Flood risk reduction by parallel flood defences – Case-study of a coastal multifunctional flood protection zone

Richard J.C Marijnissen,Matthijs Kok,Carolien Kroeze,Jantsje M Van Loon-Steensma

doi:10.1016/j.coastaleng.2021.103903

Richard J.C Marijnissen, Matthijs Kok + Show 2 more

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https://doi.org/10.1016/j.coastaleng.2021.103903

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Abstract

In this paper the safety of a double-dike system (or twin dikes) is assessed. Such a system consist of two parallel lines of flood defences. During storms the combined strength of the parallel flood defences must prevent flooding of the hinterland. A culvert can be implemented for the tidal exchange of sea water to enable new land-uses in the area between the dikes such as aquaculture, saline agriculture, salt marsh restoration and clay extraction. We develop a general framework for assessing the safety of such double dike systems and apply a simplified version to the Double Dike between Eemshaven and Delfzijl (The Netherlands) to test this method. In doing so, we aim to quantify the flood protection benefits of parallel flood defenses and enable their use in multifunctional flood protection strategies.Within the framework the transmission of hydraulic loads by the seaward dike to the landward dike in the case-study was described by overtopping, overflow and erosion of the outer slope, alongside discharge through the culvert in the event of a non-closure. For the subset of coastal double dike systems with a tall seaward dike (as in the case-study), the results show only a negligible improvement in flood protection compared to a single dike system. With the addition of a culvert in the first dike, flood risk will only be reduced by the second landward dike if its height is sufficient to retain water in the event of a non-closure during common storm events. These double dike systems are implemented for potential uses of the inter-dike zone, e.g. for nature restoration, rather than as a measure to primarily improve flood protection.

Highlights

For the subset of coastal double dike systems with a tall seaward dike, the results show only a negligible improvement in flood protection compared to a single dike system
This paper proposes a general method for assessing the safety of double dike systems and applies a simplified version to the Double Dike between Eemshaven and Delfzijl (The Netherlands) to demonstrate its applicability
The water level exceedance frequency in the interdike area due to overtopping and erosion of the first dike was obtained by the first order reliability method (FORM)-importance sampling (IS) procedure

Summary

Introduction

One method to reduce the exposure to floods is the continued heightening and strengthening of existing levees, seawalls and other structures. Such interventions may not address, or could even aggravate, other pressing problems (Elliott et al, 2019; Jeuken et al, 2014), like loss of marsh and fish habitat (Schuerch et al, 2018; Munsch et al, 2017), coastal erosion (Williams et al, 2018), limited space for urban expansion (Barbier, 2015), and a deteriorating water quality (Kiedrzynska et al, 2015). A system of multiple parallel dikes ( called double dikes or twin dikes) is one alternative being explored in this context

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