in very different ways, demonstrated by the various design strategies. Some of the installed elements are exposed to flooding (A5 Tolerating), some of them avoid it by being placed above the flood zone (A4 Placing over the water) or they move up and down with the fluctuations of the water level itself (A6 Adapting). Even though the available space is limited, the numerous design tools and measures shown here demonstrate that there are still multifarious options for intervention.

Amenity

Many of the steep, high banks of this Process Space block direct access to the water, and roads along the riverbank often present a further obstacle and nuisance. New recreational spaces by the waterside make the river visible and accessible; the fluctuations in water level can be appreciated again. Differences in level can also be exploited to create special quiet places protected from traffic noise in the middle of town. In such constrained situations, new footpaths and cycleways alongside the water are often difficult to install. However, exceptional solutions such as floating jetties or pontoons can make a contribution to creating high-quality, distinctive places in a city.

Flood protection

In Process Space A the natural extent of the river flood plain is constrained by the artificial embankment walls, and dense urban development allows few alterations to this situation. Any intervention in these spaces should consequently avoid further reducing the discharge cross-section. Terracing back the riverbank walls can help enlarge the space to some extent, and the walls can be raised to improve flood protection. In this situation temporary solutions are preferable to avoid creating an additional barrier between the city and its river.

Ecology

Most of these hard, built-up banks are of little ecological interest; there is no amphibious marginal zone at all, and little riparian vegetation. On the channel bed there are few current variations and the river is impassable for many aquatic animals. These watercourses are classified under the EU Water Framework Directive as ‘artificial’ or at least ‘heavily modified’ and are thereby subject to weaker quality criteria: the ecological habitat must be improved where possible but need not be ‘good’. Nevertheless, smallscale habitat enhancement is feasible: places for fish to shelter from the current can be built in the river channel, and small amphibious zones created as stepping-stone biotopes make it easier for fish and amphibians to migrate up- and downstream. In certain places the embankment wall can be breached to create water-land connections for animals and plants.

At the vertical embankment wall, the flood limit and the limit of self-dynamic channel development are superimposed and become practically inseparable. By building broad steps – here on Pleiße Millrace in Leipzig – these boundary lines are drawn apart and a new flood space is thus created, forming at the same time a space for recreation by the water.

50 5∂

Design Catalogue Embankment Walls and Promenades

A∂ Linear spatial expansion

∫ All design tools in A∂ can be combined with –––––––– A3.∂ Closable access A5.∂ Underwater steps A5.2 Boulders and stepping stones A5.3 Foreshores A6.∂ Floating jetties A6.3 Moored ships

This design strategy presents various linear expansion possibilities to differentiate the riverbank more strongly and at the same time create somewhat more space for the water to spread sideways. This is achieved by terracing the riverbank walls. The flood limit (green line) shifts landwards, and the waterside area is stepped in terraces or stairs. This creates differentiated spaces within the flood area allowing direct access to the water. Elements made entirely of masonry or concrete such as steps are possible, but so are grassed terraces; the important factor is their resistance to erosion, as the terraces or riverbank steps also serve as bank revetment. They determine the edge of the watercourse (red line) and at the same time allow access to the water. The terraces or steps can be confined to a small stretch, or line a longer linear run along the watercourse. Spaces thus created are exposed, according to the height of the terraces, in varying degrees by fluctuations in flood levels; the height of the terraces determines the level and frequency with which they are flooded. Fluctuations in discharge are thus appreciably different, compared to the previous steep riverbank wall; for instance, the number of submerged steps make fluctuations discernible – how many steps are underwater today? Various design tools employ this strategy and differ in the design of the height and breadth of the steps or terraces used; the decision in favour of smaller or larger steps has an immediate bearing on possible uses for the new spaces – as steps, as seating or as intermediate terraces. This reconfiguration of the riverbank makes it possible to directly interweave the urban structure with the water space and can thus elevate formerly insignificant or degraded watercourses into prominent parts of the townscape and make them accessible again. Direct contact with the water is facilitated, and uses such as swimming or canoeing are made possible.

A∂.∂

A∂.2

A∂.3

Intermediate levels

Terraces

Broad riverbank steps

Nahe, Bad Kreuznach

Rhne, Lyon

Nahe, Bad Kreuznach

A broader intermediate level offers spaces for lingering by the waterside and temporary uses such as summer cafés. This element, frequently used over longer sections of a river, is conceivable when space is limited. The divisive character of a vertical riverbank is ameliorated and the flood area is improved by broadening the cross-section. In Bad Kreuznach, such a space was created with boat hire and a café by the water.

A staged transition to the water over several broad terraces permits several uses to coexist: in Lyon, ball games areas were laid out beside tree plantings. The design emphasises the twofold function of this area, on one hand access to the river and on the other an interesting recreational area beside the river. To develop its effect fully, this approach is suitable for longer stretches of a river. A gradual transition to the adjacent urban space can be created, without a perceptibly hard borderline.

Broad riverbank steps create public space beside the water, offering direct contact with the river at various water levels. By opening new sightlines they can achieve striking connections between the urban surroundings and the river. Diverse structuring of the steps enhances their various functions as movement spaces and pleasant places to linger, similar to the tiers of a sports stadium. The new riverside steps in Bad Kreuznach open up a wide vista to the surroundings.

Rhône, Lyon ∆ ∂60

––––––––

––––––––

Nahe, Bad Kreuznach ∆ ∂84

Limmat, Zurich, Factory by the Water ∆ ∂56

Elster and Pleiße Millraces, Leipzig

Rhône, Lyon ∆ ∂60

∆ ∂50

Nahe, Bad Kreuznach ∆ ∂84

Limmat, Zurich, Wipkingerpark ∆ ∂58

+ Limmat, Zurich, Bathing Facility Oberer

IJssel, Doesburg ∆ ∂72

Letten ∆ 280

Nahe, Bad Kreuznach ∆ ∂84

–––––––– Leine, Hanover ∆ ∂54

+ Elbe, Hamburg, New Elbe Promenade ∆ 279

52 53

Design Catalogue Embankment Walls and Promenades

A2

Selective spatial expansion

∫ All design tools in A2 can be combined with –––––––– A∂.2 Terraces A3.∂ Closable access A5.2 Boulders and stepping stones A5.3 Foreshores A5.8 Submergible planting

Unlike A1, here the continuous vertical limitation on the space is breached at just one location or opened at a selected point. Narrow access to the water via a ramp or terrace is created, ending in a beach situation sloping into the riverbed. This gently sloping access can be used as bathing beach, waterside playground, slipway or canoe landing place. As with design strategy A1 (Linear spatial expansion), the flood limit (green line) is pushed back, and the space that emerges is subject to variations in the water level; fluctuations in the river volume flow rate can be appreciated, in contrast to steep embankment wall, through the water spreading sideways. Depending on how the idea is executed, the bank reinforcement (red line) can be dispensed with at the beach cove that is created. As a calm area of water is created in the shallow inlet, currents are weaker and the slower flow means that sedimentation can be expected. Depending on the type of watercourse, a gravel or sandy beach can develop or mud can accumulate. The zones with reduced flow rate act as small ecological niches. In heavily built-up urban watercourses, usually with high flow rates, such a marginal foreshore offers space for special riparian plants, the various sediments enhance biotope diversity, and waterland access is created for amphibians and mammals. Thus, despite the artificiality of the watercourse, small habitats can develop. The two design tools shown here demonstrate possibilities of making selective periodic openings to the water. Choosing the direction in which access is laid – parallel to the bank (A2.1) or perpendicular to the bank (A2.2) – has various consequences for the organisation of the surrounding space.

A2.∂

A2.2

River access parallel to the bank

River access perpendicular to the bank

Leine, Hanover

Limmat, Zurich, Factory by the Water

Wupper, Müngsten

Where the riverbank wall is breached at a single point a place to linger at the waterside can be created. One space-saving solution to access this place and overcome the height difference is with steps or a ramp from the promenade running parallel to the riverbank. In Hanover, on the socalled Hohes Ufer and next to the floating café Leine Suite, a ramp-like pathway allows immediate access to the River Leine.

In the ‘Factory by the Water’ project (Fabrik am Wasser) on the River Limmat in Zurich an old, formerly filled-in branch canal was used to re-establish terraced access to the water.

An opening to the watercourse at right angles to the bank is the spatial counter-concept to parallel access (A2.1). The foreland topography is more strongly influenced, as the access cuts into the higher land. The angle of the embankment determines the length of the access, opening charming views of the water from above. The ‘Brückenpark Müngsten’ employs an alternation of open spaces far from the waterline and beach-like inlets.

–––––––– Leine, Hanover ∆ ∂54 Limmat, Zurich, Factory by the Water ∆ ∂56

–––––––– Wupper, Wuppertal ∆ ∂68 Wupper, Müngsten ∆ 230 Ahna, Kassel ∆ 234 Soestbach, Soest ∆ 248

54 55

Design Catalogue Embankment Walls and Promenades

A3 Temporary resistance

∫ All design tools in A3 can be combined with –––––––– A∂.∂ Intermediate levels A∂.2 Terraces A2.∂ River access parallel to the bank A2.2 River access perpendicular to the bank A5.4 Submergible riverside paths A5.5 Submergible boardwalks B6.∂ High water marks

Movable flood protection elements can supplement protective walls when there is a threat of flooding and also offer an opportunity to create breaks in the walls or build walls of more moderate height. Movable elements are only used temporarily, during periods of high water. However, their permanent mountings and closable watertight flood doors or windows make design references to the high water events; the visibility of protection measures sensitises people to the danger of flooding. The use of movable elements requires a sophisticated flood protection strategy, including operational planning to erect the elements storage facilities. Sufficient advance warning of flood events is also a precondition for the use of such elements. Preserving views (A3.2 Retaining sightlines) and accessibility (A3.1 Closable access) by using movable elements means that urban spaces which need a higher level of flood protection can maintain a close relationship to the river. Depending on how high above mean water level the elements are installed, they may be intended for use only rarely, in times of extreme flood events, or may need to be installed and dismantled again fairly frequently.

A3.∂

A3.2

Closable access

Retaining sightlines

Waal, Zaltbommel

IJssel, Kampen

Openings in flood protection walls can create direct access to areas which are subject to flooding. For example, closable doors or gaps are installed that are a precondition for use of the open space in front of the flood protection line. Both movable, temporary dam beams and permanently installed watertight gates or shutters are feasible. In Zaltbommel, a gap that can be sealed with a dam beams affords access to the lower lying harbour area.

By installing removable flood bariers or window flaps, sightlines and visual connections can be retained despite the need to increase the height of flood protection structures. Along with a clear view from the city to the water, the vista from the water or across the river to historical town waterfronts can be kept open. In Kampen, using temporary elements that can be mounted on top of the wall means that views of the river IJssel and the historical townscape remain uninterrupted.

–––––––– Seine, Choisy-le-Roi ∆ ∂64

––––––––

IJssel, Kampen ∆ ∂74

IJssel, Kampen ∆ ∂74

Nahe, Bad Kreuznach ∆ ∂84 Waal, Zaltbommel ∆ ∂90

56 57

Design Catalogue Embankment Walls and Promenades

A4 Over the water

∫ All design tools in A4 can be combined with –––––––– A5.6 Surmounting the embankment wall

On rivers and streams in dense urban surroundings and subject to intensive use, overhanging platforms or balconies that extend over the water as part of the riverbank structure create additional open space. They are unaffected by high water events as they are level with the top edge of flood protection constructions, that is, above the river’s dynamic processes. They jut out into the river space and offer a good view of the river and its processes, creating added drama und heightening awareness of the river among local inhabitants. The flood protection line is unchanged by this strategy. Balconies and overhangs in the flood-secure area do not impair the discharge cross-section or the retention space of the watercourse. Platforms at the level of the bank’s upper edge blend into the adjacent open spaces and can be used all year round. Where they face the water, these platforms are usually enclosed by a waist-high parapet or railing. The design of this barrier has a crucial influence on the visual relationship with the water. Balconies and overhangs constitute striking open spaces with high recreational quality. Depending on the location, well-frequented spaces for cafés and restaurants or secluded spots for relaxation and nature observation can be created (A4.1 Balconies). In constricted situations, overhangs offer additional space (A4.2 Overhangs) or can, with projecting jetties, help create new and attractive pathways in otherwise inaccessible riverbank areas (A4.3 Suspended pathways).

A4.∂

A4.2

A4.3

Balconies

Overhangs

Suspended pathways

Ebro, Zaragoza, embankment in the city centre

Elster Millrace, Leipzig

Elster Millrace, Leipzig

Balconies jut out at specific places into the river space and invite people to linger by the river. Hanging over the water, they open up new viewpoints for visitors. The prominent location permits views across the entire river that are not possible from the bank. With a transparent construction method, as on the river Ebro in the centre of Zaragoza, the ‘floating in mid-air’ effect can be heightened.

When daylighting a culverted river or waterway, the street that lies above it can be a major problem. In Leipzig, opening the waterway to its full breadth was only possible when part of the street was built as an overhang. Through transparent railings the Elster Millrace (Elstermühlgraben), part of which runs beneath the road, is clearly visible.

In the confined space of historical city centres, buildings often stand right on the waterfront, making new pathways difficult to construct. Boardwalks that are fixed only to the walls offer both a functional and aesthetically pleasing solution. The ‘suspended’ boardwalks over the Elster Millrace (Elstermühlgraben) in Leipzig enrich the cityscape and create attractive pathways that follow the watercourse through dense urban areas. They are difficult and expensive to build but do not impair the discharge cross-section and are not affected by floods.

–––––––– –––––––– Leine, Hanover ∆ ∂54 Wupper, Wuppertal ∆ ∂68

Elster and Pleiße Millraces, Leipzig ∆ ∂50

Leutschenbach, Zurich ∆ 240 Neckar, Ladenburg ∆ 242

––––––––

+ Ebro, Zaragoza, Riverbank Improvements

Elster and Pleiße Millraces, Leipzig

in the City Centre ∆ 279

∆ ∂50

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Design Catalogue Embankment Walls and Promenades

A5 Tolerating

∫ All design tools in A5 can be combined with –––––––– A∂.3 Broad riverbank steps A2.∂ River access parallel to the bank A2.2 River access perpendicular to the bank A3.∂ Closable access A4.∂ Balconies A6.∂ Floating jetties A6.3 Moored ships B6.∂ High water marks

The most interesting and charming spaces on rivers are those right on the riverbank; to sit by the river, dangling one’s feet in the water, watching the fish and the play of the current, is one of life’s simple pleasures for children and adults alike. To open up these spaces, even though they are regularly flooded, is the challenge that the design strategy of ‘Tolerating’ addresses. Urban open spaces lying between mean and high water level have to be designed and furnished to survive temporary submersion without significant damage – i.e. to ‘tolerate’ flooding. In town and city centres, such spaces are mostly only found as narrow strips or niches at the foot of the embankment wall. Furniture, planting and hardscaping must be robust and firmly fixed to resist the power of the current and damage if struck by flotsam. As an additional shield, pilings can be fixed upstream to divert flotsam into the middle of the river. Such places on the waterside are flooded at high water level and thus unusable then, but this promotes perception of changes in the water level. Outcropping boardwalk constructions can create new places at the waterside. Artificial foreshores made by depositing substrate can be planted and offer ecological niches in fast-flowing watercourses. The marginal riparian habitats that consequently arise are usually very rare in the vicinity of steep riverside walls. Individual, smaller biotopes thus created can offer refuges for migrating organisms, so-called stepping-stone biotopes. Furthermore, the striking contrast created by plantings along the stone embankment walls is particularly aesthetically appealing. Design measures such as shores, underwater steps or stepping stones improve access to the water and enhance the potential of these areas considerably. As a result, attractive and, due to their location at the foot of the embankment walls, often relatively intimate and secluded places can be created in the centre of towns.

A5.∂

A5.2

A5.3

Underwater steps

Boulders and stepping stones

Foreshores

Limmat, Zurich, Wipkingerpark

Limmat, Zurich, Wipkingerpark

Seine, Choisy-le-Roi

A flight of steps or a platform whose lowest step is below mean water level facilitates uses at various water levels and, especially, contact with the water. In Zurich, the shallow water on the last step offers tempting opportunities for paddling. Such a solution also presents important safety aspects – as the distance one could fall from the bank into the water is very short it is often possible to dispense with visually intrusive and obstructive railings or parapets.

Boulders and stepping stones which rise above the mean water level enhance the experience of flowing water by making direct contact possible. On the river Limmat in Zurich, stones have been set several metres out into the water. They are of varying height and thus make fluctuations in the water level vividly apparent. Water flowing over their rough upper surfaces creates interesting ripples.

Zones along the edge of a watercourse are elevated by depositing soil material which is then planted; sometimes the new substrate must be secured until it is sufficiently colonised by plants, for example with a geotextile layer. A green riparian corridor develops along the hardscape edge. Such shallow, calmer zones in large rivers offer ecological stepping-stone biotopes for migratory fish and amphibians. They are particularly suitable for inner-city rivers and waterways with a hard, uniform, boxed cross section, and are also very aesthetically appealing thanks to the contrast they provide with their mostly hardscape surroundings. On the river Seine in Choisyle-Roi, a suburb of Paris, the shoreline has been restored at the waterside: a marginal planting zone serves as an intermediary between the boardwalk and river, and reduces the danger of falling into the water.

––––––––

–––––––– Limmat, Zurich, Wipkingerpark ∆ ∂58

Limmat, Zurich, Wipkingerpark ∆ ∂58

Wupper, Wuppertal ∆ ∂68

Wupper, Wuppertal ∆ ∂68

Soestbach, Soest ∆ 248

–––––––– Seine, Choisy-le-Roi ∆ ∂64 Wupper, Wuppertal ∆ ∂68 Soestbach, Soest ∆ 248

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Design Catalogue Embankment Walls and Promenades

A5.4

A5.5

A5.6

Submergible riverside paths

Submergible boardwalks

Surmounting the embankment wall

IJssel, Doesburg

Seine, Choisy-le-Roi

Rhône, Lyon

People like to be able to walk along the water's edge; a continuous promenade at the foot of the riverbank wall offers this opportunity and also creates an intimate and secluded waterside space. The possibility of lingering, both on the wall and down by the water, creates diverse views and perpectives along the river. The paths are subjected to regular flooding, and cleaning after high water events may be necessary. In Doesburg on the River IJssel a broad promenade which also serves as a landing quay for boats has been laid at the foot of the new riverbank wall.

