Storm surge attenuation in a large intertidal marsh: impact of marsh geomorphology

Jeroen Stark, Yves Plancke, Stefaan Ides, Patrick Meire, Stijn Temmerman

Wednesday 1 july 2015

8:45 - 9:00h at South America (level 0)

Themes: (T) Flood risk management and adaptation, (ST) Flood risk assessment

Parallel session: 8J. Floodrisk - Assessment

The potential of coastal wetlands to reduce flood risks is becoming more recognized, while studies on the quantification of storm surge attenuation in marshes are scarce. Here, in-situ water level observations in a tidal marsh as well as 2D hydrodynamic modeling are used to study the influence of marsh geomorphology on tidal propagation and storm surge attenuation for varying hydrodynamic boundary conditions in Saeftinghe, a 3000ha tidal marsh along the Western Scheldt estuary (SW Netherlands). Water level measurements, conducted at different locations in and around a 4 km long marsh channel during several spring-to-neap cycles and a severe storm surge, show that damping or amplification of tides within the marsh area depends on the height of the tidal wave compared to the marsh platform elevation. Undermarsh tides, with peak water levels below the platform elevation, are slightly amplified along the converging marsh channels. Overmarsh tides with peak water levels above the marsh platform are mainly attenuated, with maximum attenuation rates of up to 5cm/km along marsh channels for tides that inundate the platform by 0.5-1.0m. A hydrodynamic model is set up with TELEMAC-2D to assess a wider range of peak water levels and include simulations with variations in the marsh geomorphology. The effect of marsh vegetation is herein implemented by increased bed roughness. Model results indicate that tides are only attenuated if the channel depth and width are small and the extent of the marsh platform is large, while tides are slightly amplified if the channel depth and width are large and the influence of the platform is less. The model results confirm the dependency of flood wave damping or amplification on the peak water level relative to the marsh platform elevation for all scenarios. Finally, model simulations show that storm surge attenuation can be limited by the marsh storage area, due to blockage or reflection against levees or other man-made structures that surround the marsh area.