Influence of Wetland Restoration and Degradation on Storm Surge and Waves

Coastal areas are vulnerable to devastating storm surge and waves, a vulnerability that will increase with the ever-increasing population that seeks to reside along the coast and its accompanying infrastructure. It is generally acknowledged that coastal features such as wetlands and barrier islands can reduce surge and waves. Understanding the interaction between hurricanes and coastal landscapes is important in planning hurricane flood protection for South Louisiana. In the past, the level of protection provided by wetlands has been empirically estimated with simple “rules of thumb” that state surge is attenuated at a rate of X feet per Y miles of marsh. However, the assumption of a constant attenuation rate implies a simple balance between gravity/water surface elevation gradient and friction. The actual situation is much more complex and is dependant on many details including storm track, forward speed, size, and surrounding local bathymetry/topography, including levee configurations. Many of the complexities can be simulated with numerical models. Coupled ADCIRC-STWAVE model simulations were made for a number of landscape configurations that involved both restored and degraded wetland features. Effects of landscape features were represented by changes in elevation and frictional resistance. Restoration and degradation of marsh resulted in decreases (for restoration cases) and increases (for degradation cases) in both surge and waves. The magnitude of change was correlated with the magnitude of the horizontal extent and elevation changes in the marsh and varies with changes in storm details and surrounding bathymetry/topography. The application of numerical models to evaluate the potential of wetlands to attenuate surge and waves allows for the consideration on the transient nature of the forcing, the dynamics of the governing force balance, and the variability in local topography, which is required for a proper assessment.