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  as  a  complement  to  mitigate  the  risks  of  failure  of  engineered  flood  defense  structures such as levees. However, recent analysis of historic coastal storms revealed smaller  dike  breach  dimensions  if  there  were  natural,  high  tidal  marshes  in  front  of  the  dikes.  Since  tidal  marshes  naturally  only  experience  weak  flow  velocities  (~0-0.3  ms^-1 during  normal  spring  tides),  we  lack  direct  observations  on  the  stability  of  tidal  marsh  sediments  and  vegetation  under  extreme  flow  velocities  (order  of  several  ms^-1)  as  may  occur  when  a  dike  behind  a  marsh  breaches.  As  a  first  approximation,  the  stability  of  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.  Although  present  results  may  not  be  blindly  extrapolated  to  all  other  marsh  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  natural  barrier  reducing  flood  discharges  towards  the  hinterland,  following  a  dike  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.