In many situations there is no continuous riverside path, and a connection in the form of a submergible boardwalk is a very good way of extending links, closing gaps and at the same time allowing attractive places to develop close to the water. Such a boardwalk combined with new green foreshores has made it possible for the residents of Choisy-le-Roi to take their ease by the river Seine. The robust steel construction and solid anchoring of the wooden elements ensures that the boardwalk can resist the strong currents of high water.

Steps, ramps and jetties that link the crest of a riverbank wall with submersible waterside areas can, when attractively designed, become visual highlights and pleasant places to linger by the water. Constructions can lie alongside or at an angle to the water and must be exceptionally robust to withstand the current and impact of flotsam during high water periods. In Lyon the height difference between the embankment wall and the water was exploited with various playground apparatus. –––––––– Rhône, Lyon ∆ ∂60

––––––––

–––––––– Seine, Choisy-le-Roi ∆ ∂64

Rhône, Lyon ∆ ∂60

IJssel, Kampen ∆ ∂74

in the City Centre ∆ 279

IJssel, Doesburg ∆ ∂72

+ Limmat, Zurich, Bathing Facility Oberer

IJssel, Kampen ∆ ∂74

Letten ∆ 280

+ Ebro, Zaragoza, Riverbank Improvements

A5.7

A5.8

A5.9

Submergible furniture

Submergible planting

New embankment walls

Rhône, Lyon

Rhône, Lyon

Ebro, Zaragoza, embankment in the city centre

On riverbank promenades that are often flooded, stable foundations and the choice of exceptionally heavy or flood-resistant material for the furniture and fittings are important, and the location and alignment of this furniture should correspond to the hydraulic conditions to which it is subjected. With a distinctive form, the furniture can raise awareness of flood likelihood and effects. The riverside promenade in Lyon lies within the flood area but is nevertheless equipped with a great variety of places to sit, and play apparatus with diverse surface materials. Here, a conscious decision was made to create high-quality structures, and the need for cleaning and maintenance after high water events was also accepted.

Planting in a river’s flood area at the foot of the riverbank wall can enhance the space considerably. Many plants originating from the seasonally flooded habitats of riparian woodlands can tolerate the alternation between flood and drought well. Design possibilities are multifarious, from single shrubs through linear planting to naturalistic riparian corridor planting. Along the Rhône in Lyon a green atmosphere has been created on the new river promenade.

When embankment walls are reshaped or built anew, the form and choice of materials are crucial to the ecological and above all the aesthetic quality of the river space. Selecting attractive natural stone, special types of concrete (as on the Leutschenbach in Zurich) and the right plantings (as on the River Ebro in Zaragoza) enliven embankments areas. Leaving wide gaps in the masonry can create ecological niches for flora and fauna. In the Brückenpark Müngsten on the River Wupper, the upper edge of the masonry shows visitors the water level to be expected during a one-in100-year flood event.

–––––––– Rhône, Lyon ∆ ∂60 Seine, Choisy-le-Roi ∆ ∂64

–––––––– Wupper, Müngsten ∆ 230 Leutschenbach, Zurich ∆ 240

––––––––

+ Ebro, Zaragoza, Riverbank Improvements

Rhône, Lyon ∆ ∂60

in the City Centre ∆ 279

Seine, Choisy-le-Roi ∆ ∂64

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Design Catalogue Embankment Walls and Promenades

A6 Adapting

∫ All design tools in A6 can be combined with –––––––– A∂.∂ Intermediate levels A∂.2 Terraces A∂.3 Broad riverbank steps A5.4 Submergible riverside paths

This design strategy employs elements that float on the water surface and visibly rise and fall with water level fluctuations, while the water can flow unimpeded beneath them. Through their prominent location on the open water surface, floating elements can act as a strong visual feature in the cityscape and are therefore very important in open space design. Traditionally, this principle has always been applied for shipping piers on major rivers, but the variety of uses has increased significantly in recent years; in addition to houseboats, bathing ships and floating islands have become a permanent feature of the cityscape in many European urban centres. Depending on the degree of integration with the open spaces on the adjoining riverbank, these elements are temporarily or permanently attached to a particular place. Their direct proximity to water and dependence on the watercourse flow rate variations accentuates people's perception of the river processes – from the strength of the current to the various water levels. Because the principle of floating elements starts at the water surface and adapts flexibly to water levels, the flow resistance and the discharge cross-section of the watercourse are hardly affected. If the pontoons are to be accessible from the bank, the height difference between the fixed riverbank edge and the floating elements must be compensated for by a flexible construction. Floating elements are easier to use in slow-flowing watercourses; where the current is strong, along with robust moorings or other fixings, a protective shield against dangerous flotsam during high water levels is recommended.

A6.∂

A6.2

A6.3

Floating jetties

Floating islands

Moored ships

Elster Millrace, Leipzig

Leine, Hanover

Spree, Berlin

A frequently used and simple variation on the ‘Adapting’ theme is the floating jetty, which can serve as a landing stage for boats or bathing pontoons and create highquality recreational open space right on the waterside. These flexible constructions can be architecturally interesting and raise awareness of the water’s dynamics. The jetty on the re-exposed Elster Millrace (Elstermühlgraben) in Leipzig permits access to the water and serves as a canoe landing place.

Floating islands open up spaces that were previously inaccessible: on the edge of the old city fortifications in Hanover a floating island was built to be used as a bistro, making it possible to sit right beside the water. Such small islands are directly exposed to water level fluctuations, waves and the current, so that the water is experienced almost as if from a boat.

Decommissioned or specially constructed ships can serve as houseboats, studios, discotheques, cafés or restaurants. In Berlin and Vienna, ships containing swimming pools make it possible to feel as if one is swimming in the river. These can be moored at places where the river water quality is not good enough for bathing. These ships are not dependent on the river water level and offer a good way of enlivening inner-city water spaces.

–––––––– Leine, Hanover ∆ ∂54 Elbe, Hamburg ∆ 202

––––––––

––––––––

Rhône, Lyon ∆ ∂60

Elster and Pleiße Millraces, Leipzig

Spree, Berlin ∆ ∂66

∆ ∂50

+ Danube, Vienna, Bathing Ship ∆ 279

+ Limmat, Zurich, Women's Bath Stadthausquai ∆ 280

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Design Catalogue Embankment Walls and Promenades

B

Dikes and Flood Walls

Main, Wörth am Main

From rigid protection line to multifunctional amenity – the outer limits of the flood plain are markedly distant from the river channel and are thus only periodically affected by water. Dikes or flood walls are designed to enhance the available open spaces and to serve as the connecting interface between the river system and the protected hinterland.

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Design Catalogue Dikes and Flood Walls

B

Dikes and Flood Walls

Spatial situation

Dikes are the oldest and simplest built structures for protecting areas from the dangers of flooding. In constricted urban spaces they are frequently substituted with vertical flood protection walls. The protection system is then overlaid with infrastructure lines such as railway tracks. Floodgates at crossing points can make these barriers permeable, but basically dikes and flood walls create a powerful spatial and functional separation between the urban area and flood plain.

Operative processes

The various limiting elements such as flood protection walls or dikes are at a considerable distance from the riverbank and thus affected by water only periodically. They are engineering works that contain and fundamentally shape the flood plain space. Dikes and flood protection walls are, within a certain return period, bound to be repeatedly exposed to the fluctuations of flood events. They must then withstand the scour of the current and pressure of accumulating water. From elevated dikes, fluctuations in river flow rate and the extent of the different water levels in the dike foreland are clearly visible.

Design approaches

The aim of design measures within this Process Space is to shape the protective limits not as rigid mono-functional lines but rather as multi-functional elements that enrich their surroundings. Process Space B lies within the flood protection line (green line) along the watercourse. These flood protection lines constitute the limit of the flood plain and can lie far away from the actual water channel. The flood limit is thus not related to the limit of the river’s dynamically meandering channel (red line) and can therefore be designed separately. In the face of the more extreme flood events that may be expected as a consequence of climate change, and through the new procedures and action required by the EU Flood Risk Management Directive, many flood protection systems must be renovated or raised. In a few cases the construction of new dikes may be necessary if the river space needs to be enlarged by setting dikes back or a city district needs additional protection. Through innovative design of these protective lines, the interrelation of the watercourse space with the protected hinterland can be improved. Openings in the protective line can, for example, make it less divisive and create visual connections; when the waters rise these openings are temporarily closed. Other appropriate design tools for this Process Space are aimed at using the protective line more effectively by promoting multifunctionality or integrating it more closely in other ways. When reinforcing or raising existing systems,

B∂ Differentiating resistance

B1.1 B1.2 B1.3 B1.4 B1.5 B1.6

Dike parks Trees on dikes Reprofiling the dike section Dikes as path networks Dike steps and promenades Superdikes

B2 Vertical resistance

B2.1 Integrating flood protection walls B2.2 Influencing perceptions of the wall height

B3 Reinforcing resistance

B3.1 Invisible stabilisation B3.2 Glass walls

B4 Integrating resistance

B4.1 Using the historical city wall B4.2 Watertight facades

B5 Temporary resistance

B5.1 Portable protection elements B5.2 Attachable protection elements B5.3 Fold-out protection elements

B6 Making river dynamics evident

B6.1 High water marks B6.2 Art objects and furniture

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Design Catalogue Dikes and Flood Walls

B

Dikes and Flood Walls

for example, new ideas are called for to minimise negative impact on the surrounding area. All design strategies in Process Space B, Dikes and Flood Walls, address the issue of resistance. The flood protection elements resist the horizontal expansion of the water and thus contain the flood space. The design tools show diverse ways or organising this resistance and simultaneously integrating it within the surroundings to the best advantage.

Amenity

Due to their height and divisive effect, dikes and flood walls are dominant elements of the landscape. For most of the year they stand far from the riverbank on the margin of the flood plain where, along with their latent flood protection role, they fulfil further open space functions. The topographic elevation can, for example, be exploited to create a viewpoint, to accommodate underground parking or as a dike park. Additionally, the linear structure of the flood protection line offers possibilities of combining it with a pathway system at various levels; dike top paths are especially popular with cyclists because their elevation offers a good view of the surroundings. A combination with residential development is another possibility; instead of living ‘behind the dike’, building on its crest can create high-quality housing with river views. The buildings can be part of the dike construction or stand on separate artificial elevations.

Flood protection

The flood limit (green line) in Zaltbommel on the River Waal runs along the top edge of the new flood protection wall. The wall can be temporarily raised with mobile protection elements (green dashed line). The limits of the river channel run between the riverbank reinforcements; between these lines is the flood plain, subject to regular flooding.

Constructing a flood protection line defends the settlements and structures behind it, but also means an artificial limitation on the flood plain and thus a reduction in a watercourse’s natural retention space. This in turn increases the flood discharge peak and the danger of flooding downstream. Through the building of dikes and riverbank walls, awareness of the danger of flooding has declined in many natural flood plain areas. Residential settlements and commercial estates are built; the area behind the dike is regarded as safe – and should the dike fail, the resulting damage is therefore all the more serious. If climate research is correct in its predictions there will be more frequent incidents of very heavy rainfall in the decades to come. For many rivers, the maximum flood level has been recalculated in recent years and the requirements for flood protection made more stringent. So far there are no absolutely reliable predictions of how flood events can develop as a result of

climate change. The necessary height for flood defences had to be adjusted regularly in the past and cannot be definitively calculated – and ideas for flexible solutions are needed. Raising the traditional dike system is very costly and often difficult to implement due to lack of space. Trans-regional thinking and joint implementation planning as required by the EU Flood Risk Management Directive are also increasingly important in the light of such developments. In many cases a dike need not be made higher when additional retention areas are created upstream or when the volumetric flow rate of the watercourse can be expanded. Reducing the flood damage potential by minimising the secondary or collateral damage of a flood is another flood risk management strategy: buildings in the flood area can be designed or converted so that they sustain minimal damage from floodwater. Furthermore, there are ways of minimising the extent of flooded area with double dike lines or compartmentalising the flooded area.

Ecology

Essentially, separating a river from its natural flood plain with a flood protection line means intervening in the flood plain dynamics, which will constrict the ecologically valuable space shaped by hydraulic fluctuations. In urban areas, dike systems and flood plain areas lying before them can also constitute large connected green corridors. Accompanying measures such as restoring a seasonal flood zone at the foot of the dike contribute to the networking of important habitats. As a contrast to the moist flood plain vegetation, the steep, dry sides of the dike can increase biological diversity; with appropriate sowing and maintenance strategies, species-rich biotopes can be created. This must, however, be carefully coordinated with the requirements of flood safety.

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Design Catalogue Dikes and Flood Walls

B∂ Differentiating resistance

∫ All design tools in B∂ can be combined with –––––––– B2.∂ Integrating flood protection walls B3.∂ Invisible stabilisation C∂.∂ Setting back the dike

If there is enough space the flood protection line usually consists of dikes (or levees) – artificial earth embankments sealed on the river side with clay. They are technical structures whose construction and form have been perfected over the centuries, above all to make them as secure as possible – and have subsequently become ever broader and higher. Today they constitute dominant landscape features with powerful design potential. The cross-section of a dike is the decisive factor for its function; almost everywhere, rivers are lined with dikes in a standardised monofunctional trapezoid cross-section. By modifying the dike, new spatial arrangements within the landscape can be created. Profiling the dike section or broadening it can actually improve the dike’s stability as well as enhance its design and use potential: sunbathing lawns or varied tree plantings can be accommodated (B1.2 Trees on dikes) or, by working with the transitional zone at the foot of the dike between seasonally wet and dry areas, biotope diversity can be enhanced (B1.3 Profiling the dike section). The gentler the slope of the dike the more it blends into the landscape and becomes barely perceptible as a barrier between the river space and the hinterland. The dike can thus become a dike park (B1.1). Narrow dikes with steep banks create a feeling of floating above the landscape (B1.4 Dikes as path networks). A staggered cross-section, by contrast, can dramatize the presence of the flood protection element itself (B1.5 Dike terraces and promenades).

B∂.∂

B∂.2

Dike parks

Trees on dikes

Main, Wörth am Main

Maas, Waalwijk, Maas dike

Nahe, Bad Kreuznach

Reshaping a dike as a park creates an attractive amenity close to the water. Instead of the usual trapezoid form, the dike crosssection is graded to be much broader and flatter, whether by building a new dike or reprofiling an existing one that needs, for example, to be made higher. The flatter dike is more stable and can accommodate various open space uses. On the River Main in Wörth, for example, allotment gardens and parking spaces have been laid out on the land side of the new dike, while sunbathing meadows and seats are on the river side. Because of the gently sloping cross-section and diverse design features, the dike blends into the green surroundings and its flood protection function is hardly noticeable.

In the past, dikes were often planted with trees. These tree rows provided shade and were prominent landscape elements visible from afar. A good example is the Maas dike near Waalwijk in the Netherlands. Today, as a rule trees cannot be planted on the river side of a dike; cavities in the body of the dike or gaps in the turf made by dead roots or fallen trees can endanger the dike’s stability. If the dikes are otherwise reinforced, however, planting is possible. One method is to oversize the dike, as on the River Main in Wörth, while another is to integrate a sheet piling wall within the dike construction.

In Bad Kreuznach on the River Nahe, a sheet piling wall, which also serves to regulate groundwater levels, runs through the centreline of the dike, which is landscaped, made larger and planted with trees and shrubs. –––––––– Main, Wörth am Main ∆ ∂80 Nahe, Bad Kreuznach ∆ ∂84

–––––––– Main, Wörth am Main ∆ ∂80 Nahe, Bad Kreuznach ∆ ∂84 + Nieuwe Maas, Rotterdam, Dakpark ∆ 280

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Design Catalogue Dikes and Flood Walls

B∂.3

B∂.4

B∂.5

Reprofiling the dike section

Dikes as path networks

Dike steps and promenades

Waal, between Afferden and Dreumel

Waal, between Afferden and Dreumel

Waal, Zaltbommel

The cross-section of a dike is essentially determined by safety and stability considerations – but its form also determines the effect of the dike on and in the landscape. The section of a dike in Ladenburg was graded in steps on the side towards the water, so that it could be used as a viewing terrace. When the Waal dike near Afferden in the Netherlands had to be raised, the landscape architects gave it a more strongly tapered profile so despite the fact that the foot of the dike was the same width the dike itself appeared less massive. From the ridge the upper part of the dike embankment is hardly visible and thus the experience of the surrounding countryside is heightened. At the foot of the dike on the water side is a swale which floods at high water and functions as a seasonally wet zone.

On old dike structures, there are often traditional, established pathways along the crest of the dike. Dikes as close-to-water linear landscape elements offer themselves as pathways and long-distance networks, linking spaces within a city and also linking the city to its surroundings. Because of their prominence in the landscape and location between what are often dense urban spaces and quieter green watercourse areas, paths along the top of a dike are especially valuable. The compulsory dike defence road along the foot of the dike does not exploit this landscaping potential and is thus not unconditionally suitable as a footpath or cycleway. The new Waal dike between Afferden and Dreumel in the Netherlands doubles as part of the national cycleway network and is quite popular in particular on weekends.

In city centres, opportunities may present themselves to develop protection structures as promenades, seating steps or terraces, thus fulfilling similar functions to those of urban riverbank walls in Process Space A – the difference being that these structures lie above the city ground level. In Zaltbommel on the River Waal, as part of dike reinforcement measures a modest promenade with seating was laid along the top of the dike, offering wide views over the river. In Hamburg, the same principle is applied on a large scale for the new Elbe Promenade. At the landing piers flood defences are raised by 1.4 metres, with curving steps to sit on and a wide promenade by the water. –––––––– IJssel, Doesburg ∆ ∂72 Waal, Zaltbommel ∆ ∂90

–––––––– Waal, between Afferden and Dreumel ∆ ∂88

–––––––– Waal, between Afferden and Dreumel ∆ ∂88

Neckar, Ladenburg ∆ 242

Ebro, Zaragoza ∆ ∂98

+ Elbe, Hamburg, New Elbe Promenade ∆ 279

B∂.6 Superdikes

IJssel, Doesburg

Nieuwe Maas, Rotterdam, Dakpark

Dikes can be developed into an entire urban landscape space. In Doesburg the former dikes were replaced with a construction that offers space for a new waterside promenade, foundations for residential buildings and underground car parking in one. At the top level of the dike are apartment houses with views of the river. The entire side of the old dike towards the water was remodelled as a flood protection wall with riverbank stairs. Such dike constructions with a very large crosssection are often classified as ‘superdikes’. Their multifunctionality means that these new protection systems made necessary by climate change can thus be better integrated in the existing townscape.

