Salt marshes: habitat suitability and flood hazard reduction effectiveness
Mendoza Viteri, M.A. (2021). Salt marshes: habitat suitability and flood hazard reduction effectiveness. MSc Thesis. Delft University of Technology: Delft. xiv, 68 pp.
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Documenttype: Doctoraat/Thesis/Eindwerk
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| Abstract |
In recent years, nature-based solutions have gained recognition for their capacity to reduce flood hazard on coastal communities. These coastal threats range from waves and tides, to future storm variability and predicted sea level rise. Vegetated foreshores are one of the measures that fall under this palette of solutions, and are the focus of this thesis. While vegetated foreshores can be colonized by several plant species (mainly mangroves or marshes), this thesis presents the study of phenomena occurring at salt marshes in an attempt to determine which are relevant indicators that can predict the presence or absence of this ecosystem at a given location. Moreover, the wave height attenuation capacity of such vegetated foreshores under different conditions is also assessed for different effectiveness indicators, in a way that provides insight into whether this habitat is not just feasible, but also effective as a coastal protection measure, with respect to wave height attenuation.Results suggest that within a water system with similar sediment composition, inundation-free period and daily-occurring hydrodynamics can explain the presence/absence of vegetation at most sites, while storm conditions can the determining condition for vegetation absence in other locations. For comparison between systems, the dimensionless wave-induced excess bed-shear stress factor may serve as a relevnat proxy. These conclusions indicate the occurrence of sediment re-suspension during approximately 20% of the assessed time, at the most.From the wave attenuation effectiveness assessment, considerations for tidal regime, surge height, and type of wave forcing were used to determine the degree of effectiveness of salt marshes in the studied water systems, while defining effectiveness in terms relative to a mudflat located at MSL. Results suggest that salt marshes located in macro tidal sites, like in the Western Scheldt, can dissipate more wave energy than habitats located in micro or meso tidal areas, such as the Wadden Sea. On the other hand, results treating the different wave loads further indicates that greater attenuation may be achieved in sheltered coastal locations such as the Western Scheldt estuary, indicating that this system may be more effective in sheltered than in exposed coastal areas.Finally, the set of conclusions provided by both sections may serve to determine whether an arbitrary location is capable of hosting salt marsh vegetation, as well as the effectiveness of such nature-based solution with respect to wave height attenuation. The elaboration of an assessment tool was not achieved in this thesis; however, in order to perform a quick check for salt marsh introduction as an effective flood defense measure at a given site, results indicate the gathering of necessary data such as bed level elevations - for inundation-free period analysis - as well as occurring hydrodynamic forcing and sediment properties at such arbitrary site (excess bed shear stress determination, evaluated with the previously proposed limits). With respect to the degree of wave height reduction effectiveness, the determination of the occurring tidal regime, design surge heights, and considerations of sheltered/exposed coastal conditions may serve as a first estimate for this purpose. |
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