LIMITS OF WETLAND WAVE DISSIPATION

Abstract

Wetlands provide a natural protective buffer against coastal storms by dissipating wave energy, but there is limited quantitative information to evaluate the effectiveness of such natural features over a range of hydrodynamic conditions. Laboratory experiments were conducted in a large-scale flume with artificial vegetation. The measurements show good agreement to the wave dissipation formulation of Mendez and Losada (2004). Drag coefficients correlated well with Reynolds number for the submerged vegetation, but emergent vegetation gave higher coefficients than the submerged vegetation. Applying the drag coefficients derived from the lab, the efficiency of vegetation dissipation was explored analytically. For dense vegetation (400 stems/m2), it was found that significant reductions in wave height (50%) are possible over tens and hundreds of meters for relative depths (depth times wavenumber) less than 1.5. The distance required to dissipate wave energy increases exponentially as water depth increases. A hypothetical field example using the spectral wave model STWAVE with the Mendez and Losada (2004) expression for wave dissipation is also presented. The field example shows reduction in wave height of 70-80 percent along the shoreline for extensive vegetation coverage during severe wind and surge conditions. These hypothetical simulations demonstrate the potential for vegetation to reduce the storm intensity at the coast by reducing wave height under severe wind and water level conditions. However, the simulations cannot show whether the vegetation’s capacity to reduce storm damages can be sustained during storms or over longer periods of time as wetlands evolve.