In the old harbour area of Rotterdam a shopping and office complex is under construction, the so-called Dakpark, scheduled for completion in 2013. Its backside toward the Nieuwe Maas River is planned as a park and the whole area is part of the flood protection system. –––––––– IJssel, Doesburg ∆ ∂72 + Nieuwe Maas, Rotterdam, Dakpark ∆ 280

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Design Catalogue Dikes and Flood Walls

B2 Vertical resistance

∫ All design tools in B2 can be combined with –––––––– B∂.∂ Dike parks B∂.5 Dike steps and promenades B4.∂ Using the historical city wall B5.∂ Portable protection elements B5.2 Attachable protection elements B5.3 Fold-out protection elements

Frequently, the flood defence lines in urban areas consist of by much more costly and complicated flood protection walls rather than dikes, due to the typical lack of space. As the floodwater rises vertically it exerts stronger pressure on the wall, and these constructions must have firm foundations, be very stable and watertight. Additionally, further measures often have to be taken to block groundwater flow beneath the structure. Erecting vertical protection elements can be necessary for various reasons: within existing urban structures, space to build a new dike or broaden the existing dike is often too restricted. Vertical flood protection walls require a minimum of space and are thus particularly suitable for densely settled areas. Apart from this, walls are often easier to integrate in the townscape than dikes; they can be used to delineate spaces, as seats, to structure the space, or as noise buffers. Higher walls, however, present serious obstructions to sightlines and paths; openings in the wall, perhaps combined with temporary flood protection elements, offer a solution. A further advantage of riverbank walls is that they can be raised when flood level predictions are amended as long as the foundations had been designed to bear extra loads.

B2.∂

B2.2

Integrating flood protection walls

Influencing perceptions of the wall height

Nahe, Bad Kreuznach, Roseninsel Park

Nahe, Bad Kreuznach, inhalation area next to salina

Flood protection walls are deliberately used as park boundaries, additional places to sit or visual and/or noise shields; their multi-functionality makes them easy to integrate in existing open spaces, and their form, the choice of materials and quality of detail can enhance the townscape. In Bad Kreuznach on the River Nahe the entrances to the Roseninsel Park were dramatized by the new flood protection wall; the line of the wall structures the space with a promenade and seating niches.

Protective walls between a town and its river can be a problem when the required height is above eye level. To ameliorate the loss of important visual connections the relative height of the wall can be reduced. To achieve this in Bad Kreuznach and in Zaltbommel, the ground level on the town side of the wall was progressively raised so that views over the river from the promenade were retained. In Miltenberg the same principle was applied and the entire urban riverside space redefined as a raised promenade with lower-lying open space close to the water.

–––––––– Main, Miltenberg ∆ ∂78

––––––––

Main, Wörth am Main ∆ ∂80

Main, Miltenberg ∆ ∂78

Nahe, Bad Kreuznach ∆ ∂84

Nahe, Bad Kreuznach ∆ ∂84 Waal, Zaltbommel ∆ ∂90

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Design Catalogue Dikes and Flood Walls

B3 Reinforcing resistance

∫ All design tools in B3 can be combined with –––––––– B∂.∂ Dike parks B∂.2 Trees on dikes B∂.3 Reprofiling the dike section B5.∂ Portable protection elements B5.2 Attachable protection elements B5.3 Fold-out protection elements

In many towns and cities, flood protection systems are currently being reinforced or raised in response to the EU Flood Risk Management Directive and predictions of the increased likelihood of extreme high water events as a consequence of climate change. The necessary height of flood defences has consequently been recalculated for many rivers over the last few years and requirements for flood protection redefined. Raising the height of existing dikes and protection walls is often plagued with difficulty: it can lead to interference with visual connections and paths in town, and raising dikes, especially, is very expensive – roads, bridges, weirs and locks all have to be raised accordingly. Raising a dike also makes broadening the cross-section necessary, and the space required is often not available. When trees are established on older dikes they have to be felled to make way for raising the dike. This can detract from the open space quality and – especially with large-scale clearances in urban areas – stir up protest and objection from local people. The design measures presented here minimise or avoid such negative effects.

B3.∂

B3.2

Invisible stabilisation

Glass walls

Isar, Munich

Rhine, Cologne, Flood Protection in Westhoven

If a dike is not strong enough it is possible, instead of widening the cross-section, to stabilise it with a wall of concrete or steel running along its core. This measure is not usually visible. In Munich, the priority was to retain the trees along a popular leisure path along the River Isar while stabilising the dike.

When it is necessary to raise the flood protection line in front of promenades or terraces, glass walls can be set on top of the existing protection elements. In this way visual connections between the river and its flood plain can be retained. The extremely robust watertight glass elements also offer interesting views of the water during flooding. In Cologne on the River Rhine, installing glass walls meant that views from private gardens into the surrounding countryside could be retained.

–––––––– Nahe, Bad Kreuznach ∆ ∂84 Isar, Munich ∆ 260

–––––––– + Rhine, Cologne, Flood Protection in Westhoven ∆ 28∂

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Design Catalogue Dikes and Flood Walls

B4 Integrating resistance

∫ All design tools in B4 can be combined with –––––––– B2.∂ Integrating flood protection walls B5.∂ Portable protection elements B5.2 Attachable protection elements B5.3 Fold-out protection elements B6.∂ High water marks

In compact town and city centres, historical old towns or residential settlements close to water, flood protection elements that are inserted between a town and its river are not desirable, and so in many places the flood protection line is integrated in the existing built structures. City walls or windowless house fronts are altered to be able to resist the rising water. Individual elements are sealed, stabilised and secured against underflow with steel piling walls. The existing structures are converted so that the flood protection line is hardly noticeable in the townscape and very few use limitations are created. Another possibility is to deliberately put the new protection systems in a dramatic setting and thus lend a town a special visual accent. This also raises and sustains local public awareness of the dangers of flooding. In seeking the best line it can make sense to leave certain buildings and infrastructure elements outside the protected area. Financial compensation for the lower level of protection, individual protection for specific buildings or installing flood-tolerant fittings and furnishings can be acceptable solutions for owners of less well protected properties; as their houses had hitherto been vulnerable to flooding in any case their situation is no worse than before. The existing (historical) structures acquire an additional and important function through such conversions: synergies between flood protection and conservation can make a contribution to retaining historic buildings.

B4.∂

B4.2

Using the historical city wall

Watertight facades

Main, Wörth am Main

Historical fortifications such as the remains of old city walls can be utilised for flood protection; their continuous line and location make them suitable structures for conversion, whereby the structural stability of the walls must be altered to meet the higher demands, and their surfaces treated to be watertight when river levels rise. In Wörth on the River Main the entire old city wall was replaced with a new wall of ferroconcrete that was then faced with the old stone in some places. In Kampen on the River IJssel, this was not necessary; the existing wall could be sealed and reinforced. –––––––– IJssel, Kampen ∆ ∂74 Main, Wörth am Main ∆ ∂80 Waal, Zaltbommel ∆ ∂90

IJssel, Kampen

Making building facades flood-resistant is an elegant way of improving flood protection when other measures are difficult to implement due to lack of space. The existing facades are sealed. In Kampen on the River IJssel this was achieved by watertight pointing of the masonry and the installation of windows with special safety glass, while the doors are protected with mobile flood protection elements. –––––––– IJssel, Kampen ∆ ∂74 Main, Wörth am Main ∆ ∂80

80 8∂

Design Catalogue Dikes and Flood Walls

B5 Temporary resistance

∫ All design tools in B5 can be combined with –––––––– B2.∂ Integrating flood protection walls B2.2 Influencing perceptions of the wall height B3.2 Glass walls B4.∂ Using the historical city wall B4.2 Watertight facades B6.∂ High water marks

Temporary flood protection elements either close gaps in flood defences or raise the flood protection level when the water rises. They are also called mobile flood protection elements because they can be moved. This principle makes it possible to leave openings in the flood protection line or to design it to be lower or less massive. Temporary flood protection systems are only fitted when the water rises to dangerous levels; the rest of the time they either stand open where they are installed or are removed and stored elsewhere. When the water rises they close the defence line and protect the hinterland from flooding. During periods of medium or low water they either disappear entirely from the townscape or, if they remain, are a constant reminder of the town’s location within a flood area. With these mobile elements, the path networks between the watercourse flood plain subject to flooding and the hinterland remain intact. Floodgates close roads, footpaths and cycleways only when the water rises to flood level. Protective walls can be built to moderate heights or with small gaps which are then raised or sealed respectively when floods threaten. The visual connection between town and river are preserved. Temporary flood protection may be divided into separate mobile elements that can be completely removed and stored elsewhere, attachable protection elements whose foundations remain on site, and fold-out elements permanently mounted on site to be opened and closed as necessary. Using moveable elements needs efficient logistics to mount them in good time when floods threaten: storage space, a trained team to install them and a good flood warning system are essential, delivering and fitting the elements must be perfectly organised and rehearsed; the period of advance warning determines how much time the team has to set up the moveable elements.

B5.∂

B5.2

B5.3

Portable protection elements

Attachable protection elements

Fold-out protection elements

Danube, Regensburg, Mobile Protection Elements

Waal, Zaltbommel

Main, Wörth am Main

Portable protection elements need no framework constructions or foundations and can be used at short notice and at any location as required. Along with the traditional sandbag barriers, much more manageable systems made of plastic and metal are available today, along with innovative systems such as enormous tubes filled with water. These elements are either a permanent component of the existing flood protection concept or serve as specific defences against unexpected flood disasters. Local authorities also use them in transitional phases when their protection systems are still in the planning or construction stage, such as on the Danube in Regensburg. In this case, when the water rises, light weight plastic elements are used.

Mobile barrier elements are fixed temporarily on permanent mountings. The flood protection line is only recognisable from these mountings under normal water conditions. They can replace or supplement permanent protection systems and often serve to close access ways and doors. Such elements can also be used over long distances. In Zaltbommel on the River Waal, for example, aluminium dam beams are set up along the existing flood protection wall to raise it during extreme high water events.

Openings in flood defence walls, dikes or buildings can be secured with shutters and gates when the water rises. Such defences are very impressive but also expensive as they are usually one-off constructions, specially designed and built for every individual gap in the flood protection line. In Wörth on the River Main a watertight cover was made for every entrance and every window in the old city wall. The large gates are today a well-known landmark feature of the city. ––––––––

–––––––– Limmat, Zurich, Factory by the Water ∆ ∂56

IJssel, Kampen ∆ ∂74

IJssel, Kampen ∆ ∂74

Elbe, Hamburg ∆ 202

Main, Wörth am Main ∆ ∂80

Main, Miltenberg ∆ ∂78 Main, Wörth am Main ∆ ∂80

––––––––

Nahe, Bad Kreuznach ∆ ∂84

+ Danube, Regensburg, Mobile

Waal, Zaltbommel ∆ ∂90

Protection Elements ∆ 279

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Design Catalogue Dikes and Flood Walls

B6

Making river dynamics evident

∫ All design tools in B6 can be combined with –––––––– A3.∂ Closable access A5.4 Submergible riverside paths A5.7 Submergible furniture B4.∂ Using the historical city wall B5.3 Fold-out protection elements

!

Along with practical flood protection measures, raising awareness of potential flood dangers among people living in flood-prone areas is an important part of a town’s safety concept; an appropriate response to a flood warning can save lives. Flood protection in many European countries today is, however, so technically sophisticated that people living behind the flood protection line are often unaware of the dangers of flooding. Design measures can make a contribution to keeping public awareness of the neighbouring river’s dynamic awake. A very effective measure is to deliberately create a dramatic setting for the flood protection line itself. Flood protection walls and gates are a subtle daily reminder of possible flooding; the height and structure of the walls show how high floodwaters could rise. Even if there have been no critical incidents of flooding for years, the danger is still present. One relatively simple measure is to install marks of former flood levels on buildings or at the riverside. This sensitises people to the danger, makes history more tangible and contributes to local identity. Objects or art installations in and on the water can also make a contribution to raising awareness of a river dynamic that is difficult to appreciate in daily life.

B6.∂

B6.2

High water marks

Art objects and furniture

IJssel, Doesburg

Petite Gironde, Coulaines

High water levels can be marked on buildings with scales and commemorative plaques but also with creative responses that go beyond the simple act of recording. The artistic and design quality of the place make the invisible visible thereby raising awareness of the watercourse dynamics and potential danger. In Choisy-le-Roi on the banks of the River Seine stands a sculpture of corten steel on which water levels are marked. The most recent high water level can still be seen from the changing colour of the rust. In Doesburg on the River IJssel, white stones in the black riverbank wall mark the various water levels reached in the year 1995. A plaque explains the installation.

In the retention basin of the Petite Gironde, designed as a park, one finds flood-tolerant seats whose highest point corresponds with the maximum water level.

–––––––– Seine, Choisy-le-Roi ∆ ∂64 IJssel, Doesburg ∆ ∂72 Waal, Zaltbommel ∆ ∂90

–––––––– Waal, Zaltbommel ∆ ∂90 Waal, Zaltbommel

Petite Gironde, Coulaines ∆ 2∂4

Art objects and installations that address the element of water or its processes enhance the value of the watercourse. In Zaltbommel two figures, one in front of and one on the dike promenade, indicate with their hand the protection level of the dike, making a subtle reference to the possibility of flooding and drawing attention to the dramatic fluctuations in the river’s water level. Giving furniture in flood-sensitive areas a particular form is another way for artists and designers to establish connections between open space and water dynamics.

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Design Catalogue Dikes and Flood Walls

C Flood Areas

Ebro, Zaragoza

From monofunctional flood plains to submersible landscapes with multiple uses. The area between the flood limit and the river is so designed that, despite naturally occurring periodic flooding, it can be used as open space for recreation while serving as a natural habitat for many riparian species at the same time.

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Design Catalogue Flood Areas

C Flood Areas

Spatial situation

Through the industrial uses, settlement and intensification of agriculture that went with dike construction and artificial land elevation in preceding centuries, many natural flood plains have been lost. Retaining and creating new zones along rivers that can be flooded in the event of high water is today an important contribution to flood protection. These retention areas create space into which the river can expand as the water level rises. Along with flood protection, flood plains in urban spaces mainly serve recreational purposes; for many towns and cities, amenities along watercourses with direct access to the water offer the principal local recreational area. A system of cycleways and footpaths along the flood plain also represents a connection between urban and rural spaces outside the city. Process Space C may also be described as the submersible space between the reinforced, i.e. fixed limits of the river (red line), which also constitutes the limits of the self-dynamic river channel development, and the flood limit (green line): this is the river plain, which is flooded when the water level is high.

Operative processes

This Process Space is flooded at irregular intervals. The water level can, according to the river’s size, vary by several metres. It is possible to extend the flood area by shifting the flood protection line back into the hinterland. Another possibility is to excavate the flood plain. The extent of this space influences the water level reached during high water events; if the flood area of a river is extended the water level in this stretch of the river sinks. Furthermore, the flood surge is slowed and weakened where the water can spread out; the area serves as a retention space. Water levels commonly rise following long periods of rain during the winter months and after the snowmelt in spring, but also after heavy sudden rain in summer, and the flood plain can be under water for anything between a few days and several weeks. Away from the main current the flow rate decreases, sediment can settle and, especially in lowland watercourses, lead to slow but continuous raising of the flood plain area. After each flood the higher areas dry out, while in the lower-lying places ponds and marshy areas remain that store water for longer periods. If they have contact with the groundwater they survive as isolated patches of water all year round.

Design approaches

The challenge for design is to find the best possible combination of the various functions – water retention, nature and recreation – of these spaces. Setting back the dikes and lowering the dike foreland enlarges the retention space and creates new retention areas for water, but at the same time also affects the ecological

C∂ Extending the space

C1.1 C1.2 C1.3 C1.4 C1.5 C1.6 C1.7

Setting back the dike Branches Flood channels Reprofiling the flood plain Backwaters Polder systems Retention basins

C2.1 C2.2 C2.3 C2.4 C2.5

Mounds Mound principle with buildings Buildings on piles Escape routes Cableways

C3.1 C3.2 C3.3 C3.4 C3.5 C3.6 C3.7 C3.8

Paths within the flood plain Sports facilities and playgrounds Flood-tolerant buildings Parks within the flood plain Extensive natural areas Agriculture Camping and caravan sites Events grounds

C2 Placing over the water

C3 Tolerating

C4 Evading

C4.1 Warning signs and barriers C4.2 Electronic warning systems

C5 Adapting

C5.1 Floating and amphibious houses C5.2 Marinas

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Design Catalogue Flood Areas

C Flood Areas

potential and usability of the riverside spaces that are thus created. The basic prerequisite for designing these spaces is adjusting the planned use to the frequency and duration of high water events and the different degree and depth of flooding according to various probabilities (from annual high water events through to a one-in-a-100-year flood, HQ100). Along with this, flood-resistant access which is also to some extent usable during flooding is necessary; alternatively, evacuation via elevated escape routes triggered by various warning and information systems can be devised. Although the river plains offer themselves primarily for recreational uses, more and more flood-tolerant constructions such as raised or floating structures or buildings on mounds are being built in flood plains. This can be understood as the first indications of increasing interest in integrating flood hazard areas in the townscape – and conversely regarding part of the townscape as a flood area. Extensive spaces that carry no water at mean water level but are flooded at high water can be enhanced with flood-adapted uses. The design tools and measures shown illustrate the possibilities of extending, accessing or using the Process Space intelligently and practicably. The interventions and uses adapt to fluctuations of the river in various ways. The design strategy can consist of allowing flooding more space, tolerating temporary flooding, or evading it. Floating elements, for example, can adapt to fluctuations in water level within the flood plain. For the measures in Process Space C, it is mostly the objectives of flood protection or nature conservation that dominate. Above all, in large-scale projects with an eye on care and maintenance costs, a combination of expanding retention spaces and developing natural flood plain landscapes is most attractive, while in city centres recreational uses dominate.

Amenity

The spaciousness and closeness to water of flood plain areas means that attractive open spaces can evolve that are easily accessible and offer a good variety of possible uses. These flood plains offer space for events, sports and leisure facilities, sunbathing meadows, barbecue areas and playgrounds. These uses are temporary as they are restricted to flood-free periods. Rivers that are enhanced as nature conservation areas or extensive cultural landscapes can provide high quality leisure and recreation environments if they are appropriately accessible in densely populated urban spaces. They constitute durable open space structures, as they are on the one hand indispensible for flood protection and on the other hand exclude the possibility, with a few exceptions, of permanent uses such as housing or commerce and industry. For these reasons floodsensitive areas represent a chance for the long-term securing of recreational space for city dwellers, and their development can signify a positive impetus for urban development. Good zoning to avert conflicts between nature conservation and leisure seekers is important. The linearity of these spaces means that they can be a structurally important part of a city’s greenspace system.

