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Grant, Alastair 
ORCID: https://orcid.org/0000-0002-1147-2375 and Cooker, Mark J.
(2023)
Reductions in water level over coastal wetlands during storm surges and tsunamis: an analytical result and a critical review of the literature.
Coastal Engineering, 183.
ISSN 0378-3839
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Abstract
Understanding how saltmarshes and mangroves reduce movement inland of high water levels resulting from storm surges and tsunamis is vital for disaster planning and predicting impacts of sea level rise. This has been examined using both site-specific hydrodynamic models and field measurement, but an overarching theoretical framework is lacking. To address this, we provide a simple analytical solution for the propagation of a surge across a uniform wetland surface. The distance that flooding penetrates inland is reduced by the presence of vegetation and increases with both surge height and duration. After 4 hours, 1 m and 5 m floods will travel 5.5 and 19.1 km inland over saltmarshes respectively. Penetration distances are approximately 2.5 times smaller for mangroves and are lower for short duration events, emphasising the effectiveness of mangroves in reducing impacts of tsunamis. The (absolute) reduction in water level per km depends strongly on surge duration and vegetation type, but only weakly on surge height. In a surge rising by 2m over one hour, the reduction in water level over a saltmarsh can be as high as 45 cm/km, but reduces as surge duration increases or where the width of the low-lying land is small enough for the surge to fill “the bathtub”. So water level reductions will be greatest during short duration events or when large widths of wetland are maintained. Several studies report much higher rates of attenuation, but these involve up to 50-fold extrapolation from measurements made over short distances or misinterpretations of primary sources. Once these are excluded, field data are consistent with our model results.
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