Graduation of Marco Mendoza Viteri

27 January 2021 14:30 till 16:00 - Location: ZOOM - By: Webredactie | Add to my calendar

Salt marshes: habitat suitability and flood hazard reduction effectiveness

  • Professor of graduation: Prof. dr. ir. S.G.J. Aarninkhof

  • Supervisors of graduation: Dr. ir. V. Vuik (TU Delft), dr. ir. A.P. Luijendijk (TU Delft/Deltares), dr. ir. B.C. van Prooijen (TU Delft)

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 the provides insight into whether this habitat is not just feasible, but also effective as a coastal protection measure, with respect to wave height attenuation.

The study is divided into two main sections, characterized by different methodologies: one treating the habitat suitability and one addressing the effectiveness with respect to wave height attenuation rates. The habitat suitability chapter evaluated daily conditions occurring during the months of Aprl-July (2020) at 28 different sites located within two water systems in The Netherlands: the Western Scheldt and the Wadden Sea. Locations were characterized by mature salt marshes, emerging salt marshes (patches of vegetation), bare intertidal fringing flats, and intertidal shoals. The assessment was performed through numerical modelling of occurring water levels (inundation-free periods), flow velocities, waves, and excess wave-induced bed-shear stresses. Additionally, boundary conditions were affected in the model used to determine flow velocities in the Western Scheldt in order to simulate storm conditions.

Results from the habitat suitability analysis suggest that within a water system with similar sediment composition, inundation-free period and daily-occurring hydrodynamics, such as flow velocities and wave height, can explain the presence/absence of vegetation, whereas storm conditions are the determining condition for salt marsh absence. However, a generalization of these findings was not possible between the two water systems, which was likely due to the several differences found between them. This occurrence led to the introduction of the dimensionless wave-induced excess bed-shear stress factor, where positive values indicate sediment re-suspension (erosion). Final results suggest that at sites across both water systems where this dimensionless factor exceeded the 2.5, 4.2 and 5.3 values at 10%, 1%, and 0.1% of the assessed time, respectively, were indeed non-vegetated sites. Furthermore, these conclusions indicate the occurrence of sediment re-suspension during approximately 20% of the assessed time in the most energetic vegetated sites (Western Scheldt), and as little as 1% in the least energetic locations (Wadden Sea). This analysis shows that a generalization of habitat suitability indicators has to include both external loading and bed resistance, providing initial values for excess bed-shear stress occurrence.

The wave attenuation effectiveness assessment was likewise performed by numerical modelling of waves propagating over a vegetated field, under different imposed boundary conditions. This assessment, assisted by the spectral wave model SWAN, covered indicators such as relative vegetation height (hv/h), wave height to water depth ratio (H/h) and tidal regime. Imposed boundary conditions consisted of deep (kh > 3.0) to shallow water (kh > 0.3) conditions, as well as characteristic values described as 'Exposed coast, local wind waves' (Hs = 1.0 m; Tp = 4.0 s); 'Exposed coast, swell waves' (Hs = 1.0 m; Tp = 12.0 s); and 'Sheltered coast, local wind waves' (Hs = 0.5 m; Tp = 4.0 s). Moreover, different surge levels and tidal regimes with characteristic amplitudes were included so as to account for different water depths over the foreshore (micro = 0.5 m; meso = 1.5 m; and macro = 2.5 m).

From this 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. Output of the simulations suggests that salt marshes that develop 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. This is due to the fact that foreshores accrete at higher elevations with respect to MSL, due to sediment deposition on higher levels occurring during MHWS. On the other hand, results treating the different wave loads (swell waves, sheltered locally wind-generated waves, and exposed wind-generated waves) further indicates that greater wave height attenuation may be achieved in sheltered coastal locations such as the Western Scheldt estuary; which is is due to the fact that longer swell waves are dissipated by other mechanisms (such as wave breaking and bottom friction), while shorter waves can be largely be dissipated by the vegetated foreshore itself. This observation suggests that introduction of this system as a flood protection measure may be more effective in sheltered than in exposed coastal areas.

Finally, the set of conclusions provided by both sections proposed generally applicable indicators that may serve to determine whether an arbitrary location is capable of hosting salt marsh vegetation as a flood defense measure, 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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