Flood protection

River flood plains make an important contribution to flood protection. Extending the flood plain can be an economical alternative to raising the height of dikes; as the water level rises the river can spread out over the retention space, thus reducing the flow rate and high water surge, and relieving the burden on downstream areas. There are various possibilities for enhancing this relief effect: measures such as shifting the flood protection line towards the hinterland or expanding the flood plain volume by excavation increases its retention capacity. Polder systems can use the available space more effectively. The shaping of the flood space influences the hydraulic roughness and thus also the interplay of currents during high water periods. High roughness causes the water to back up, thus raising the water level and delaying discharge. If the flood plain is restricted, for the purposes of flood protection all elements that could further delay discharge need to be removed, and tree planting or development of riparian woodland cannot be permitted. When such areas are redesigned this can lead to conflicts of interest with the aims of nature conservation, urban development or recreational uses. One solution can be to oversize the retention space to compensate for higher roughness.

Ecology

Natural river spaces that develop without human restraint are particularly precious because of their rarity and high structural diversity. With the variety of biotope types in variably moist areas these riparian woodlands, marshy areas and water meadows are home to an enormous diversity of flora and fauna. The areas are therefore particularly suitable for nature enhancement, especially because intensive agriculture or construction of new residential or commercial estates in flood plains is possible in only a few exceptional cases. Creation of more flood spaces can also happen on a large scale; the Dutch flood protection programme ‘Ruimte voor Rivieren’ (Room for the Rivers), for example, pursues the idea of creating an interlinked system of quasi-natural ever-changing landscapes along the major rivers wherever new flood containment areas are needed, complemented by flood-adapted urban development in the towns and cities they pass through. Implementing and marketing riverside building projects at selected sites can also make a contribution to financing more extensive river development measures elsewhere. The areas developed this way have become valuable nature conservation areas and charming urban and recreational landscapes.

In Zuera on the Gallego a new branch of the river was dug to bring the water closer to the town. The flood limit (green line) was also defined in such a way that the new bullring is flooded in high water periods – the water remains in the arena for a few days afterwards and creates a lake.

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Design Catalogue Flood Areas

C∂ Extending the space

∫ All design tools in C∂ can be combined with –––––––– B∂.3 Reprofiling the dike section B∂.4 Dikes as path networks C2.∂ Mounds C2.3 Buildings on piles C3.∂ Paths within the flood plain C3.2 Sports facilities and playgrounds C3.4 Parks within the flood plain C3.5 Extensive natural areas C3.6 Agriculture C4.∂ Warning signs and barriers C4.2 Electronic warning systems C5.∂ Floating and amphibious houses C5.2 Marinas E∂.∂ Removing riverbank and riverbed reinforcement E2.∂ Reprofiling the channel cross-section E3.∂ Creating meanders

In the spirit of flood protection but also for ecological reasons, any current river project aims to create more flood space for a typically tightly constricted watercourse. Various methods can be applied to allow the water to spread. They differ primarily in the space required and in the possibilities of controlling the flood event. Setting back the dike line or creating new retention basins extend the flood space horizontally, while excavating the existing flood plain creates a larger discharge cross-section for the floodwaters without infringing on the old limits – the reinforced river channel or the flood protection line. Excavating the flood plain can, in densely populated areas, offer the only option to create new retention spaces. Retention areas are either controllable flood polders, opened to reduce flood peaks in the river, or they flood progressively as the water level rises without human intervention. An unregulated scheme has two different effects on the high water event. One is that the expanded flood space causes the water level to sink in a specific section of the river and it provides a retention space in which the water can be temporarily stored. The other effect is that a larger discharge cross-section means that the high water event is guided downstream faster. Polders, on the other hand, can be kept closed until the highest water level is reached and then flooded. This means that in emergencies the high water peak can be capped and floods averted. The frequency, duration and depth of flooding in unregulated areas are determined by the topography and hydraulic characteristics of the river. These factors influence the possible design and usability of the areas. The spectrum of high water events ranges from regularly occurring high water levels to extreme situations that occur only every 50 to 100 years. For the safety of users in these spaces the advance warning period, the speed of the flood and its height are above all relevant. The expansion of water-susceptible habitats with a high water dynamic creates potential for nature enhancement close to town as well. As the flood areas may not, as a rule, be built on, creating flood channels, for example, can secure open space and recreational areas and enhance a town or city’s greenspace system.

C∂.∂

C∂.2

C∂.3

Setting back the dike

Branches

Flood channels

Emscher, Dortmund, retention basin Mengede

IJssel, Zwolle

Elbe, Magdeburg, Flood Diversion System

This measure is relatively expensive as a new dike line has to be built. Old dike structures or former low summer dikes can be productively integrated in the new plans as embankment paths. Setting the dike further back from the water is a very effective way of easing bottlenecks in the system. The extensive new retention areas on the River Emscher, where the dikes that previously ran directly alongside the canalised watercourse were set back by up to several hundred metres, now offers a possibility that extensive natural areas will develop in the middle of the Ruhr region.

Dividing the discharge and creating a new branch results in excavating the flood plain, as has happened on the Vreugderijkerwaard near Zwolle on the River IJssel. The retention space of the riparian landscape expands. A branch that carries water at normal levels creates new water areas and banks that are not subject to the strict rules of the navigable main channel. Shallow areas of water and dynamic morphological processes can be allowed and thus valuable natural areas created. The division of the water volume and type of connection to the main channel – on one or both sides, in or against the flow direction – must be precisely determined for each case to guarantee a minimum navigable depth for shipping or, with a small watercourse, to ensure that it carries the minimum necessary flow.

Flood channels are alternative routes which can carry away water in extreme conditions and thus avert flood damage. Water that overflows the bank at the spillway is fed into the channel and back into the river further downstream. They can run through the city, as does the Fulda flood ditch in Kassel, or divert floodwater through the hinterland around sensitive areas of the city, as the Elbe channel does in Magdeburg. Within the area of flood channels, as a rule, no building development is possible which can help to secure open urban space. Regularly submerged areas, if appropriately laid out and maintained, can develop into valuable biotopes, as it was the case at the Kyll mouth in Trier.

–––––––– Bergsche Maas, between Waalwijk and Geertruidenberg ∆ ∂94 Emscher, Dortmund ∆ 256

–––––––– Kyll, Trier ∆ 2∂0 Schunter, Braunschweig ∆ 266

–––––––– Gallego, Zuera ∆ 204

+ Elbe, Magdeburg, Flood Diversion

IJssel, Zwolle ∆ 208

+ Fulda, Kassel, Flood Ditch Area ∆ 279

System ∆ 279

Waal, Gameren ∆ 224

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Design Catalogue Flood Areas

C∂.4

C∂.5

C∂.6

Reprofiling the flood plain

Backwaters

Polder systems

Isar, Munich

Ebro, Zaragoza

Rhein, Brühl

Excavating the flood plain profile creates a larger discharge cross-section for high water surges without the need for additional flood areas. Reshaping the shore and bank areas with more frequently submerged marginal areas and more gently sloping banks is a possibility which can enhance both the ecological value of the flood plain areas, often used as farmland, and its accessibility. In Munich such reprofiling led to the creation of broad gravel beaches and shallow water areas that are inviting places to play.

In the flood plain, ponds, scrapes and oxbow lakes can be dug that will develop into valuable wetland biotopes. During high water events they fill with water; if they have no contact with the groundwater they will dry out periodically. They represent a refuge for riparian flora and fauna and enrich the experience of the flood plain. Their depth and extent can be determined according to ecological aims and with regard to the needs of specific species. In Zaragoza’s Parque del Agua, illegally dumped building waste was removed from the Ebro flood plain, and small riparian biotopes were laid out and planted up.

Polders are spaces enclosed by dikes on all sides with a sluice gate to control the influx and outflow of water. Regulated polders make highly effective flood protection possible by capping the peak water level. Because of the technology required for the sluice gate, constructing a regulated polder system is relatively expensive. Designing the polder’s limits and the influx/ outflow sluice gate is also a crucial determining factor in the evolution of the landscape. The Koller Island polder near Brühl on the River Rhine has developed, through its conversion, from a purely agricultural area to a nature and recreational space with an emphasis on equestrian sport.

–––––––– IJssel, Zwolle ∆ 208 Kyll, Trier ∆ 2∂0 Waal, Gameren ∆ 224

–––––––– Ebro, Zaragoza ∆ ∂98

––––––––

Seille, Metz ∆ 246

Kyll, Trier ∆ 2∂0

Rhine, Brühl ∆ 2∂8

Isar, Munich ∆ 260

Waal, Gameren ∆ 224

+ Rhine, Ingelheim, Polder Ingelheim ∆ 28∂

Losse, Kassel ∆ 264

C∂.7 Retention basins

Petite Gironde, Coulaines

A retention basin holds back the discharge volume of a river when the water level rises. Such basins can be laid out so that the water flows through them (direct connection) or so that the basin lies beside the river and is filled via a connecting channel (side connection). By dividing a retention basin into several chambers to be filled in succession it is possible to use less often flooded areas more intensively; the first basin will be under water most often. By building in sections it is possible to integrate the dikes and chambers into the landscape less intrusively by averting the need for a very high dike. For instance, in the rainwater retention basins on the stream Petite Gironde, which runs in a gutter through the centre of the basin, the first basin is strongly reinforced as the water flows into it with considerable force. The second and subsequent basins are flooded less often and designed as a public park. The overflows between the chambers are reinforced to resist erosion. Another type of retention basins are stormwater retention areas, as used on the Seille River in Metz, which are designed to store stormwater for a short time to reduce discharge into the river itself. –––––––– Petite Gironde, Coulaines ∆ 2∂4

94 95

Seille, Metz ∆ 246 Emscher, Dortmund ∆ 256

Design Catalogue Flood Areas

C2 Above the water

∫ All design tools in C2 can be combined with –––––––– C∂.∂ Setting back the dike C∂.3 Flood channels C∂.4 Reprofiling the flood plain C∂.6 Polder systems C∂.7 Retention basins

Using flood plains as settlement and commercial areas has always demanded specially adapted built forms. The oldest human settlements in river spaces used the principle of placement on higher ground, above the water; in north Germany the first settlements arose on natural elevations such as dunes with the church at the highest point. Later, artificial earth dwelling mounds were thrown up, known in German as Warft or Wurt, to build and live on. When high water threatened everyone fled to the mounds, and people and animals could hold out above the floodwaters until they receded. Another possibility for living on the waterside is building on piles. On Lake Constance such buildings were used as long ago as the Stone and Bronze Ages, and in countries where water is a dominant element of the landscape such as Vietnam and the Philippines this is still a very common way of building. In Europe, the flood-endangered areas in front of the dikes were regarded for a long time as unsuitable for new settlements. Today, the question of whether flood plains can be used for new residential or commercial development has resurfaced, due to their attractive waterside location, and the old principles are being resurrected: new housing areas on mounds and buildings on piles are emerging, while elevated pathway systems or boardwalk constructions are being developed to provide safe access to these areas. In the event of a disaster they serve as escape routes to evacuate people or valuable goods. A relatively rare but very attractive means of access high above the water is offered by cableways. They can set a very special accent in urban river spaces and have proved their worth not only as temporary attractions at horticultural shows but also as permanent connections in a riverside path network. The design tools presented here are all suitable for application in flood plain areas; they contribute to the emergence of new landscapes in the river plains, which have been rediscovered as places to live and work. The elevated paths and mounds create new visual relationships and views. During a flood the landscape changes: the mounds become islands or peninsulas, while the lower-lying areas are transformed into open water.

C2.∂

C2.2

Mounds

Mound principle with buildings

Rhine, Brühl, horse ranch on mound

Waal, Gameren

Elbe, Hamburg

As mentioned above, the mound principle is very old, examples are known dating from the 3rd century CE. They are earthworks, settlement mounds built of earth. Near Brühl on the River Rhine in Germany this strategy has been revived to locate a horse ranch in a flood polder. In the Overdiepse Polder on the Bergsche Maas in the Netherlands a dike was shifted back from the water and the farmhouses along the dike rebuilt on new mounds. This concept makes it possible to submerge parts of the existing farmland to relieve the flood pressure when water levels rise. The mounds remain accessible via the dike. Mounds consist, exactly like dikes, mostly of a sand core covered with a layer of clayey soil and planted with grass.

In designating and developing nature conservation areas, refuge hills are built in the flood area to which animals can flee to as the floodwaters rise. When land is used for grazing by horses or cattle to keep vegetation down, as in the Gamerense Waard on the River Waal in the Netherlands, this is essential for the herds’ survival. For wild animals, too, refuges above flood level are important in nature conservation areas.

In former port areas in Hamburg outside the flood protection line a new city district, the HafenCity, has been built. Along the quays, mounds enclosed with walls were built in which the buildings were directly integrated. The buildings on the edges of the mounds stand on a base storey with underground car park entrances, shops and cafés which, according to HafenCity flood protection regulations, have to be protected by flood-proof seals. The road system also lies at mound level and is extended with new bridges and footpath connections providing escape routes from HafenCity to the city centre behind the flood protection wall.

–––––––– Bergsche Maas, between Waalwijk and Geertruidenberg ∆ ∂94 Rhine, Brühl ∆ 2∂8 Waal, Gameren ∆ 224

–––––––– Elbe, Hamburg ∆ 202

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Design Catalogue Flood Areas

C2.3

C2.4

Buildings on piles

Escape routes

Rhine, Mannheim, lido restaurant

Wantij, Dordrecht

Elbe, Hamburg

Pile dwellings are to be found in many parts of the world near the coast or in marshy areas. This method is also popular for new buildings close to water. They are thus protected from floods and the floodwaters can flow underneath them; the discharge is impeded very little by the piles. In Mannheim on the Reiß Island the new beach restaurant has been built on stilts and according to this principle. If the Rhine is running high the building is no longer accessible but protected from the floodwaters.

In Dordrecht in the Netherlands a residential area has been built in this way, within the flood plain in front of the dike line. The roads, paths and terraces are at dike top level, so the houses are still accessible during floods. The occupants’ boats can be anchored directly in front of their terraces.

To make it possible to leave the houses even when water levels are high, new housing developments in flood risk areas can employ entire access systems or an additional escape route that lies above the high water level. In the HafenCity in Hamburg this was realised in the form of high-level bridge and boardwalk constructions that make it possible to live in the flood area by guaranteeing connections to the mainland and safety, while providing attractive footpath connections all year round. Similarly, in the new housing development on piles on the River Wantij in Dordrecht, all the access roads were laid out at the top level of the dike.

–––––––– Maas, Maasbommel ∆ 2∂2 Rhine, Mannheim ∆ 220 Wantij, Dordrecht ∆ 228

–––––––– Elbe, Hamburg ∆ 202 Wantij, Dordrecht ∆ 228

C2.5 Cableways

Wupper, Müngsten, handcar in the Brückenpark

Rhine, Cologne, Rhine Cablecar

One very attractive river feature is a cableway by which one can cross the flood plain and river, swinging high above them. On the River Wupper in the Brückenpark Müngsten, a small suspended ferry that can be moved across the river by the visitors themselves using a hand-operated mechanism (draisine) is a special attraction and an important link in the local circular footpath.

The Rhine Cablecar in Cologne, which has linked the banks of the Rhine since the 1957 Federal Horticultural Show, offers spectacular views over the river and is both a tourist attraction and an alternative means of transport. –––––––– Wupper, Müngsten ∆ 230 + Rhine, Cologne, Rhine Cablecar ∆ 28∂

98 99

Design Catalogue Flood Areas

C3 Tolerating

∫ All design tools in C3 can be combined with –––––––– C∂.∂ Setting back the dike C∂.2 Branches C∂.3 Flood channels C∂.4 Reprofiling the flood plain C∂.5 Backwaters C∂.6 Polder systems C∂.7 Retention basins C4.∂ Warning signs and barriers C4.2 Electronic warning systems

Amenities in the flood plain must be able to withstand the force of floodwater and tolerate temporary submersion. For most of the year, when rivers run at medium or low water levels, unlimited use of the flood area is possible. At high water level these places are no longer or only partially accessible. For this reason temporary uses must be found that can tolerate high water such as sports facilities, playgrounds and campsites, festival grounds or also quasi-natural areas with typical riparian vegetation. The facilities are permanently installed and the materials selected to be able to withstand longer flood periods. Elements that cannot resist flooding must be removed for storage elsewhere in case of a high water event. Permanently installed elements in the flood area may not obstruct floodwater discharge. At a larger scale, the type and location of the open spaces in the topography play an importatnt role in the design of flood areas. More complex, costly and high-maintenance areas such as sports facilities can be planned for higher-lying areas of the flood plain because these are flooded less frequently. Thus the facilities can be used for most of the year and need not be cleaned of sediment after flooding so often.

C3.∂

C3.2

C3.3

Paths within the flood plain

Sports facilities and playgrounds

Flood-tolerant buildings

Ebro, Zaragoza, embankment in the city centre

Gallego, Zuera, bullring

Elbe, Hamburg, Fish Auction Hall

Setting up a road and pathway concept involves zoning and differentiating between inaccessible, seldom used and frequently used areas of the flood plain. The path networks can be built as narrow footpaths or broad, surfaced access. Complex and costly boardwalk constructions, such as in Zaragoza on the River Ebro, dramatise the natural flood plain. Cleaning the path of deposits after flooding may be necessary depending on the sedimentation patterns of the river. Low dike lines within the flood plain that restrain less dramatic summer high water events are, because of their linear character and elevated position, well-suited for use as the foundations of pathways.

Sports and play facilities promote recreational uses of flood plain areas. They can be designed in very diverse ways, ranging from the use of a meadow for ball games through more complex sports grounds or golf courses through to exceptional uses like the bullring in Zuera in Spain, whose round arena is submerged by up to 1 m during high water events. At the waterside there are often playgrounds that, though they may not use the water directly, still enhance the water experience.

