Coastal marsh die-off and reduced attenuation of coastal floods: A model analysis

Global climate change is expected to increase the risks of coastal flood disasters due to accelerating sea level rise and increasing intensity and frequency of storm surges. Coastal marsh vegetation is considered, on the one hand, to increase resistance to a landward propagating flood wave, such as a storm surge, and hence to protect against flood disasters. On the other hand, coastal marsh vegetation is dying off at several places around the world due to accelerating sea level rise. Here we present hydrodynamic model simulations of flood attenuation by a tidal marsh, with particular focus on the effects of spatial patterns of vegetation die-off. It is shown that a same percentage of marsh die-off but occurring as different spatial patterns of marsh break-up has largely different effects on flood attenuation. Patches of die-off that are directly connected to tidal channels have a much greater effect on increased landward flood propagation, while a same percentage of marsh die-off, but occurring at inner marsh locations disconnected from tidal channels, has only a minor effect. This implies that a random pattern of up to 50% of marsh die-off still provides a considerable flood attenuating effect. However with increasing percentage of random marsh die-off the flood attenuating effect decreases exponentially, since the chance for vegetation die-off occurring directly adjacent to tidal channels increases. This study demonstrates that tidal marsh die-off, which may increase with ongoing global change, is expected to have non-linear effects on reduced coastal protection against flood waves.