Salt marshes for flood risk reduction: Quantifying long-term effectiveness and life-cycle costs

Abstract

Flood risks are increasing worldwide due to climate change and ongoing economic and demographic development in coastal areas. Salt marshes can function as vegetated foreshores that reduce wave loads on coastal structures such as dikes and dams, thereby mitigating current and future flood risk. This paper aims to quantify long-term (100 years) flood risk reduction by salt marshes. Dike-foreshore configurations are assessed by coupled calculations of wave energy dissipation over the foreshore, sediment accretion under sea level rise, the probability of dike failure, and life-cycle costs. Rising sea levels lead to higher storm waves, and increasing probabilities of dike failure by wave overtopping. This study shows that marsh elevation change due to sediment accretion mitigates the increase in wave height, thereby elongating the lifetime of a dike-foreshore system. Further, different human interventions on foreshores are assessed in this paper: realization of a vegetated foreshore via nourishment, addition of a detached earthen breakwater, addition of an unnaturally high zone, or foreshore build-up by application of brushwood dams that enhance sediment accretion. The performance of these strategies is compared to dike heightening for the physical boundary conditions at an exposed dike along the Dutch Wadden Sea. Cost-effectiveness depends on three main factors. First, wave energy dissipation, which is lower for salt marshes with a natural elevation in the intertidal zone, when compared to foreshores with a high zone or detached breakwater. Second, required costs for construction and maintenance. Continuous maintenance costs and delayed effects on flood risk make sheltering structures less attractive from a flood risk perspective. Third, economic value of the protected area, where foreshores are particularly cost-effective for low economic value. Concluding, life-cycle cost analysis demonstrates that, within certain limits, foreshore construction can be more cost-effective than dike heightening.

Towards Resilient Vital Infrastructure Systems: Challenges, Opportunities, and Future Research Agenda

“…As another method of increasing infrastructure resilience, risk assessment has been commonly used in the studies conducted by Ruijters and Stoelinga (2016); Hall et al (2016); Do and Jung (2018); Mao et al (2018); Wang et al (2019); and Tsavdaroglou et al (2018). The selected studies also highlight that within the water sector, combining green and grey infrastructures (nature-based solutions) is the most frequently used approach to increase system's resilience (e.g., Hulscher et al, 2014;Augustijn et al, 2014;Demuzere et al, 2014;Borsje et al, 2017;Augustijn et al, 2018;Beery, 2018;Vuik et al, 2019).…”

Enhancing the Ecological Value of Sea Dikes

“…Mangroves, salt marshes and sea grass show wave damping capacities that depend on vegetation properties, e.g., the vegetation height and width, and hydraulic conditions, e.g., the water depth [34][35][36][37]. Additionally, increased sediment accretion was observed for these foreshore vegetations [38][39][40].…”

“…Due to spatial and temporal variation in ecosystems and not yet resolved knowledge gaps, integration of coastal protection services of ecosystems in design processes still poses a challenge [45,46]. However, combining hard coastal structures with foreshore ecosystems can decrease the hydrodynamic loads on the built infrastructure, thus increasing the design life and reducing necessary maintenance efforts [8,37,47].…”

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

“…In many deltas, "grey" solutions such as dikes, levees, and storm surge barriers have been implemented [1][2][3]. In general, these measures are designed to function over long periods, yet are relatively inflexible to unforeseen accelerated sea-level rises [2,[4][5][6]. Moreover, many grey solutions can be detrimental to ecosystems by confining the intertidal area (coastal squeeze) or affecting the natural hydro-morphological processes [2,7,8].…”

“…However, mitigating flood risk with nature-based solutions alone may not always be feasible. As a result, hybrid flood defences, incorporating both traditional flood defences structures and natural elements, are an attractive strategy to protect deltas [4][5][6].…”