Expected outcome
Probabilistic framework to compare and assess the shared-use of flood defences and guidelines for their implementation.

Range of functions that can be encountered on/near a flood defence including housing on a riverine dike (top photo), vegetated foreshores in coastal areas with adapted design for the extraction of deposited clay (bottom-left) and more (unusual) objects that can be found (bottom-right). (Photos accordingly available from molendatabase.nl, Regional Water Authority Hunze en Aa's and Richard Marijnissen).
Motivation and practical challenge
My contribution both as engineer and researcher is to understand the middle ground between man and nature. Flood defences protect us from the water but are also integrated with many other functions such as housing and nature. Depending on the location, some functions are more common than other ones (see photo). This shared-use of the flood defences often comes with conservative estimates for a robust design that guarantees the flood safety. While having multi-functional flood defences generally impose restrictions on the design, some elements in the natural foreshores or flooded areas near the dike can contribute to reducing flood risk. For example, vegetated foreshores in coastal areas can damp the amount of wave energy that reaches the dikes. Aside from natural values, coastal salt marshes can also capture clay over time which may be used in future dike reinforcements. With the new probabilistic risk approach recently adopted in the Netherlands, we may be able to quantify the effects on flood safety of both standard and nature-based reinforcement measures. Yet, an integrated risk assessment framework is required.
Research challenge
The objective of this research project is to gain more insight into the shared-use of flood defences on flood risk reduction. I do so by using the new probabilistic risk approach to determine the safety level of flood defences that are shared with traditional functions such as housing. I further assess the effects on flood safety of some nature-based measures applied in innovative dike reinforcements.

Research components to devise a framework for integrating additional functions within the new risk standards.
Innovative components
My research integrates already existing work in flood protection, risk management and nature-based engineering into a risk assessment framework. To develop this framework, I used existing assessment procedures to estimate the probabilities of dike failure for typical mechanisms (top-left figure). Under presence or absence scenarios of housing and trees, I compared the effects of multiple reinforcement measures such as broadening or heightening the dike for a more robust design (top-centre figure). I further extended the framework to assess the effects of dike reinforcements that include salt marsh development as nature-based adaptation measure for sea-level rise (top-right figure).
I am applying the concepts of this framework into two innovative case studies (bottom figures): the Wide Green Dike, and the Double Dike. How are the dike designs adapted for additional functions such as clay mining and salt marsh development? Moreover, what are the synergies and risks that these functions provide to dike safety? In the final step, insights from developing the extended framework, as well as the application cases are synthesised. Thereby, I can provide some recommendations for assessing the effects of other (nature-based) functions within the new flood risk standards.
Relevant for whom and where?
The results are beneficial for users involved in designing, maintaining, assessing and drafting policies for flood defences within the new risk-based approach. All-Risk is supported by the Regional Water Authorities in the north of the Netherlands who are keen to integrate the results into current and future dike reinforcement projects.
The research components are applied into two innovative projects for dike reinforcement located in the north of the Netherlands.
Progress and practical application
Conservative estimates of failure probabilities for flood defences with multiple functions lead to a systematic underestimation of the reliability of these dikes. Instead, the extended approach incorporates the probability of different scenarios in which elements such as houses and trees affect the flood defence to evaluate its safety. In some situations, the probability of a dike failure may turn out to be 100 times stronger than would conservatively be estimated. Not only for traditional functions but also nature-based functions, the framework can be more adaptable. Natural foreshores can expand over time and partially mitigate the effects of future sea-level rise on the flood defence. Moreover, the deposited sediments may provide a source of material for future reinforcements. Recognising the potential of multiple functions is vital to strengthen the Dutch flood defences for the future efficiently. For a detailed description of the findings, click on the related outputs below.
Status for day-to-day practice
Numerous combinations of functions can be integrated into a flood defence system with shared-uses. The goal of this project is to provide an extended framework for recognising the risks and synergies of different uses with flood protection.
Next steps
In the next steps, we will continue to research promising multi-functional dike concepts beyond housing and nature conservation.
Last modified: 19/10/2020
Contributing researchers

Richard Marijnissen
Wageningen University
A wide green perspective on dikes
Modelling the salt marsh and borrow pit helping a hand against the sea-level rise

Richard Marijnissen
Wageningen University
Project outputs
A sustainable adaptation scheme for a wide green dike

Feasibility of extracting salt marsh sediment for dike reinforcement as a climate adaptation strategy in several sea-level rise scenarios.
02/06/2020 by Richard Marijnissen et al.
Bevat: Publication open access journal
The sensitivity of a dike-marsh system to sea-level rise

Model-based exploration of the future need for dike heightening due to: (1) sea-level rise, (2) changes in sediment concentration, (3) a retreat, and (4) compaction of the salt marsh.
10/01/2020 by Richard Marijnissen et al.
Bevat: Publication open access journal
Re-evaluating safety risks of multifunctional dikes with a probabilistic risk framework

An additional step to the basic assessment allows estimating the probability of the presence and absence scenario of nature and housing for the reinforcement of a typical Dutch river dike for flood safety.
10/04/2019
Bevat: Publication open access journal
Blogs
Bringing All-risk to practice: Martin’s recommendations
08/10/2020 by Wim Kanning
This blog is based on an informal interview between Martin Schepers (manager flood safety projects) and Wim Kanning (All-Risk researcher) on the development of...