Shared-use of flood defences

Summary

The primary purpose of flood defences is protect people and their livelihoods from flooding. Nevertheless, flood defences are often integrated with many other functions such as housing and nature. This shared-use of the flood defences often comes with conservative estimates for a robust design that guarantees the flood safety standards are met. While in current practise multifunctional use generally imposes strict restrictions on the design of flood defences due to safety concerns, multifunctional-use may in some cases contribute to reducing flood risk instead. With the new probabilistic risk approach recently adopted in the Netherlands, it is possible to improve the methodology for assessing multifunctional flood protection. To do so an integrated risk assessment framework is required. Therefore, the aim of this research is: to investigate how combining different activities on or near flood defences affects the safety provided by flood defences assessed with a risk-based approach. 

For this research, four research questions are addressed through case studies: 1) How can a dike assessment framework be adapted to probabilistically evaluate multifunctional use of a flood defence? 2) How can additional defences constructed for multifunctional use of the flood protection zone be incorporated and evaluated within a probabilistic evaluation framework? 3) How can a probabilistic framework account for long-term climate adaption benefits of multifunctional use within a flood protection zone?

To answer the first question a scenario-based approach already suggested in grey-literature was compared to the existing conservative approach. Assessments made of a dike containing of common multifunctional elements (house and tree). As the analysis shows, the current conservative approach can lead to a significant overestimation of the probability of failure compared to a scenario-approach, up to several orders of magnitude. This is most notable when the dominant failure mechanism is directly affected by the multifunctional use. Furthermore, the influence of multifunctional use on safety is proportional to the size and protection level of the flood defence itself. The safety of large flood defences will thus be impacted less by multifunctional use than conventional assessments would suggest, but flood protection benefits of multifunctional use will also be limited.

The inclusion of additional defences as part of a multifunctional flood protection concept was explored through the case of the Double Dike between Eemshaven and Delfzijl. Further analysis shows not all double dike configurations may improve flood protection. When the size of a seaward defence is much larger than the landward defence, the bulk of the safety is provided by the seaward defence. Furthermore, additional elements like culverts introduce a new mechanism for flooding the hinterland.  To evaluate benefits of double dikes, it is proposed to treat the seaward defences as transmitting loads to the final seaward defence in the assessment framework. This way, different configurations of dike heights and culvert sizes can be explored, while the development of failures across the parallel defences during storm events remains accounted for. 

Long-term benefits are often not included in risk assessments. The Wide Green Dike case in the Ems-Dollard region is an exception by incorporating the sediment dynamics on the foreshore in its design and construction and was therefore analysed in this research. The results indicate extracting clay from pits on the salt-marsh foreshore is feasible to obtain sufficient material for future dike reinforcements, provided a salt marsh can persist under sea-level rise. Still, uncertainty in marsh accretion due to compaction of the foreshore (e.g. by grazing) or the reduction of available sediment directly affects the need for future dike heightening, up to several additional millimetres each year. Results stress the importance of management of the foreshore in future dike reinforcements against sea-level rise.

With the results from the cases above, this study identifies four types of design components linking safety with shared-use: 1) the foreshore 2) the geometry of the flood defence system, 3) the (state of) materials used for flood protection elements, and 4) objects in, on or near the flood protection zone. Changes in dike composition and geometry can already be accounted for in the assessment procedure. Foreshores need to be evaluated by the transmission of hydraulic loads. Objects introduced for multifunctional use are the most complicated to incorporate as these may directly affect hydraulic loads and the dike’s resistance as well as introduce additional uncertainty. A scenario-based approach is needed to quantify the effect on flood protection across different potential states of the object during an extreme event.

This research gives prospects for assessing the safety of flood defences shared with other uses using a risk-based approach. This includes common features, additional defences to enable other uses, as well as climate adaptation designs through analysing the ramifications of four design components.  For  a full risk-based analysis the use of advanced hydrological, geotechnical, statistical and morphological models is required. Still, the developed framework can be improved by including the effects of other uses on reducing the vulnerability or exposure of the hinterland. With a growing need for integrated flood protection solutions due to climate change and urbanisation, the shared-use of flood defences is only expected to increase. This research is a starting point for developing new integrated flood protection solutions within a probabilistic risk-based approach.