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A2) Shared-use of flood defences

Start: 09/2017
End: 09/2021
Status: Active

Contact details

Richard Marijnissen

Wageningen University

Download here the PhD thesis related to this project.

Outcome

By analysing three case studies, we developed an updated framework for the probabilistic assessment of dikes considering traditional uses such as housing but also nature-based measures for the innovative reinforcement of dikes. On the one hand, we assessed how probabilistic effects of other uses could be expressed directly by scenarios and (hydraulic) transmission models. On the other hand, we explore how the dike geometry and composition can increase synergies with flood safety while considering additional uses. Finally, the vegetated foreshore of coastal dikes was found to present substantial long-term benefits for flood protection and thus should be considered in flood protection strategies.

Figure 1. 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 an 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 flood defences often comes with conservative estimates for a robust design that guarantees 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 dampen 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.

Figure 2. Research components to devise a framework for integrating additional functions within the new risk standards. Based on schemes prepared by Richard Marijnissen.

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 (Figure 2, top-left). 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 (figure 2, top-centre). I further extended the framework to assess the effects of dike reinforcements that include salt marsh development as a nature-based adaptation measure for sea-level rise (Figure 2, top-right).

I am applying the concepts of this framework into two innovative case studies (Figure 2, bottom): 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, check the related outputs.

Recommendations for practice

  • Incorporate scenarios or transmission of shared use whenever feasible.
  • Shared-use affects flood risk proportional to the safety level of the flood defence.
  • Consider sediment management along muddy coasts within the vegetated foreshore of coastal dikes.
  • Consider both short-term and future flood protection benefits in concepts like the Double Dike.

Last modified: 10/12/2021

Contributing researchers

Richard Marijnissen

Wageningen University

Supervisory team

dr. ir. Jantsje M. van Loon Steensma

Wageningen University

prof.dr. Carolien Kroeze

Wageningen University

prof.dr.ir. Matthijs Kok

Delft University of Technology

Contributing partners

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

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Project outputs

Flood risk reduction by parallel flood defences

Double dikes should be assessed using the transmitted loads of parallel defences rather than the failure of individual defences. The case study of this parallel defence consists of a low dike landward of a tall dike at the coast that, although designed with multi-functional purpose via a culvert, has a negligible improvement in flood protection.

01/08/2021

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Bevat: Publication open access journal

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.

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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.

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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

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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...

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Events

17/05/2021

Reflection: Double Dikes, twice the protection with twice the responsibility?

During the All-Risk webinar, the questions were discussed how a double (twin) dike can contribute to flood risk protection, and what the division of water management responsibilities between goverments is regarding this concept. In the reflection below you will find what has emerged from the discussion between science and practice.

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News

PhD Defence - Shared-use of Flood Defences

06/10/2021 by Richard Marijnissen

Past october 6, 2021 at 16:00, Richard Marijnissen defended his PhD research at Wageninge University in a ceremony that included an (online) mini-symposium with...

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Videos

The Wide green dike a pilot reinforcement project on the Wadden Sea area

21/04/2020

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