Design principles of multifunctional flood defences

Nieuws - 23 mei 2017

Multifunctional flood defences are structures that primarily protect land from being covered by water coming from oceans, seas, rivers, lakes and other waterways, and that simultaneously serve other purposes. The 'other purposes' are commonly fulfilled by hydraulic engineering structures (for example, discharge sluices), infrastructures (roads, cables, pipes) and buildings, objects & shared use (houses, trees, sunbathing meadows). The research focused on the combination of flood protection with functions that are fulfilled by means of buildings and objects (other than hydraulic engineering structures and infrastructures), with a high degree of structural integration. This can typically be found in the urban context, where the combination of long-term flood protection and spatial quality is considered crucial for the viability of cities along rivers and seas.

Buildings and objects in multifunctional flood defences are combined with the structural elements that are primarily intended to contribute to the flood-protecting function. The composition of such a combined structure is more complex and diverse than of plain flood defences, so the design of these multifunctional flood defences requires extra attention. The objective of the research therefore was to develop a method for the design of multifunctional flood defences. The method concentrates on the verification of the flood protection function. The verification is a step in the design process that ensures a feasible and functioning result. Design projects for the realisation of flood defences are usually part of a more extensive strategy to reduce flood risks, formalised in regional, national or international policies. The research objective was achieved by answering three research questions that are related to the three levels of taking care of flood risk reduction.

1. What issues determine the chosen strategy for flood risk reduction in the Netherlands?
Literature was studied to understand the context in which strategies of flood risk reduction have been developed in the Netherlands. It appears that the flood protection strategy has always been the result of available technological knowledge, experience and organisational capabilities. An efficient and effective strategy of flood risk reduction was often hampered by the lack of funding for construction or maintenance. Interest in flood protection has varied due to political conflicts and policy changes, which often impeded a long-term strategy. A major obstruction for an effective strategy has been the mismatch between the geographical scale of the problem and the geographical expanse of governing institutions. This only gradually improved in the nineteenth century, after the foundation of national committees like, such as the Dutch governmental agency, Rijkswaterstaat, in 1798, and a Royal Advisory Committee instated in 1809 by King Louis Napoléon Bonaparte.

The influence of society on decision-making regarding flood defences has changed in 1970s. Instead of basing the strategy only on economic (cost-benefit) arguments, values of landscape, nature and culture were gradually involved. This can be explained by a shift of societal well-being. Meanwhile, society has become more complex and more people have become involved in policy-making and management. These governance aspects pose challenges to those who are involved in developing effective and efficient flood risk reduction strategies.

2. What method can best be used for the integrated and sustainable design of multifunctional flood defences?The methodology of design was studied in general and more specifically, it was attempted to find an approach that would be suitable to the design of integrated and sustainable multifunctional flood defences. The engineering and the spatial design approaches have grown apart since the 1970s in academics as well as in parts of practice. This has led to a sub-optimal process of first engineering flood defences and then attempting to let them improve the spatial quality, or the other way around. The challenge for this dissertation was therefore to integrate the systematic approach of the engineering method and the creative and learning character of the spatial design method. The advantages of an integrated approach are that it aims at solving a specific societal problem, it can be sub-divided into phases and it can be applied by a multi-disciplinary project team. The integrated design method developed in this dissertation consists of seven main steps, and is cyclic and highly iterative. It enables creativity, experimenting and learning from developing concepts, and offers possibilities to organise the process. It takes landscape, nature and cultural values into account, includes stakeholder participation and involves multiple disciplines in the design process. The method ensures that feasible and functional results are reached.

The proposed method has been tested by student design teams and indeed appears to be systematic, intuitive and creative. If attention for several aspects of application of the proposed method is taken into account, an integrated design is guaranteed.

3. How should design concepts of multifunctional flood defences be verified (concentrating on the flood-protection function)?Most importantly, it should explicitly be verified whether multifunctional flood defences will be able to fulfil their flood-retaining function. The verification needs extra attention, because the structural composition of multifunctional flood defences deviates from plain flood defences. Therefore, a method was developed for a qualitative functional verification, making use of generic structural element types that can be distinguished in multifunctional flood defences. Examples of these structural element types are water-retaining elements, erosion protecting elements and supporting elements. With the help of these element types, it can be verified whether a specific structure will potentially be able to function as a flood defence. Twenty-eight existent cases were studied to give reasonable proof that the derived element types could indeed be recognized and that there are no elements that don't fit in the derived typology.

The functional verification of a design concept should be followed by an in-depth quantitative structural verification to ensure structural integrity and constructability. This structural verification is similar to regular flood defences, but the influence of the 'other' function on existing failure mechanisms has to be included. Furthermore, potential new failure mechanisms have to be considered. Especially scour around buildings and materials unusual for flood-retaining structures are points of attention. A sequence for the structural verification of multifunctional flood defence was developed.

Four cases were studied in more detail to validate the verification method proposed in this dissertation. The case studies concentrate on the functional verification of the flood protecting function, but include other design steps as well, to demonstrate how the functional verification is embedded in an entire design loop. The first case concerns the sea defence of the coastal town of Katwijk aan Zee, which is combined with a parking garage. The second case concerns a shopping complex in Rotterdam, called the Roof Park, which is combined with a river dike. A boulevard along a river in Rotterdam, called the 'Boompjes', combined with sports and leisure functions, forms the third case. The last case studies a river dike in Sliedrecht, where houses on both sides of the crest impede simple dike reinforcement.

From the case studies it is concluded that the verification method as described and validated in this dissertation is workable and useful. The overall method for integrated and sustainable design has been tested with student groups and the functional verification has been validated in the present dissertation. Apart from the technical aspects shown in the cases, it is, however, recommended to solve the problems concerning governance aspects, because they still seem to be the main impediment for the design and realisation of multifunctional flood defences. The present dissertation, however, offers a helping hand by proposing to vary the roles of structural elements during the development of concepts, and in this way provide insight in the consequences for governance and make them open to discussion.

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