The Sensitivity of a Dike-Marsh System to Sea-Level Rise—A Model-Based Exploration

Abstract

Integrating natural components in flood defence infrastructure can add resilience to sea-level rise. Natural foreshores can keep pace with sea-level rise by accumulating sediment and attenuate waves before reaching the adjacent flood defences. In this study we address how natural foreshores affect the future need for dike heightening. A simplified model of vertical marsh accretion was combined with a wave model and a probabilistic evaluation of dike failure by overtopping. The sensitivity of a marsh-dike system was evaluated in relation to a combination of processes: (1) sea-level rise, (2) changes in sediment concentration, (3) a retreat of the marsh edge, and (4) compaction of the marsh. Results indicate that foreshore processes considerably affect the need for dike heightening in the future. At a low sea-level rise rate, the marshes can accrete such that dike heightening is partially mitigated. But with sea-level rise accelerating, a threshold is reached where dike heightening needs to compensate for the loss of marshes, and for increasing water levels. The level of the threshold depends mostly on the delivery of sediment and degree of compaction on the marsh; with sufficient width of the marsh, lateral erosion only has a minor effect. The study shows how processes and practices that hamper or enhance marsh development today exacerbate or alleviate the challenge of flood protection posed by accelerated sea-level rise.

The four components to combine flood protection with other functions

“…the higher roughness of a marsh substrate) can work in conjunction to decrease loads (e.g. through wave breaking and friction) (Marijnissen et al, 2020b). The case of the Double Dike requires a shift in perspective, as the first dikes can also be perceived as a foreshore to the final landward dike (Marijnissen et al, Submitted).…”

Resilient Flood Defenses

“…Next, based on model results it turned out that traditional structures, which were widely implemented in the past, proved to be most effective to stimulate marsh growth. Additionally, the recognition of the large temporal scale on which sea level rises acts was key to understand the resilience of flood defense systems as studied by Marijnissen et al [5]. At a low sea-level rise rate, the marshes can accrete such that dike heightening is partially mitigated.…”

“…Most multifunctional dikes are hybrid structures that combine hard elements such as breakwaters [6], rock revetments [7] or rip-rap [1] and soft material such as sand [1,2], clay on the dike [5,8], or sandy and clayey soils as salt marshes in front of the dike [3,4]. Hard dike covers are often applied in cases of extreme wave loads, but they are also used as a practical solution because the fundamental understanding of the strength of dike covers based on vegetated soil is missing.…”

Shared-use of flood defences

“…The thesis shows that the safety of flood defences shared with other uses can be assessed using a risk-based approach. This includes common features (Chapter 2), additional defences to enable other uses (Chapter 3), as well as climate adaptation designs (Chapters 4,5) through analysing the ramifications of four design components (Chapter 6). For a full riskbased analysis the use of advanced hydrological, geotechnical, statistical and morphological models is required.…”

“…• Hoe kan een probabilistisch beoordelingskader rekening houden met klimaatadaptatievoordelen van multifunctioneel gebruik van de waterveiligheidszone op de lange termijn? (Hoofdstuk 4,5) Hoeveel een conservatieve benadering het veiligheidsoordeel van een multifunctionele kering kan beïnvloeden is onderzocht in hoofdstuk 2. Een scenario-aanpak wordt geïntroduceerd binnen het beoordelingskader om zowel voor-als nadelen voor waterveiligheid te kwantificeren.…”