An appropriately adapted and equipped flood-tolerant building can be subjected to flooding without sustaining significant damage. This means, for example, tiled floors and walls and specially protected electrical installations. Traditionally, many buildings in flood areas such as the centre of Cologne or the Fish Auction Hall in Hamburg were adapted in this way; today these concepts are being discussed anew in the light of more stringent flood protection regulations. In Kampen on the River IJssel in the Netherlands, sections of the existing built area were not included in the new flood protection system due to their awkward locations and the buildings were equipped to be flood-tolerant instead. A tolerant attitude to flooding can reduce the costs and effort involved in building protection systems.

–––––––– Ebro, Zaragoza ∆ ∂98 Gallego, Zuera ∆ 204 IJssel, Zwolle ∆ 208 + Ebro, Zaragoza, Riverbank Improvements in the City Centre ∆ 279

–––––––– Gallego, Zuera ∆ 204 + Rhine, Düsseldorf, Lausward Golf Course ∆ 28∂

–––––––– IJssel, Kampen ∆ ∂74 + Elbe, Hamburg, Fish Auction Hall ∆ 279

∂00 ∂0∂

Design Catalogue Flood Areas

C3.4

C3.5

Parks within the flood plain

Extensive natural areas

Petite Gironde, Coulaines

IJssel, Zwolle, flood plain with highland cattle

IJssel, Zwolle, birdwatching hide

A flood plain can be designed as a floodresistant park. Plantings and furniture have to be designed and built to tolerate several days of submersion – examples are swamp cypress and heavy stone benches. Such features lend the park a striking and distinctive character. A drainage system can also make the park’s green spaces usable again shortly after flooding – at Petite Gironde in Coulaines, for example, drainage pipes were laid under the entire lawn area to ensure this.

River forelands are suitable sites to allow new natural flood plain environments to emerge from former agricultural areas. To permit natural succession until the now rare riverside woodland reappears is often not possible, as the dense woodland obstructs floodwater discharge. In the Netherlands, for example in the Vreugderijkerwaard or the Gamerense Waard, for this reason extensive grazing systems with ponies, highland cattle and wild horses were developed. The animals seem to have little fear of people and enrich the landscape in a very special way. As they cannot prevent all growth through grazing alone, however, alternative ideas are currently under discussion such as clear-cutting every 10-15 years.

Between these interventions the vegetation can develop freely. In some places, carefully planned access systems guide visitors to relieve pressure on sensitive zones so that the area can also be used for recreation. In the Vreugderijkerwaard, by contrast, access is strictly limited, although the area can be crossed on a boardwalk leading to a birdwatching hide.

–––––––– Besòs, Barcelona ∆ ∂96 Ebro, Zaragoza ∆ ∂98 Gallego, Zuera ∆ 204 Petite Gironde, Coulaines ∆ 2∂4 Seine, Le Pecq ∆ 222 Wupper, Müngsten ∆ 230

–––––––– Ebro, Zaragoza ∆ ∂98 IJssel, Zwolle ∆ 208 Kyll, Trier ∆ 2∂0 Waal, Gameren ∆ 224 Emscher, Dortmund ∆ 256 + Elbe, Lenzen, Large-Scale Nature Conservation Project in the Elbe Valley ∆ 279

C3.6

C3.7

C3.8

Agriculture

Camping and caravan sites

Events grounds

Rhine, Ingelheim, Polder Ingelheim

Rhine, Mannheim, caravan site on Reiß Island

Neckar, Ladenburg, permanent stage

Depending on how frequently they are flooded, today many areas in river plains are used for extensive grazing or even arable purposes, as this view from the dike across the polder near Ingelheim on the River Rhine shows. Financial incentives encouraged farms to permit flooding of their lands again. On the Bergsche Maas in the Netherlands the farmers themselves initiated setting the dikes further back so that the Overdiepse Polder could once again serve as a flood space. The farm buildings were moved on top of mounds. In areas close to towns and cities, agricultural uses can enhance local recreational areas and release the municipality from the burden of maintenance costs.

Camping and caravan sites in riparian landscapes are very attractive because the location directly adjacent to the water is convenient for water sports and generally regarded as conducive to recreation. This campsite on the Reiß Island in Mannheim is particularly attractive because of the extensive gravel beach beside it. The pitches are mainly used in the summer months when the risk of flooding is low, but to exclude the hazard completely the possibility of flooding has to be communicated clearly to visitors so that the site can be evacuated quickly in the event of an unexpected flood. In Maasbommel on the Gouden Ham the campsite also rents out small bungalows on piles that can remain on site during flooding.

River plains, retained as extensive open spaces in cities, can provide sites for open air concerts and other major events, such as the annual ‘Rheinkultur’ Music Festival near Bonn, which would be difficult to stage elsewhere because of the space required and the noise it causes. These events usually happen in the dry summer months and only a few fixed installations such as the stage foundations are needed; festival marquees, toilet facilities and food and drink stands can be set up and dismantled. In Ladenburg, a permanent events stage was installed as a flat, light construction on the bank of the River Neckar that presents no obstruction to high water discharge and is used for the annual festivals.

–––––––– Bergsche Maas, between Waalwijk and Geertruidenberg ∆ ∂94 Rhine, Brühl ∆ 2∂8 + Rhine, Ingelheim, ‘Polder Ingelheim‘ Floodwater Retention ∆ 28∂

–––––––– Maas, Maasbommel ∆ 2∂2 Rhine, Mannheim ∆ 220

–––––––– Main, Miltenberg ∆ ∂78 Petite Gironde, Le Mans ∆ 2∂4 Neckar, Ladenburg ∆ 242 + Fulda, Kassel, Flood Ditch Area ∆ 279 + Rhine, Bonn, ‘Rheinkultur‘Music Festival ∆ 28∂

∂02 ∂03

Design Catalogue Flood Areas

C4 Evading

∫ All design tools in C4 can be combined with –––––––– C∂.6 Polder systems C∂.7 Retention basins C3.∂ Paths within the flood plain C3.2 Sports facilities and playgrounds C3.4 Parks within the flood plain C3.5 Extensive natural areas C3.6 Agriculture C3.7 Camping and caravan sites

Safety aspects play an important role in decisions on the use of flood plain areas. High water predictions, barriers, warning signs and digital flood warning systems make the temporary use of flood-sensitive areas possible. The determining safety factors are, for one thing, how quickly and how deep the space could be flooded, and for another how recognisable the danger is. If users are aware of the potential danger they can escape to safe areas in time when a flood warning comes. Information to visitors therefore plays a crucial role for their safety. Another important aspect is how often the flood happens; if areas are frequently flooded this is planted firmly in residents’ awareness. Permanently installed warning systems refer to the potential hazards of rising water levels. These can be supplemented by additional high water announcements, while another contribution can be made by design that emphasises that one is within a flood-sensitive area. It is almost always possible to reduce the dangers from flooding to a minimum through appropriate warning systems. The prerequisite is an attitude of acceptance towards the residual risk and the will not to exclude the use of flood areas from the start. Examples such as the retention basins on the Petite Gironde and the project on the River Besòs in Barcelona illustrate this stance. With this double usability, retention spaces are secured both as flood protection and as amenity for the city.

C4.∂

C4.2

Warning signs and barriers

Electronic warning systems

!

!

Schanzengraben, Zurich, Promenade

Ihme, Hanover, closed-off cycleway

Besòs, Barcelona

In many cases, when water levels rise gradually, predictably and visibly, local authorities consider it sufficient to encourage public awareness of potential flood risks, leaving people to take responsibility for their reaction to flooding. This general information of the public can be supported by simple signs or explanatory information boards at the entrance to or within the river flood plain. A further possibility is the use of an expressive design, for example in the form of dramatic accentuation of the sluice gates or the dikes, or affixing high water markings.

Closing off the flooded areas when the waters rise is the safest measure; responsibility for this, however, lies entirely with the local authority. Individuals do not decide for themselves when staying in the area would be dangerous. Furthermore, this approach requires continuous monitoring of the water level and intervention on the part of the local authority. However, at sites which are difficult to monitor, such measures are essential.

Warnings of the danger of flooding can come from an electronic monitoring system that receives and processes high water predictions. In Barcelona on the River Besòs such a warning system is linked with electronic signs and gates at the entrances to the flood plain, permitting or prohibiting access. This system can respond very quickly to the strong, sudden flood surges of the Besòs, and this is the only way in which using the space as a park is possible.

––––––––

––––––––

+ Schanzengraben, Zurich, Promenade

Besòs, Barcelona ∆ ∂96

∆ 282

∂04 ∂05

Design Catalogue Flood Areas

C5 Adapting

∫ All design tools in C5 can be combined with –––––––– C∂.2 Branches C∂.5 Backwaters

According to the Bible, Noah built the Ark to save people and animals, which can be regarded as one of the first uses of this strategy: floating elements rise with the floodwaters. Floating structures are currently enjoying a renaissance, especially for residental developments in flood plains; similarly, schemes for new marinas for yachts and motor boats often make use of floating jetties. When located on oxbow lakes or gravel pits within the river space, the floating elements are largely protected from strong currents. If access from the river is also safely possible in times of flooding then the elements can be used all year round. In any case they are protected from damage as they rise with the water. The floating structures have minimal effect on the retention capacity of the flood plain, making this strategy a sensible component of large-scale flood protection concepts. Furthermore, the river plain landscape is enhanced and fluctuations in water level are immediately visible.

C5.∂

C5.2

Floating and amphibious houses

Marinas

Maas, Maasbommel

Waal, between Afferden and Dreumel

Wantij, Dordrecht, existing marina

The forms of floating habitation have become very diverse in recent years. On a branch of the River Waal between the Dutch towns of Afferden and Dreumel near the municipality of Beneden-Leeuwen some 20 floating houses are moored. They range from converted cargo barges to floating luxury villas with terraces and boat moorings, thus demonstrating a broad spectrum of possible floating homes. Unlike forms of floating habitation on the inner city quays (A6.3 Moored ships) in Process Space A, Embankment Walls and Promenades, the larger water areas of flood plains present an opportunity to establish entire large-scale floating settlements on the water. In the Netherlands several such floating residential areas are under development or even in construction.

In Maasbommel at the Gouden Ham water sport and leisure amenity there are 14 permanently floating and 32 ‘amphibian’ houses that move up and down with the water level in the River Maas. The FürstPückler-Land International Building Exhibition in the Lausitz in Brandenburg held a design competition for floating houses and holiday homes to enliven the new lakes in the former open-cast mining areas in eastern Germany. Two boat designs from the competition have been built as prototypes.

By building marinas for sailing and motor boats in flood plain areas, creating parking spaces, restaurants and cafés and in some cases overnight accommodation, stimuli for more intensive leisure uses in river landscapes can be provided. The jetties together with the moored boats rise with the water level. Direct links to access roads at dike level means that they can still be reached during high water. Even if the marina is not accessible during high water this is only a minor restriction on use, as leisure boating is rare during such periods. The Plan Tij Housing Estate was built on piles in the flood plain area of the River Wantij and next to an existing marina. Behind the houses there are floating jetties that are directly accessible from the residents’ terraces.

–––––––– Waal, between Afferden and Dreumel ∆ ∂88 Maas, Maasbommel ∆ 2∂2 + IJsselmeer, Amsterdam, IJburg ∆ 280 + Geierswalder and Gräbendorfer Lakes, Großräschen, Floating Houses ∆ 280

–––––––– Maas, Maasbommel ∆ 2∂2 Wantij, Dordrecht ∆ 228

∂06 ∂07

Design Catalogue Flood Areas

D

Riverbeds and Currents

Birs, Basle

From a completely regulated to a varied and artificially enlivened riverbed – new design reinstates the natural dynamic channel processes that had been lost in a canalised flow. A diversified current profile evolves with eddies, calmer areas, rapids and marginal zones. ∂08 ∂09

Design Catalogue Riverbeds and Currents

D

Riverbeds and Currents

Spatial situation

Process Space D comprises the riverbed between the banks or the riverbank reinforcements themselves, whose lines are immovable, thus forming the limit of self-dynamic river channel development (red line). The defined flood limit (green line) and the flood plain area between riverbank line and flood limit thus lie outside this Process Space and are not addressed here. Rather, the possibilities are described of reshaping canalised, straightened watercourses within a constricted area to allow morphodynamic processes to re-emerge. For this, unlike in Process Space E, the flood plain cannot be used as space for the river channel to develop self-dynamically, and the fixed riverbank lines can be shifted only slightly, if at all, because open space close to the water is scarce or non-existent, or because it fulfils other functions such as containing underground cables and pipeline conduits. Sometimes there is no flood plain at all, for instance in the case of rivers that had been culverted. In many towns and cities, however, the aim of restoring straightened and canalised rivers persists.

Operative processes

Just as in Process Spaces A, B and C, temporary flow fluctuations are relevant as they make themselves felt in both vertical rising of water levels and horizontal expansion into the flood plain. Above and beyond this, within this Process Space morphodynamic sedimentation processes on the riverbed are permitted and/or encouraged; erosion and sedimentation leads to constant changing of the riverbed with the creation of cut banks and slip-off slopes, scour holes, rapids and islands that convey the impression of a varied, near-natural river. Unlike in Process Space E, however, no alterations in the river’s course through its inherent dynamics are permitted, as the urban river flood plain of Process Space D with its existing built-up surroundings or infrastructure allows no room for natural development. Therefore the river, like its former condition, a drastically straightened watercourse, remains completely secured and controlled, and sediment shifts are only possible within a fixed framework.

Design approaches

Within this defined area, then, conditions are created that resemble those of a natural river. As a rule this concerns small and medium-sized rivers whose channel may be influenced by interventions on the riverbed that affect the flow. In reshaping the riverbed, the typical characteristics and processes of a natural watercourse, which had been eradicated in the canalised river, are artificially restored. The aim of many a river intervention in the past was to inhibit erosion as much as possible and to induce constant and steady current, as in a canal. The important design tools to instigate the

D∂ Deflecting the current

D1.1 D1.2 D1.3 D1.4 D1.5 D1.6 D1.7

Large single rocks Dead wood Laid stone groynes Piled stone groynes Bioengineered groynes Submerged groynes Riverbed sills

D2 Grading the channel

D2.1 Widening the channel D2.2 Extending the flow length

D3 Varying the riverbed

D3.1 Sand and gravel beaches on inner bends D3.2 Sand and gravel beaches in bays D3.3 Creating scour holes

D4 Varying the bank reinforcement

D4.1 D4.2 D4.3 D4.4 D4.5 D4.6

Partially naturalising the riverbank Living revetment Stone revetment Terraced stone revetment Masonry riverbank revetment Building over the existing reinforcement

D5 Varying the riverbed reinforcement

D5.1 Fish passes D5.2 Varying the riverbed and transverse structures D5.3 Ramps and slides

∂∂0 ∂∂∂

Design Catalogue Riverbeds and Currents

D

Riverbeds and Currents

greatest possible riverbed dynamic are current-altering elements to produce a differentiated flow scenario with eddies, calm zones and rapids. Through this, sediment shifts can be deliberately encouraged. By combining various current-deflecting elements and remodelling the medium and low water course, an artificial sinuous course with different depths of water evolves between the fixed banks. Further design measures address the differentiated shaping of the indispensible bank and riverbed reinforcements. The aim is to design a river within the given limits in which as many natural water processes as possible can happen. Installing current-deflecting elements such as groynes or islands causes differentiation of the current, making its direction and the flow rates of the river apparent and tangible: rapids and eddies, but also calm zones. Variations in the current activate natural sediment shifting processes that can be guided by specific installation of current-altering elements. Depending on the flow rate, sediment of various particle sizes will be removed or deposited, and varying riverbed substrates and depths will develop. For this to happen, a change of the existing ground sills is necessary. Elements to secure the riverbed’s stability will still be needed, but with differentiated, more permeable structures that help reinstate the passability of the river for living organisms as well as facilitate sediment transportation. As a rule these sediment movements are reversible. At low water levels there is more aggradation of riparian deposits, while at high water levels such deposits are carried off by the stronger flow rate. It is only in very sheltered places that permanent aggradation zones arise. Erosion processes that alter the course of a river channel are prevented by the bank and riverbed reinforcements. Taken as a whole, by deliberately permitting certain morphodynamic processes within Process Space D, a differentiated riverbed with shallow zones, rapids, scour holes and gravel banks is created. For city dwellers, the reshaping of a canalised river is a great enrichment of their habitat: within a small space, accessible, living and varied watercourses appear, giving the townscape a new appearance and providing quality recreational space.

Amenity

The interventions in Process Space D make diversified river structures possible in densely built-up urban spaces. Reshaping the riverbank edges and installing multifunctional current-deflecting elements creates a multiplicity of accessible places in and on the watercourse. Boulders and groynes to walk on make direct contact with the water possible, aggradation zones encourage children to play in and on the water. On larger rivers, popular places for bathing are established on gravel- and sandbanks – the living river spaces offer a contrast to the paved and built-up urban space and are valuable close-to-town recreational areas for the city population.

Flood protection

When the riverbed is remodelled, the discharge cross-section changes, and disruptive elements, current-deflecting objects and plant growth mean that resistance to the flow is higher and flow rates will be slowed. This can raise water levels locally during high water periods. By the same token, these measures often create more retention capacity, thus relieving pressure on downstream areas. Where higher water levels cause problems they can be compensated for with a combination of measures to enlarge the discharge cross-section, such as setting back the dike line or excavating the flood plain, if possible (see Process Space C).

Ecology

Variations in the current, differentiations in the riverbed substrate and various water depths offer wide structural diversity in the river and lead to the creation of diverse biotopes in the marginal and aquatic zones. The location, forms and materials of the installed elements create new habitats; with bioengineering measures or in the gaps between loose stone embankments, sheltered places develop for various species of flora and fauna. A further ecological objective of the Process Space D projects presented in this book is the formation of a low water channel in the river so that lower-lying sections of the riverbed still carry water as fish habitats even during longer dry periods. Also, the pass-

ability of the watercourse is reinstated, for instance by replacing ground sills with ramps, allowing fish and tiny organisms to migrate naturally. From an ecological point of view, a process of ecological revitalisation sets in, as various species of fish, amphibians and dragon- and damselflies return to the city.

On the River Birs in Basle, the space within the limits of the river channel (red line) was designed anew. The flood plain up to the flood limit (green line) could not be included in the design because of pipelines that lay there. Ground sills were removed and the river’s course was differentiated with the help of current-deflecting elements.

