Stability of a Tidal Marsh Under Very High Flow Velocities and Implications for Nature-Based Flood Defense
Authors: Schoutens Ken , Stoorvogel Marte , van den Berg Mario , van den Hoven Kim , Bouma Tjeerd J. , Aarninkhof Stefan G.J. , Herman Peter M.J. , van Loon-Steensma Jantsje M. , Meire Patrick , Schoelynck Jonas , Peeters Patrik , Temmerman Stijn 
Published: February 2023

Nature-based&#160; strategies,&#160; such&#160; as&#160; wave&#160; attenuation&#160; by&#160; tidal&#160; marshes,&#160; are&#160; increasingly &#160;proposed&#160; as&#160; a&#160; complement&#160; to&#160; mitigate&#160; the&#160; risks&#160; of&#160; failure&#160; of&#160; engineered&#160; flood&#160; defense &#160;structures such as levees. However, recent analysis of historic coastal storms revealed smaller&#160; dike&#160; breach&#160; dimensions&#160; if&#160; there&#160; were&#160; natural,&#160; high&#160; tidal&#160; marshes&#160; in&#160; front&#160; of&#160; the &#160;dikes.&#160; Since&#160; tidal&#160; marshes&#160; naturally&#160; only&#160; experience&#160; weak&#160; flow&#160; velocities&#160; (~0-0.3&#160; ms-1 during&#160; normal&#160; spring&#160; tides),&#160; we&#160; lack&#160; direct&#160; observations&#160; on&#160; the&#160; stability&#160; of&#160; tidal&#160; marsh &#160;sediments&#160; and&#160; vegetation&#160; under&#160; extreme&#160; flow&#160; velocities&#160; (order&#160; of&#160; several&#160; ms-1)&#160; as&#160; may &#160;occur&#160; when&#160; a&#160; dike &#160;behind&#160; a&#160; marsh&#160; breaches.&#160; As&#160; a&#160; first&#160; approximation,&#160; the&#160; stability&#160; of &#160;a tidal marsh sediment bed and winter-state vegetation under high flow velocities were tested in a flume. Marsh monoliths were excavated from Phragmites australis marshes in front of a dike along the Scheldt estuary (Dutch-Belgian border area) and installed in a 10 m long flume test section. Both sediment bed and vegetation responses were quantified over 6 experimental runs under high flow velocities up to 1.75 ms-1 and water depth up to 0.35 m for 2 hours. These tests showed that even after a cumulative 12 hours exposure to high flow velocities, erosion was limited to as little as a few millimeters. Manual removal of the aboveground vegetation did not enhance the erosion either. Present findings may be related to the strongly consolidated, clay- and silt-rich sediment and P. australis root system in this experiment. During the flow exposure, the P. australis stems were strongly bent by the water flow, but the majority of all shoots recovered rapidly when the flow had stopped.&#160; Although&#160; present&#160; results&#160; may&#160; not&#160; be&#160; blindly&#160; extrapolated&#160; to&#160; all&#160; other&#160; marsh &#160;types, they do provide a strong first indication that marshes can remain stable under high flow conditions, and confirm the potential of well-developed tidal marshes as a valuable extra&#160; natural&#160; barrier&#160; reducing&#160; flood&#160; discharges&#160; towards&#160; the&#160; hinterland,&#160; following&#160; a&#160; dike &#160;breach. These outcomes promote the consideration to implement tidal marshes as part of the overall flood defense and to rethink dike strengthening in the future.