Stability of a Tidal Marsh Under Very High Flow Velocities and Implications for Nature-Based Flood Defense

Nature-based strategies, such as wave attenuation by tidal marshes, are increasingly proposed a complement to mitigate the risks of failure engineered flood defense structures levees. However, recent analysis historic coastal storms revealed smaller dike breach dimensions if there were natural, high marshes in front dikes. Since naturally only experience weak flow velocities (~0-0.3 ms -1 during normal spring tides), we lack direct observations on stability marsh sediments and vegetation under extreme (order several ) may occur when behind breaches. As first approximation, sediment bed winter-state tested flume. Marsh monoliths excavated from Phragmites australis along Scheldt estuary (Dutch-Belgian border area) installed 10 m long flume test section. Both responses quantified over 6 experimental runs up 1.75 water depth 0.35 for 2 hours. These tests showed that even after cumulative 12 hours exposure velocities, erosion was limited little few millimeters. Manual removal aboveground did not enhance either. Present findings be related strongly consolidated, clay- silt-rich P. root system this experiment. During exposure, stems bent flow, but majority all shoots recovered rapidly had stopped. Although present results blindly extrapolated other types, they do provide strong indication can remain stable conditions, confirm potential well-developed valuable extra natural barrier reducing discharges towards hinterland, following breach. outcomes promote consideration implement part overall rethink strengthening future.