∂∂2 ∂∂3

Design Catalogue Riverbeds and Currents

D∂ Deflecting the current

∫ All design tools in D∂ can be combined with –––––––– D3.∂ Sand and gravel beaches on inner bends D3.2 Sand and gravel beaches in bays D3.3 Creating scour holes D4.∂ Partially naturalising the riverbank D4.2 Living revetment D4.3 Stone revetment D4.4 Terraced stone revetment D4.5 Masonry riverbank revetment D5.2 Varying the riverbed and transverse structures D5.3 Ramps and slides

Deflecting the current as a design approach is derived from engineering of navigable rivers. By means of groynes or embankments that extend out into the river channel, the current is diverted away from the riverbanks into the centre. This serves both to protect the banks and to keep the shipping channel in the middle open. By contrast, in the projects of Process Space D presented in this book the principle is used for the process-oriented revitalisation of the watercourse. Using elements that disturb and divert the current creates variation in the flow and thus initiates morphological sediment shifting processes. Pushing the current away from the riverbanks means that hard construction measures such as reinforcing them with stones is unnecessary. The elements that disrupt and guide the current can be installed directly on the banks or in the middle of the river, and purposeful arrangements of several elements can create diversified current patterns; by staggering them, sinuous flow paths can emerge, while two groynes opposite each other create a straight, accelerated current in the centre of the watercourse. The angle of the groynes, pointing upstream or downstream, is decisive for the direction in which the current will be guided and the creation of aggradation zones or scour holes. It is also crucial whether the groynes are always covered with water or higher than the mean water level: if the river flows over them the path of the water is diverted at right angles to the groyne and a depression is scooped out behind it. If the water flows around the groyne, vortices are created at its end, along with a calm area behind it where sediment deposit can occur. Completely submerged groynes hardly impede discharge during high water periods at all, and so if high water levels are already a problem submerged groynes are preferable. There are diverse variants of current-deflecting elements such as groynes, large boulders and stepping stones, or dead wood fixed in the river; the way they are employed has a bearing on the open space design and ecological added value. Through the choice of form and material, such elements can blend into their surroundings or deliberately contrast with them to accentuate the intervention. It is also possible to make these elements accessible for various open space uses: groynes that one can walk on and that offer seats are popular places to relax beside the river, and stepping stones permit direct contact with the water.

D∂.∂

D∂.2

D∂.3

Large single rocks

Dead wood

Laid stone groynes

Wupper, Müngsten

Isar, Munich

Birs, Basle

Current diverters in the form of stones are placed in the riverbed either singly or in small groups. Their shapes and the choice of material can easily be suited to the river and its surroundings, for instance by choosing locally occurring types of stone. Current diverters can, however, also be deliberately dramatised by using artificial elements that stand out in contrast with the surroundings. The rocks must be of sufficient size and weight to withstand the strongest expected current and remain in position. On the River Wupper in Müngsten, the current diverters can also be used as stepping stones and thus encourage direct contact with the water. The smallscale flow variations and substrate differentiation allow riparian habitats for waterdependent small life forms to develop.

Tree trunks with or without branches can be installed as current-deflecting elements in a watercourse. The dead wood is fixed in position, either by embedding it partially in the riverbank or anchoring it in the riverbed with steel cables and stakes. The alignment of the trunk can be at right angles to the current or angled downstream. As a special case, it is possible to fix a trunk at just one end so that it can swing freely in the current. A large tree stump fixed in the riverbed of the Isar in Munich is both an attractive place to play and visually enriching.

When a groyne is to be used as a place to sit it makes sense to build it of stones laid to fit together, as on the River Birs in Basle; broken or hewn stones are laid to create a fairly level top surface, which also offers hydraulic advantages. The stones should be large enough to withstand the current. Such a construction is markedly more labourious and expensive than a piled stone groyne (D1.4). Groynes of this type were used for current deflection also on the River Wiese in Basle. While sitting on these large stones, the river and its banks, but also the currents and differences in temperature can be observed.

–––––––– Isar, Munich ∆ 260 Werse, Beckum ∆ 270

–––––––– Birs, Basle ∆ 238 Wiese, Basle ∆ 250

–––––––– Wupper, Müngsten ∆230 Ahna, Kassel ∆ 234 Alb, Karlsruhe ∆ 236 Neckar, Ladenburg ∆ 242 Wiese, Basle ∆ 250

∂∂4 ∂∂5

Design Catalogue Riverbeds and Currents

D∂.4

D∂.5

D∂.6

Piled stone groynes

Bioengineered groynes

Submerged groynes

Wiese, Lörrach

Birs, Basle

Wiese, Lörrach

A groyne made of loosely piled-up stones, preferably of various sizes, is relatively easy to construct in forms varying from very narrow spurs that hardly extend out into the water at all to triangular groynes with a broad baseline along the bank that jut out a long way into the river. Piled stone groynes can also be built in combination with bioengineered groynes (D1.5). The loosely piled-up stone groynes on the Wiese in Lörrach enhance the current diversity in the river but cannot be walked on.

Groynes can be made of living woven willow, fascines or diagonally-laid willow branches. Because willows will spread their roots through the construction and keep growing, these natural groynes offer a valuable habitat and refuge for various organisms. The willow functions as a pioneer species; other shrubs follow and establish themselves to further stabilise the groyne which, as a green lining to the riverbank, is hardly recognisable as a technical construction, as here on the River Birs in Basle. Combinations of stone or other ‘hard’ materials with living vegetation offer a wide range of design opportunities, although for relatively large groynes there are limits on the use of living materials, as strong currents can only be withstood by solid built constructions.

Groynes of piled stones that are not joined to the riverbank and over which the water flows are very useful for shaping the currents of medium-sized and even large rivers; their angle to the main flow determines the current patterns that emerge and water processes such as flow eddies, aggradation and scour holes. On the River Wiese in Lörrach, by building several submerged groynes in a funnel form the main current was so diverted that distinctive riverbed and channel features (such as scour holes and sandbanks) emerged to enhance the river both structurally and visually.

–––––––– Wiese, Lörrach ∆ 265

–––––––– Birs, Basle ∆ 238

–––––––– Wiese, Lörrach ∆ 250

D∂.7 Riverbed sills

Birs, Basle

Measures to secure the riverbed and prevent the watercourse from cutting deeper into the substrate can also be used to shape the current. Sills across the riverbed, usually made of large stones, can be set at an angle to the main flow and thus deflect and shape it. The flow is always diverted at right angles to a cross-river bar. These sills should be varied in height to create deeper areas with strong currents and shallower, calmer areas in the river. A series of several sills can be arranged staggered on both river banks, so that the alternately angled sills divert the current from one riverbank to the other. At low and medium water levels the current ‘swings’ downstream, and the flow distance is longer because the current meanders. As opposed to engineered ground sills, these new riverbed sills are of varying height and thus passable for water organisms. –––––––– Birs, Basle ∆ 238 Wiese, Basle ∆ 250

∂∂6 ∂∂7

Design Catalogue Riverbeds and Currents

D2 Grading the channel

∫ All design tools in D2 can be combined with –––––––– D3.∂ Sand and gravel beaches on inner bends D3.2 Sand and gravel beaches in bays D5.2 Varying the riverbed and transverse structures D5.3 Ramps and slides

Enhancing variation of the entire watercourse can go beyond setting features to deflect the current to encompass complete structural remodelling of the medium water channel; immediately after construction, the river will look completely different within this section with its course made longer, with varied water depths and altered riverbanks. As a result, sediment shifting processes are set in motion that lead to variations in current strength, riverbed substrate and cross-section of the river channel. Widening the river creates zones with weaker currents where fords can develop. When widening a river it must be ensured that, even at low water level, there is enough depth of water for migrating organisms. A sinuous formation encourages current variation and the deposit of sediments of various particle sizes. Flattening out the riverbank improves access, and creates ecologically interesting transitional zones and calmer shallow areas along the river. Where the current can be diverted away from the banks by built structures, it is possible to dispense with strong bank reinforcements. On a large scale this is also possible on the slip-off banks of major rivers, as these become natural aggradation zones virtually unaffected by erosive forces. Such grading creates a structurally rich riverbed with gently sloping foreshores and gravel banks, marginal areas and deeper zones for fish. By differentiating the river channel it is possible to create amenities, for example frequently used recreational areas such as sandy beaches or quiet secluded spots for the enjoyment of nature.

D2.∂

D2.2

Widening the channel

Extending the flow length

Alb, Karlsruhe

Ahna, Kassel

Widening the central riverbed channel is possible if the bank reinforcements can be set back into the flood plain somewhat. Such a measure can be made either at one spot as a bay or inlet, or as just a slight broadening of the watercourse. On the River Alb in Karlsruhe the widening was only possible on one side as there was no space on the other. The widening lets the water flow more slowly at this point, and sand- and gravel banks arise. In calm zones the banks were flattened and small beach areas were created. The central channel can also be dramatically widened to divide the stream and create an island. A further benefit of widening is that it increases floodwater discharge capacity.

A combination of current-altering elements and structural grading of the central channel deflects the flow into a series of curves. This leads to more variation in the channel and the development of cut banks and slip-off banks. The length of the low- and medium water channel is extended. These graded sections must be protected against erosion. Similarly to the way a river would develop naturally, the watercourse becomes more varied. On the River Ahna in Kassel, lush riverside vegetation has colonised the space so that the river appears like a green oasis in the townscape. –––––––– Ahna, Kassel ∆ 234 Birs, Basle ∆ 238

–––––––– Alb, Karlsruhe ∆ 236

Leutschenbach, Zurich ∆ 240 Soestbach, Soest ∆ 248

Birs, Basle ∆ 238

∂∂8 ∂∂9

Design Catalogue Riverbeds and Currents

D3 Varying the riverbed

∫ All design tools in D3 can be combined with –––––––– D∂.2 Dead wood D∂.3 Laid stone groynes D∂.4 Piled stone groynes D∂.5 Bioengineered groynes D2.∂ Widening the channel D2.2 Extending the flow length D4.∂ Partially naturalising the riverbank D4.4 Terraced stone revetment

A differentiated riverbed with various types of riverbank situations in the form of beaches, islands and scour holes corresponds with sound notions of river renaturalisation. These can, however, only be produced indirectly through current-deflecting devices, the purposeful exploitation of aggradation and erosion processes, and structural grading of the water channel. To encourage sedimentation processes that create islands and beaches, current-altering elements or widening the channel at certain points are useful. These diffuse the current in specific sections, and according to the flow rate the tractive force changes and the suspended load up to a certain particle size is deposited. Only when the current is very slow can fine material such as sand settle, and thus influencing the current can determine whether a gravel or a sandy aggradation emerges. Conversely, the creation of scour holes can be influenced by using current-deflecting elements to create zones with stronger flow that intensify the erosion process. Beaches, islands and scour holes are the results of morphodynamic processes, and show that a dynamic flow equilibrium has established itself within the riverbed. This also means, however, that under high water conditions, and a thus altered interplay of currents, aggradations can shift or vanish completely. Only in sheltered bays or behind groynes permanent aggradation zones can establish themselves, which are especially suitable as amenities in the summer in the form of easily accessible gravel- and sandy beaches that need little maintenance, whereas artificially laid beaches at inappropriate points on a river have to be completely renewed after every flood event. In the light of dramatically improved water quality, especially, the development of bathing places on rivers is becoming increasingly important.

D3.∂

D3.2

D3.3

Sand and gravel beaches on inner bends

Sand and gravel beaches in bays

Creating scour holes

Rhine, Mannheim, beach on Reiß Island

Rhine, Mannheim, bay at Rhine promenade

Birs, Basle

In calm zones along the slip-off slopes of rivers aggradation can occur and, depending on the flow rate, gravel or sand beaches are created. Natural beaches, for example on the Reiß Island in the Rhine in Mannheim, are very popular places for play and recreation. By setting current-deflecting elements in flowing water, artificial aggradation zones can be created and so, even in very narrow rivers, little beaches can emerge. In navigable waters rows of groynes can be placed along the edge of the channel between which little beaches can develop.

Widened bays in the banks and artificial inlets create calm zones in the river and thus encourage sediment deposit, creating aggradation zones that can be used as bathing beaches at accessible points. On the River Rhine at Mannheim, a beach situation could actually be developed under the cut bank. It is important to protect the upstream corner of the bay against erosive eddies with a reinforcement. The aggradation is determined by the form and size of the widened bank section.

Current-deflecting elements can be used to increase the flow rate in sections of a watercourse. If the riverbed is not sealed, as on the Birs in Basle, this concentration of the current carries off bed material at one point and a scour hole is created. Such deeper points in the watercourse offer a special habitat and serve as refuges for fish. A river’s current interplay alters with its flow fluctuations; erosion and sedimentation processes cause constant changes in the form of the scour holes.

––––––––

––––––––

Wupper, Müngsten ∆ 230

Birs, Basle ∆ 238

––––––––

Neckar, Ladenburg ∆ 242

Wiese, Lörrach ∆ 252

Rhine, Mannheim ∆ 220

Wiese, Basle ∆ 250 Wiese, Lörrach ∆ 253 + Rhine, Mannheim, Rhine Promenade ∆ 28∂

∂20 ∂2∂

Design Catalogue Riverbeds and Currents

D4 Varying the bank reinforcement

∫ All design tools in D4 can be combined with –––––––– D∂.∂ Large single rocks D∂.3 Laid stone groynes D∂.4 Piled stone groynes D∂.5 Bioengineered groynes D3.∂ Sand and gravel beaches on inner bends E4.∂ ‘Sleeping’ riverbank reinforcement E4.2 Bank reinforcement as needed E4.3 Selective bank reinforcement

In Process Space D the bank reinforcement and hence the limit of self-dynamic river channel development (red line) must be retained, as the flood plain is not available for the river to develop into. Various ways of securing the riverbank used alternately can, however, offer many and diverse design possibilities. An open-pored surface with gaps and hollows, such as loosely piled stones along the bank, can offer diverse habitats for aquatic and amphibian organisms. Near-natural, bio engineering bank reinforcements of living willow are ecologically valuable and create the impression of a green, natural riverbank. With combinations of near-natural construction methods and explicitly artificial elements such as sitting terraces or riverbank steps, aesthetically appealing contrasts can be created while at the same time securing the banks and offering places to linger by the river. Depending on the design of a stretch of riverbank, diverse flowing water processes can be induced or highlighted: along smooth riverbank walls, the current of flowing water is hardly perceptible, while riverbanks that are planted or secured with piled stones are rough and cause variations in the flow and eddies that make the current visible.

D4.∂

D4.2

Partially naturalising the riverbank

Living revetment

Birs, Basle

Waal, Zaltbommel

Birs, Basle

Removing bank reinforcement to create a natural bankside is only possible on slipoff slopes or in bays where currents are weaker and thus the erosive forces are less strong. Such zones can also be artificially created using current diverters; even in restricted spaces, riverbanks can be partially naturalised, as on the River Birs in Basle. Large-scale renaturation is, especially where the current is powerful, only possible on the slip-off slope or by using groynes over a large area.

On the River Waal at Zaltbommel small beaches have developed between groynes that were originally built to improve navigability. Using these measures requires precise knowledge of the discharge and current dynamics in the watercourse and can make additional work necessary to secure the banks on endangered stretches of the river.

Securing riverbanks with the help of living plants is a near-natural solution to prevent erosion. For this, willow constructions, in places combined with stone or timber constructions, are frequently used, for instance to form a surface as spilings made of woven or planted withies, or at specific points as single trees on the riverbank line. Along with protecting the banks, this living reinforcement offers a diverse habitat, and the green bank creates a good blend of the river into the landscape. It should be noted that a certain development period is needed until such bank reinforcement is fully robust.

–––––––– Waal, Zaltbommel ∆ ∂90 Rhine, Mannheim, Reiß Island ∆ 220 Alb, Karlsruhe ∆ 236 Birs, Basle ∆ 238 Wiese, Basle ∆ 250

–––––––– Birs, Basle ∆ 238

∂22 ∂23

Design Catalogue Riverbeds and Currents

D4.3

D4.4

D4.5

Stone revetment

Terraced stone revetment

Masonry riverbank revetment

Gallego, Zuera

Birs, Basle

Sarthe, Le Mans, Île aux Planches

Securing the riverbank with set stones offers, above all, attractive design possibilities as in the new riverbank reinforcements on the artificial river branch in Zuera. The materials for the revetment, often large blocks of stone, can be deliberately selected from a range of sizes, forms and types, depending on the desired design effect and the specifics of the site. As with the more economical piled stone banks, where the stones are not set but loosely tipped on site, these riverbanks are relatively inaccessible and offer few ecological niches. The stones can, if they are of compatible format, be laid as tightly as possible, like a wall, or it is possible to choose various formats to leave gaps that can be planted or left to be colonised naturally and thus enhance the riverbank ecology.

In some sections, riverbanks threatened with erosion can be secured with terraced stone revetments that also function as steps or seats. Along with their hydraulic importance, such an element can also create a place to linger directly on the waterside although, unlike the design feature of broad riverbank steps (A1.3), the principal function is bank reinforcement. These steps can also go on down into the water and make paddling possible or serve as a place to land canoes. Especially where the watercourse channel has cut down into the terrain, this is one way to overcome the barrier of a steep riverbank. The choice of materials and the dimensions of these artificial elements on the waterside can exert a strong influence on the river space appearance. During the revitalisation of the Birs several riverbank sections particularly prone to erosion were reinforced with natural stone steps.

Masonry riverbank revetments create vertical walls that leave enough space to, for instance, lay a path immediately beside the water. They constitute a steep bank edge that permits direct access to the river. Even though this solution is of little ecological value it can make sense when it borders an intricately designed park such as that on the river island Île aux Planches on the River Sarthe in Le Mans. After a flood, however, the surfaces usually have to be cleaned of sediment. A sloping top surface and the avoidance of calm zones in the river ensure that not too much material is deposited.

