THE CONNECTION BETWEEN WIND, WAVES, AND POND EXPANSION ON THE MISSISSIPPI RIVER DELTA, LOUISIANA

Over the last century, the Mississippi River delta has lost 25% of its wetlands and may be undergoing total collapse. There are currently two theories for marsh collapse: drowning by sea-level rise and edge erosion. Unlike sea level rise, the edge erosion theory has not been fully evaluated in the Mississippi River Delta. Collapse by edge erosion occurs when wind-generated waves in a marsh basin erode the marsh boundaries, creating a positive feedback loop of larger fetches and more powerful waves. The edge erosion hypothesis has only been tested on idealized elliptical ponds subjected to a steady wind. Here we test the connection between wind, waves, and edge erosion on ponds of varying shape complexity subjected to an unsteady wind field. The objective of this study is to model the forces created by wind-driven waves on marsh pond boundaries found in the Terrebonne Basin in southern Louisiana. To accomplish this, we created a numerical model using the Delft3D software suite that simulates wind-driven waves and currents. The models are based on the pond boundaries from five ponds in the Terrebonne basin, and use offshore wind data from the US Army Corps of Engineers Wave Information Study from the years 1990 to 2014, neglecting the role of tides. In nearly circular ponds, the position of maximum impulse per unit area is dominantly a function of the frequency of occupation by an erosive force rather than the magnitude of those shear stress values. Locations along the margin with high occupancy and a moderate value for median erosive shear stress have a higher impulse per unit area than those locations with the highest median shear stress. The locations of the high impulse per unit area are also along the 30-year pond expansion vector which suggests that pond expansion is driven by regularly occurring meteorological events that produce northerly winds. In ponds with complex geometries the same principles apply.