–––––––– Gallego, Zuera ∆ 204 Wupper, Müngsten ∆ 230

–––––––– Ahna, Kassel ∆ 234 Birs, Basle ∆ 238

–––––––– Seille, Metz ∆ 246 Wiese, Lörrach ∆ 252 + Sarthe, Le Mans, Île aux Planches ∆ 28∂

D4.6 Building over the existing reinforcement

Rhône, Lyon

On many rivers there are old bank reinforcements of armour stones or concrete which would be too difficult or expensive to reshape or remove along their entire length. As the river would then be remote and inaccessible it can be a possibility to lay a path on top of the existing reinforcement, enhanced with occasional elements that jut out into the river, on which one can walk ‘over’ the water and enjoy views of the river. Additionally, along the way there can be niches to linger, sheltered from the elements. In Lyon at the southern end of the newly designed Berges du Rhône, small timber balconies with benches were installed, accessible via a few steps from the riverside path and offering direct connection with the water. –––––––– Rhône, Lyon ∆ ∂60 Seille, Metz ∆ 246

∂24 ∂25

Design Catalogue Riverbeds and Currents

D5

Varying the riverbed reinforcement

∫ All design tools in D5 can be combined with –––––––– D∂.∂ Large single rocks D∂.2 Dead wood D∂.3 Laid stone groynes D∂.4 Piled stone groynes D∂.5 Bioengineered groynes D∂.6 Submerged groynes D∂.7 Riverbed sills D2.∂ Widening the channel D2.2 Extending the flow length

In many rivers, the channel and the water level are regulated with artificial transverse constructions such as sills, barrages or weirs. They serve to slow the flow rate, to protect the riverbed and to overcome height differences – but through these cross-stream obstacles the basis for river processes and thus the natural self-dynamic development of the river channel is lost. Furthermore, the ecological passability of the watercourse is impaired and fish migration routes blocked; movement upstream into tributaries, essential for the reproduction of certain fish species such as salmon, is prevented. By varying or reshaping the riverbed reinforcement, a river becomes passable again and the basis for natural hydraulic processes is reinstated. Many transverse constructions cannot be entirely removed but can be reshaped or replaced by ramps and river bottom slides; this permits more variety in the water levels and higher flow rates, and sets natural water processes in motion again to a certain degree, while also making the watercourse passable. Converting transverse structures to semi-natural ramps and slides allows more diverse rivers to evolve; through the height differences and the babbling and flowing over large stones, the experience of the water becomes vivid and attractive for the observer. If transverse structures cannot be demolished, the situation should be improved with bypass channels or fish ladders.

D5.∂

D5.2

D5.3

Fish passes

Varying the riverbed and transverse structures

Ramps and slides

Neckar, Ladenburg, fish ladder in the Kandelbach

Wiese, Basle

Nahe, Bad Kreuznach

Transverse structures like weirs and locks present impassable obstacles to migrating fish; fish ladders, also known as fishways and fish passes, either as technical or nearnatural devices, can ensure the passability of a river. The most common near-natural fish passes are rough ramps and bypass channels of natural stone bars. Technical fish migration aids are slot or pool-andweir constructions of concrete or natural stone, or fish elevators. A naturally designed bypass channel can blend into the landscape better depending on the design context, while in urban spaces a technical gutter with its concentrated current offers welcome variety, such as in Ladenburg; it can become a dramatic attraction. In Gambsheim on the Rhine a fish ladder was constructed so that the migrating fish could be seen through windows.

Existing riverbed and transverse structures can be partially retained and the watercourse can nevertheless be allowed to develop further. On the River Wiese in Basle the riverbed reinforcement function was kept and the central section of a low weir removed down to the riverbed, creating a different cross-section with a stronger current variation function. It should be noted that when reshaping a riverbed construction the stretch of river directly up- and downstream must be taken into account and also altered. Changed currents may also require securing of the riverbank and -bed.

Ramps and slides are effective measures to preserve the beds of flowing waters and can replace disruptive transverse structures. A low weir is replaced by a ramp with a gradient of between 1:10 and 1:100. Stones of various sizes still armour the riverbed. Ramps and slides lend the river a natural appearance and ensure its passability. The gaps between piled stones offer habitats for life forms that thrive in flowing water. On the River Nahe in Bad Kreuznach the loud rushing noise of the water and visible swirling of the current over the new ramp make it a distinctive section of the river.

–––––––– Birs, Basle ∆ 238

–––––––– Nahe, Bad Kreuznach ∆ ∂84

Wiese, Basle ∆ 250

Birs, Basle ∆ 238 Neckar, Ladenburg ∆ 242 Wiese, Lörrach ∆ 252

–––––––– Alb, Karlsruhe ∆ 236

Werse, Beckum ∆ 270

Neckar, Ladenburg ∆ 242 Wiese, Lörrach ∆ 252 Isar, Munich ∆ 260 Werse, Beckum ∆ 270 + Rhine, Gambsheim, Fish Pass ∆ 280

∂26 ∂27

Design Catalogue Riverbeds and Currents

E

Dynamic River Landscapes

Isar, Munich

From constrained channel to dynamically meandering river – the static riverbank reinforcement at the mean water line is shifted or removed, and a river space with its own developmental dynamics evolves in which, over a long period, the banks and the position of the entire channel can continually change. ∂28 ∂29

Design Catalogue Dynamic River Landscapes

E

Dynamic River Landscapes

Spatial situation

In Process Space E the existing bank reinforcements restricting the self-dynamic river channel development – the red line – are removed or set back to permit the entire watercourse to follow a development driven by its own dynamic. In contrast to Process Space D the flood plain – the space between the red and green lines – is available for this. This means that, for one thing, the flood plain must still exist and for another that it may not be occupied by other uses that would preclude the possibility of stronger fluvial water dynamics. Not many rivers have such a large area to develop in the urban centres; sufficient space is often only to be found in the suburbs or within local recreational areas.

Operative processes

Removing the structurally static bank reinforcements that lie directly on the mean water level (the red line) is the basic prerequisite for designing with rivers that develop according to their own dynamics. In this way the riverbanks can be carried away at some points by erosive forces, especially during high water events. At other points aggradation zones can emerge. Sedimentation and erosion processes are not restricted to the riverbed but extend into the flood plain; over a longer period the position of the entire watercourse can change. Fords and scour holes are created in the riverbed, and the current varies dramatically. More extensively and more dynamically than in Process Space D, a varied river cross-section evolves, with cut banks, slip-off banks and various riverbed substrates. Depending on the type of river and the boundary conditions, these developments unfold at different speeds; dynamic upland streams can, for instance, change faster than slow-flowing lowland rivers, and measures must therefore be devised for each specific watercourse.

Design approaches

The design approaches in this type of space are an exception for the urban context, as allowing self-dynamic river channel development generates an unaccustomed wildness and free growth. Through the stronger differentiation of the river, with gently sloping banks, sandbanks, naturally eroding banks and new branches, its liveliness in contrast to the static townscape is highly prized. A thus modified riverscape permits intense contact with the water, makes new uses and experiences possible, with recurring change after every high water event. The measures are characterised by the fact that, once the initial construction work is completed, the result of the design intervention is not immediately apparent but that its special qualities evolve in the course of time. Dynamic river landscapes are subject to constant development, changing their characteristics (including discharge capacity, state of the banks, location of the channel) time and again. This

E∂ Allowing channel migration

E1.1 Removing riverbank and riverbed reinforcement E1.2 Semi-natural riparian management E1.3 Regulating water extraction

E2 Initiating channel dynamics

E2.1 Reprofiling the channel cross-section E2.2 Introducing disruptive elements E2.3 Adding bed load

E3 Creating new channels

E3.1 Creating meanders E3.2 Incorporating a straightened channel E3.3 Creating multiple channels

E4 Restricting channel dynamics

E4.1 ‘Sleeping’ riverbank reinforcement E4.2 Bank reinforcement as needed E4.3 Selective bank reinforcement

∂30 ∂3∂

Design Catalogue Dynamic River Landscapes

E

Dynamic River Landscapes

means that planners and the responsible authorities need a certain amount of courage to allow developments within the town or city that have no clear pre-defined final result but constitute a spatial and temporal process. Constant monitoring of these changes and, possibly, interventions to influence them are necessary under certain circumstances, especially when the river comes too close to the green limit line that it may not breach. Such an open-ended design process can often only be integrated into standard planning procedures with difficulty. If, however, the ongoing supervision and future development of the respective projects can be ensured, the quality can be better than in plans that strive for a fixed end result. Deliberate encouragement of self-dynamic river channel development is frequently applied within revitalisation and restoration measures. This chapter presents design strategies and tools that make the balancing act of dynamic river development within the built urban environment possible. As a rule, the smaller the watercourse the easier it is to put this approach into practice. The example of the River Isar in Munich shows, however, that with a powerful vision, the concept of fluvial dynamics can also be applied to a larger river in the middle of a major city. A clear understanding of the natural water processes is, in every case, the basic prerequisite for this kind of river development. The design strategies in this Process Space are distinguished above all according to the type of initial intervention: Is it just a case of removing the old bank reinforcements, or will the river’s development be further initiated, accelerated or encouraged with construction measures? Within an urban environment, the completely unrestricted development of a river is seldom possible. In the category of design strategy E4 (Restricting channel dynamics) alternative concepts to secure the banks are presented that define a clear framework within which development is permitted. In Process Space E we find projects and measures with primarily ecological aims, although spaces are also created with very high recreational value. The ecological restoration interventions can be beneficially combined with flood protection measures, as excavating the flood plain leads to the creation of new backwaters and marginal zones that have a positive effect on floodwater discharge.

Amenity The self-dynamic development of a river creates multifarious spatial situations in the river channel and on its edges. A varied, multifunctional open space evolves: beaches allow direct access to the water, inviting people to barbecue, sunbathe and swim, sand- and gravel banks or areas of shallow water are popular natural playgrounds and make it possible to experience nature in the middle of town. Potential conflicts with nature conservation objectives should be averted at the planning stage through well-considered zoning of the river plain. Rambling, cycling and horse riding along such a freely developing river is rich in variety, while dogs and their owners have attractive spaces for exercise. Aggradation and erosion processes make the dynamics of a river landscape visible and tangible through the year. In urban areas, contrasts between permanent built elements and self-transforming natural elements can be very appealing. Measures that reveal the redesign of the watercourse – for example retaining parts of the former bank reinforcements or an old industrial relict – contribute to the legibility of the townscape. Flood protection

Through the river’s own dynamics, a broad, shifting riverbed with gently sloping banks evolves. This can have various consequences for high water situations. If the flood plain is excavated to expose both the banks and the terrain close to the channel to a larger extent to the impact of water, this enlarges the river discharge cross-section. It makes it possible for floodwater to flow on faster and thus improves flood protection at this point. The emergence of a meandering course, however, can also increase the flow drag (roughness) and this reduces discharge capacity. The retarded discharge, on the other hand, expands the retention space of the watercourse in this section and thus relieves pressure on downstream areas during periods of high water: such factors should be taken into consideration during the planning process. Harmonising interventions within a regional flood protection concept must be addressed at an early stage. Dynamic alterations in the watercourse can mean that the discharge cross-

section is constantly changing on a small scale, for instance when driftwood lodges or is washed up. In light of all this, when determining a river’s discharge capacity a safety margin must be included, and regular monitoring of the discharge patterns should be carried out, especially in urban areas threatened by flooding.

Ecology

The morphodynamic processes of flowing water lead to the emergence of a great variety of structures and aquatic, amphibian and land habitats with rich biodiversity. Habitats that are repeatedly subjected to rejuvenation processes through regular flooding such as gravel- and sandbanks, beaches or undercut banks have become rare on most rivers. They provide, however, ecological niches for species of plants and animals that are adapted to such extreme conditions. Also ecologically interesting are the backwaters separated from the main channel by silting and flooded at high water. In revitalising streams, parts of the old streambed can be used. Creating ecologically valuable areas in urban spaces where pressure on space from recreational uses is high can lead to conflicts between uses, which can be defused by visitor guidance – although excluding people completely from these newly created areas would make them monofunctional enclaves within the urban landscape scenario, a wasted opportunity to raise awareness of the surrounding ecosystems. Such reconfigurations can also be planned as mitigation measures for interventions that impact on nature and the landscape in the spirit of national environmental impact assessment regulations.

Since the old riverbank reinforcements were removed the River Isar has been able to shift its course, creating islands and beaches. As safety measures, in the flood plain underground ‘sleeping’ reinforcements were built (red dashed line) so that the stability of the dikes (flood limit indicated by green line) is not endangered.

∂32 ∂33

Design Catalogue Dynamic River Landscapes

E∂

Allowing channel migration

∫ All design tools in E∂ can be combined with –––––––– C∂.4 Reprofiling the flood plain C3.5 Extensive natural areas E2.∂ Reprofiling the channel cross-section E2.2 Introducing disruptive elements E2.3 Adding bed load E3.∂ Creating meanders E3.3 Creating multiple channels E4.∂ ‘Sleeping’ riverbank reinforcement E4.2 Bank reinforcement as needed E4.3 Selective bank reinforcement

The simplest and most radical way of restoring a river to its natural fluvial dynamics is to remove the existing riverbank reinforcements and thereby the limits on its self-dynamic development. The riverbank revetment and low weirs that seal the bed and define the river’s course are taken out. One objective of this strategy is that the watercourse can attain a quasi-natural state with a meandering channel without additional human intervention, solely through its own dynamic processes. If a river is capable of regaining its dynamic equilibrium within a reasonable period, then this approach without artificial interventions is advantageous for both economic and ecological reasons. If a river has been drastically straightened, however, just removing the riverbank reinforcement is often insufficient; when a watercourse has very little dynamic of its own it can take decades to achieve semi-natural structures, while if the flow rates are fast, with correspondingly high tractive forces, there is a danger that the watercourse will cut too deeply into the bed at first until a form develops within which dynamic equilibrium can evolve. When removing the riverbank reinforcement, adapting the channel maintenance regime is an important aspect. Traditional river channel maintenance is usually concerned with keeping the discharge cross-section clear and thus passing water downstream as fast as possible. Responsibility for channel maintenance rests with the respective owner – in Germany the municipal water and ground authority for smaller watercourses and the national waterways and shipping administration for major rivers – whose traditional role was limited to maintaining a defined condition of each river – the riverbed was regularly desilted, aggradations and scour holes removed, the banks reinforced or shored up after collapsing, and riverbank vegetation regularly mown. Today channel maintenance – in Germany following the new version of the water resources law (WHG) of 31 July 2009 – pursues the aim of ecological enhancement of rivers with stronger self-dynamic channel development, encouraged through the introduction of ‘disruptive’ elements (see also E2 Initiating channel dynamics). Crucially, it must be realised that this self-dynamic development requires sufficient space. Removing steep riverbank reinforcement has direct and positive consequences for the accessibility of the river. Permitting natural vegetation and aggradation processes enhances the experiential quality but can also convey an impression of unkemptness and running wild. To ensure that such measures are welcomed by local residents and to assuage their concerns, a broad-based information campaign and opportunities for citizen involvement are recommended.

E∂.∂

E∂.2

E∂.3

Removing riverbank and riverbed reinforcement

Semi-natural riparian management

Regulating water extraction

Wahlebach, Kassel

Losse, Kassel

Isar, Munich

Taking out the riverbank and riverbed reinforcement can happen either in its entirety or in part – the type and extent of the intervention will influence the river’s inherent dynamic and its channel development. Removal of bank reinforcements can, for example, be on just one side, as on the River Isar in Munich, or on both sides along a considerable stretch of the watercourse as on the Wahlebach stream in Kassel. It is possible to reuse the removed materials as disruptive elements on the riverbed or to build a new flood protection line.

More extensive, adaptive channel maintenance permits natural development processes in flowing water. Abandoning measures such as regular desilting and the removal of aggradations, scour holes and bank collapses while permitting or deliberately introducing disruptive elements such as dead wood or aquatic plants leads to differentiation in the flow pattern and works on the riverbed to initiate sedimentation and erosion processes. The river’s self-dynamic channel development of a meandering course with cut banks and slip-off banks begins. On the delta of the River Losse in Kassel, once the works had been completed, refraining from regular maintenance has actually been the design tool that allows the river’s own dynamic to create new branches and vegetation zones. It is only when the situation is critical, for example when floodwater backs up or erosion goes too far, that conventional riparian management measures are employed.

Large amounts of water are taken from some rivers by hydroelectric power stations with supply canals, as along the River Isar in Munich, and for the irrigation of agricultural land in dry areas, leaving – especially in the dry summer months – very little water (‘residual water’) in the watercourse. On the one hand this has a negative effect on the appearance and ecology of the river, and on the other it is insufficient to drive the self-dynamic river channel development. By adjusting water extraction the residual water quantities can meet the ecological and aesthetic needs of the river; this can, as in Munich, mean curbing or even interrupting power station operations in especially dry months.

–––––––– Emscher, Dortmund ∆ 256 lsar, Munich ∆ 260 Schunter, Braunschweig ∆ 266 Wahlebach, Kassel ∆ 268 Werse, Beckum ∆ 270

–––––––– Isar, Munich ∆ 260

–––––––– Kyll, Trier ∆ 2∂0 Emscher, Dortmund ∆ 256 Losse, Kassel ∆ 264 Schunter, Braunschweig ∆ 266 Werse, Beckum ∆ 270

∂34 ∂35

Design Catalogue Dynamic River Landscapes

E2

Initiating channel dynamics

∫ All design tools in E2 can be combined with –––––––– C∂.4 Reprofiling the flood plain C3.5 Extensive natural areas E∂.∂ Removing riverbank and riverbed reinforcement E∂.2 Semi-natural riparian management E3.∂ Creating meanders E3.2 Incorporating a straightened channel E3.3 Creating multiple channels E4.∂ ‘Sleeping’ riverbank reinforcement E4.2 Bank reinforcement as needed E4.3 Selective bank reinforcement

Selective interventions in the river, in the form of specific shifting of substrate along the banks or on the riverbed, stimulate morphodynamic processes. This strategy is applied when the desired effect of channel development out of the river’s inherent dynamic would come about only after a very long time. Its aim is to optimise the framework conditions for the inherent dynamics to create a structurally rich watercourse more quickly. These redeployments, then, serve merely to prime a strong internal dynamic under which the initial condition of the river will quickly disappear. Within a short period, varied habitats can arise that facilitate speedy colonisation by flora and fauna. At the same time the appearance of the river can also alter quickly, making it easier to communicate the intervention to the public although the actual course of the river channel is not changed at first. Most design tools to introduce morphodynamic processes exert an influence on the currents in the river, setting erosion and sedimentation processes in motion through their specific variation. Broadening the river at certain points slows down the flow and thus creates aggradations, while disruptive elements placed in the current can deflect it towards one bank and initiate erosion. As design leitmotif, it is possible to strive for a semi-natural appearance, for instance by using dead wood, or conversely to make the deflection of the current obvious by using plainly artificial elements. Introducing disruptive elements also means expanding habitat diversity, and stepping stones can facilitate direct contact with the water. A further option is to influence the framework conditions for the discharge and sediment transport processes. Direct mechanical deposition of sediment on the riverbed makes it easier for sand- and gravel banks or natural beaches to arise, while stimulating the discharge dynamics reinforces the operative forces and thus accelerates developments.

E2.∂

E2.2

E2.3

Reprofiling the channel cross-section

Introducing disruptive elements

Adding bed load

Isar, Munich

Schunter, Braunschweig

Isar, Munich

Excavating the flood plain and flattening out the banks makes it possible for the river to develop beyond the former riverbank line. Selective excavations or shifting substrate within the riverbed creates sinks or shallow water zones; varying the crosssection raises the flow variation and a low water channel is created in the fasterflowing areas which serves to sustain the continuity of flow even during an extreme drought. Depending on the speed of the current, sediments of various particle sizes settle on the riverbed, as on the Isar in Munich, ranging from sand through gravel to larger stones – thus further enhancing the river’s structural diversity.

Disruptive elements are set at specifically chosen places in the riverbed to deflect the current towards the bank and catalyse erosion processes, or to encourage sedimentation on their sheltered downstream side. Disruptive elements can be just as well attached to the bank as placed in midstream. On the Schunter in Braunschweig fixed dead wood is used to divert the current and thus erode the opposite riverbank and create a steep slope. The disruptive elements enhance structural diversity and provide places to play and linger by on the waterside.

Many urban rivers lack natural sedimentation. Weirs or other cross-stream structures hinder the movement of bed load and the rivers suffer from sediment deficit. Deliberate deposition of sediments typical for the river provides material for riverbed enhancement. On the Isar in Munich, artificial sand or gravel banks were dumped, to be carried further downstream by the next high water event. If the lack of bed load is caused by weirs and ground sills, single openings in the structure can let the bed load overcome the barriers and contribute to the development of the next stretch of river.

–––––––– Isar, Munich ∆ 260

––––––––

Losse, Kassel ∆ 264

Emscher, Dortmund ∆ 256

Schunter, Braunschweig ∆ 266

Isar, Munich ∆ 260

Werse, Beckum ∆ 270

–––––––– Isar, Munich ∆ 260

Wahlebach, Kassel ∆ 268 Werse, Beckum ∆ 270

∂36 ∂37

Design Catalogue Dynamic River Landscapes

E3 Creating new channels

∫ All design tools in E3 can be combined with –––––––– C∂.2 Branches C∂.4 Reprofiling the flood plain C3.5 Extensive narural areas E∂.∂ Removing riverbank and riverbed reinforcement E∂.3 Regulating water extraction E2.∂ Reprofiling the channel cross-section E2.2 Introducing disruptive elements E2.3 Adding bed load

This design strategy foresees an artificial reshaping of a straightened watercourse channel within a longer stretch of the river. Its aims are to quickly reinstate natural morphodynamic processes and thus pre-empt the protracted developmental process from a straightened to a meandering course and to create a more dynamic initial situation for the river’s further development. Alongside, or instead of, the straightened watercourse, a new riverbed with a meandering course is laid. With major rivers, their use as shipping routes often means that the main channel may not be interfered with; in this case the alternative is to create new branches in the adjoining river plain that can develop more freely. The earthworks require the use of excavators and possibly trucks. To determine the natural meandering course, natural rivers in similar landscape contexts can serve as models. If desired it is also possible to reconstruct the original course or at least the breadth of the natural meanders on the basis of old maps or surviving historical landscape features. Once construction of the new riverbed is completed, its subsequent development is driven by natural fluvial dynamics. On non-navigable watercourses the old, straightened section can become a branch or cut-off section and thus develop into a special habitat, while on navigable rivers only the new branch can be developed in this way.

E3.∂

E3.2

E3.3

Creating meanders

Incorporating a straightened channel

Creating multiple channels

Wahlebach, Kassel

Werse, Beckum

Losse, Kassel

Building a meandering stretch of watercourse, as on the Wahlebach in Kassel, creates a near-natural appearance. Such a construction is relatively costly and constitutes a radical intervention; if it is possible to link up existing sinks in the flood plain or to reinstate an old watercourse channel this will save considerable earthmoving and costs. As the watercourse takes up more space and is clearly apparent in the landscape, such a design measure is suitable for a large-scale enhancement of landscape or urban spaces.

When building a new watercourse channel, sections of the old straightened channel can be included as backwater or flood plain biotope; the sections are connected to the new channel so that they are affected either permanently or only during high water by the fluctuations in water volume and the current of the river. In this way, valuable amphibian refuge zones for flora and fauna are created; as on the River Werse in Beckum, parallel to the flowing watercourse a linear standing backwater evolves, separated by an island. If the old riverbed is fitted with a sill at its upper end, over which the river flows only at high water, it can carry off part of the floodwater discharge and increase the flow rate. The old course can thus compensate for the higher roughness caused by the new meandering form of the watercourse.

Building a parallel channel or dividing the river can markedly improve its structural diversity and recreational quality. The design can address both ecological and aesthetic considerations. It must, however, be noted that splitting the river channel should not lead to a significant widening of the low water cross-section, as a water level that is too low can limit passability. Dividing the course, as on the Losse in Kassel, created several islands; their inaccessibility making them a refuge for birds and amphibians while enlivening the landscape.

–––––––– IJssel, Zwolle ∆ 208 Emscher, Dortmund ∆ 256 Losse, Kassel ∆ 264 Wahlebach, Kassel ∆ 268 Werse, Beckum ∆ 270

–––––––– IJssel, Zwolle ∆ 208 Isar, Munich ∆ 260 Losse, Kassel ∆ 264

–––––––– Losse, Kassel ∆ 264 Wahlebach, Kassel ∆ 268 Werse, Beckum ∆ 270

∂38 ∂39

Design Catalogue Dynamic River Landscapes

E4

Restricting channel dynamics

∫ All design tools in E4 can be combined with –––––––– D4.2 Living revetment D4.3 Stone revetment D4.4 Terraced stone revetment E∂.∂ Removing riverbank and riverbed reinforcement E∂.3 Regulating water extraction E2.∂ Reprofiling the channel cross-section E2.2 Introducing disruptive elements E2.3 Adding bed load

In the light of the intense use of urban areas, the space available for a river to develop a new course out of its natural fluvial dynamics is often limited; somewhere or other within the river’s natural sphere of influence is usually a building, a road or underground cables or pipes that may not be affected by the water. Safeguarding concepts define the space within which a watercourse can freely exercise its morphodynamic processes without endangering its surroundings. ‘Sleeping’ riverbank reinforcement and the practice of creating a bank reinforcement only as and where needed are design possibilities to work within this (usually invisible) restrictive frame. In Process Space E, the project vision is usually relatively close to natural models, and massive technical constructions are undesirable. ‘Sleeping’ reinforcements in the river flood plain, for example in the form of underground stone reinforcement, limit the desired development of the river’s course to the space available; should erosion reach this hitherto invisible safeguard and expose the stone reinforcement, this then functions as fixed riverbank reinforcement. The solution of securing the riverbank only as and where needed does not set an initial built limit to the way the river’s course develops. Based on assessments of what structures need protection, such as dikes, roads or buildings, safety margins for limiting the river’s self-dynamic channel development are calculated. These safety margins must be constantly reassessed, especially for high water events, in the procedures of riparian management; if the river threatens to encroach upon the safety margin through erosion and natural developments in its course, only then is the riverbank secured through such measures as stone embankments or vegetation fascines. This praxis demands constant monitoring and assessment of the river, from which important knowledge can be gathered over the years for optimising its development. These measures make it possible to systematically divide the flood plain into zones which in the long term could be overrun by the river’s development and those which should be protected. These spaces can be designed correspondingly and in various ways. At narrow sections of the watercourse channel at which no self-dynamic development of the banks is possible, there are possibilities of selective bank reinforcement in combination with concepts from Process Space D such as a terraced stone revetment (D4.4).

E4.∂

E4.2

E4.3

‘Sleeping’ riverbank reinforcement

Bank reinforcement as needed

Selective bank reinforcement

Isar, Munich

Wahlebach, Kassel

Isar, Munich

A hidden or ‘sleeping’ bank reinforcement consists of an underground stone barrier that prevents further development into the flood plain when the shifting watercourse channel reaches it. On the River Isar in Munich, material from the old riverbank reinforcements was reused for this ‘sleeper’. A combination of underground bank reinforcement and a surfaced path along the top can be very interesting, although a rough path of unprotected bare earth may not be allowed to run along the reinforcement structure; it is important that the vegetation cover or the path surface protects the earth above the sleeping reinforcement from erosion during high water, as it could otherwise be washed away and its function debilitated.

Here, a limit is defined for the development of a river’s course with a safety margin that places buildings, roads or dikes out of danger. This limit is not, however, a permanent built structure but only set through continual monitoring and the application of bank reinforcement measures when and where they are needed. This means that the responsible water authority estimates from experience how far a watercourse channel may shift through erosion without endangering sensitive areas during the next high water period; furthermore they regularly document its development. If there is a danger that the watercourse will overrun the defined limit, specific local interventions stop further erosion. The free watercourse of the Wahlebach in Kassel threatened to damage a footpath and was therefore constrained with willow plantings. The measure aims to minimise the intervention and to avoid spending on preventive but possibly superfluous safety measures.

At narrows or on directly used riverbanks there is often no alternative to securing the bank with built structures within a concept of free river channel development. This need not have a negative effect on the overall quality of the project; such places can be used to create interesting places to linger at the waterside. The reinforcements can be very small-scale, just enough to deflect the erosive force of the current away from the sensitive bank. On the Isar in Munich, the necessary protection for the bridge supports was built as a series of waterside steps, emphasising the lively urban character of the river in the immediate vicinity of the bridge.

–––––––– Birs, Basle ∆ 238 Isar, Munich ∆ 260

–––––––– Isar, Munich ∆ 260 Wahlebach, Kassel ∆ 268

–––––––– Wahlebach, Kassel ∆ 268

∂40 ∂4∂

Design Catalogue Dynamic River Landscapes

Project Catalogue

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Introduction ∂44 ∂45

Project Catalogue Introduction

34 03 04

Process Space A: Embankment Walls and Promenades

––––––––

A

1 2 3 4 5 6 7 8

Process Space B: Dikes and Flood Walls

––––––––

B

9 10 11 12 13 14 15

16 Bergsche Maas, between Waalwijk and Geertruidenberg, the Netherlands: Overdiepse Polder ∆ ∂94 17 Besòs, Barcelona, Spain: Ecological Restoration ∆ ∂96 18 Ebro, Zaragoza, Spain: Parque del Agua ∆ ∂98 19 Elbe, Hamburg, Germany: HafenCity ∆ 202 20 Gallego, Zuera, Spain: Parque Fluvial ∆ 204 21 IJssel, Zwolle, the Netherlands: Vreugderijkerwaard ∆ 208 22 Kyll, Trier, Germany: Renaturation of the Kyll Mouth ∆ 2∂0 23 Maas, Maasbommel, the Netherlands: Floating Homes in Gouden Ham ∆ 2∂2 24 Petite Gironde, Coulaines, France: Parc de la Gironde ∆ 2∂4 25 Rhine, Brühl, Germany: Koller Island Polder ∆ 2∂8 26 Rhine, Mannheim, Germany: Riverbank Renaturation and Lido Restaurant on Reiß Island ∆ 220 27 Seine, Le Pecq, France: Park Corbière ∆ 222 28 Waal, Gameren, the Netherlands: Gamerense Waard Flood Plain Renaturation ∆ 224 29 Wantij, Dordrecht, the Netherlands: Plan Tij Housing Estate ∆ 228 30 Wupper, Müngsten, Germany: Müngsten Bridge Park ∆ 230

Process Space D: Riverbeds and Currents

––––––––

D

31 32 33 34 35 36 37 38 39

Ahna, Kassel, Germany: Renaturation ∆ 234 Alb, Karlsruhe, Germany: Near-natural Restoration ∆ 236 Birs, Basle, Switzerland: Birsvital ∆ 238 Leutschenbach, Zurich, Switzerland: Restoration ∆ 240 Neckar, Ladenburg, Germany: Green Ring ∆ 242 Seille, Metz, France: Parc de la Seille ∆ 246 Soestbach, Soest, Germany: Daylighting of the Soestbach ∆ 248 Wiese, Basle, Switzerland: Revitalisation ∆ 250 Wiese, Lörrach, Germany: Wiesionen ∆ 252

Process Space E: Dynamic River Landscapes

––––––––

E

IJssel, Doesburg, the Netherlands: IJsselkade Residential Area ∆ ∂72 IJssel, Kampen, the Netherlands: Flood Protection in Kampen-Midden ∆ ∂74 Main, Miltenberg, Germany: Flood Management Concept ∆ ∂78 Main, Wörth am Main, Germany: Flood Management for the Old Town ∆ ∂80 Nahe, Bad Kreuznach, Germany: Flood Management Concept ∆ ∂84 Waal, between Afferden and Dreumel, the Netherlands: Tapered Dike ∆ ∂88 Waal, Zaltbommel, the Netherlands: Waalkade Promenade ∆ ∂90

Process Space C: Flood Areas

––––––––

C

Elster and Pleiße Millraces, Leipzig, Germany: New Riverbanks ∆ ∂50 Leine, Hanover, Germany: Leine Suite ∆ ∂54 Limmat, Zurich, Switzerland: Factory by the Water ∆ ∂56 Limmat, Zurich, Switzerland: Wipkingerpark ∆ ∂58 Rhône, Lyon, France: Berges du Rhône ∆ ∂60 Seine, Choisy-le-Roi, France: Quai des Gondoles ∆ ∂64 Spree, Berlin, Germany: Bathing Ship ∆ ∂66 Wupper, Wuppertal, Germany: Wuppertal 90° ∆ ∂68

40 41 42 43 44 45

Emscher, Dortmund, Germany: Retention Basin Mengede and Ellinghausen ∆ 256 Isar, Munich, Germany: Isar-Plan ∆ 260 Losse, Kassel, Germany: Losse Delta ∆ 264 Schunter, Braunschweig, Germany: Restoration ∆ 266 Wahlebach, Kassel, Germany: Near-natural Restoration ∆ 268 Werse, Beckum, Germany: Near-natural Development ∆ 270

Project selection criteria

This book presents 45 extremely diverse projects that address the natural dynamics of rivers in innovative ways. All the projects were either completed or under construction at the time of publication. Selection of the projects took place as part of research that concentrated on Switzerland, France, Spain, the Netherlands and Germany, on the basis of personal recommendations, study of current publications in the fields of water management and landscape architecture, internet research and attendance at conferences. As a matter of principle all projects were visited, and most of them were discussed on site with planners, ecologists and the responsible public authorities. The final selection aimed to present as many different design approaches as possible, and this means that not only particularly spectacular projects have been documented; if a project, for instance, illustrated an aspect that appeared in no other example, then this was a criterion to include it in the selection. The spectrum ranges from small-scale interventions such as enhancing a riverside promenade with a floating café or steps down to the water through to large projects such as the revitalisation of streams which had been culverted into pipes for several kilometres or the creation of several hectares of retention space to buffer cities against flood surges. What all the projects have in common is a multifunctional approach that addresses at least two of the three principal aims described in Part 1: flood protection, ecological enhancement and amenity.

Arrangement of the projects

The Project Catalogue was arranged according to the Process Spaces outlined in Part 2 of the book, and thus projects in similar spatial situations are grouped together, such as riverside walls and promenades in Process Space A or dikes and flood protection walls in Process Space B. Within each Process Space, the projects are in alphabetical order by the name of the river; various projects along the same river are thus adjacent even though they may be located in different countries. The projects are always allotted to the Process Space that defines its main aspects and hence determines the design approach. This Process Space, in which one can also find the design tools most relevant for the project, is indicated by a dark blue tab on the left margin. In large-scale projects or those with a succession of different topographical features, design measures from different Process Spaces may be found; the River Isar, for example, appears in Process Space E, Dynamic River Landscapes, because the main point of interest is permitting and initiating self-dynamic processes in the river space. At some places within the project, however, the dikes were reinforced or single flights of steps were laid out to strengthen the riverbank, measures which are typical of other Process Spaces. These secondary Process Spaces are indicated by light blue tabs.

Documenting the projects

∂46 ∂47

Project Catalogue Introduction

The most important link between a project description and the Design Catalogue is made in the listing on the left-hand margin of all the design tools and measures applied to the project. Via their identification by Process Space letter and numbering system, these descriptions can be found quickly in the first volume, leading to the discovery of further design tools suited to the Process Space (and hence a similar spatial situation) and of other projects that apply the same design tools and/or measures. Each project is illustrated with photographs and schematic sections that use the same colour scheme as in Part 2: the colour green indicates the flood limit and the colour red the limit of the self-dynamic river channel development. Marking these limits, which are always artificially defined in any urban space or cultural landscape, makes it possible to develop a deeper understanding of the projects with respect to their water processes, and justifies their assignment to a particular Process Space. The project description explains the motivation and aims behind the project and places special emphasis on the question of how the planners have addressed the dynamic fluvial processes in their project. To place it in geographical context, each project description includes a small map of varying scale on the left-hand margin that indicates the project’s location within the urban space, and next to this the principal data on the watercourse in the project area: size of catchment area, mean and one-in-100-year discharge, and width of the riverbed and flood plain conveying an impression of the size and character of the watercourse.

For the projects in Germany the stream type is indicated according to the classification of 25 different stream types by LAWA – Länderarbeitsgemeinschaft Wasser [Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit, 2008]. For the examples located in the Netherlands the ‘Survey of natural water bodies’ was used [stowa Stichting toegepast onderzoek waterbeheheer, 2005]. Since no comparable classification systems were apparent for the projects in other countries, no stream type is indicated there. The geographic coordinates are denoting the project’s exact location. A list of individuals, companies and agencies that contributed to each project, as far as they were made known to us, can be found in the appendix.

References Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit – BMU (Federal Ministry for the Environment, Nature Conservation and Nuclear Safety), 2008: Wasserblick: Bund-, Länder-, Informations- und Kommunikationsplattform. German Stream Types. Berlin: http://www.wasserblick.net/servlet/is/24739/ ?lang=en, 31 March 2010 stowa (Stichting toegepast onderzoek waterbeheheer) 2005: Overzicht natuurlijke watertypen 2005-08, Utrecht: self-published. http://www.stowa.nl/uploads/themadownloads2/ mID_4910_cID_3900_97529197_gids%20totaal.pdf

A

Embankment Walls and Promenades

∂48 ∂49

Project Catalogue Embankment Walls and Promenades

Leine, Hanover

1

Elster and Pleiße Millraces New Riverbanks, since 1996 Leipzig, Germany River data for project area Stream type: Artificial millrace Catchment area: