Morphological responses of the Wax Lake Delta, Louisiana, to Hurricanes Rita

Abstract

This study examines the morphodynamic response of a deltaic system to extreme weather events. The Wax Lake Delta (WLD) in Louisiana, USA, is used to illustrate the impact of extreme events (hurricanes) on a river-dominated deltaic system. Simulations using the open source Delft3D model reveal that Hurricane Rita, which made landfall 120 km to the west of WLD as a Category 3 storm in 2005, caused erosion on the right side and deposition on the left side of the hurricane eye track on the continental shelf line (water depth 10 m to 50 m). Erosion over a wide area occurred both on the continental shelf line and in coastal areas when the hurricane moved onshore, while deposition occurred along the Gulf coastline (water depth < 5 m) when storm surge water moved back offshore. The numerical model estimated that Hurricane Rita’s storm surge reached 2.5 m, with maximum currents of 2.0 m s–1, and wave heights of 1.4 m on the WLD. The northwestern-directed flow and waves induced shear stresses, caused erosion on the eastern banks of the deltaic islands and deposition in channels located west of these islands. In total, Hurricane Rita eroded more than 500,000 m3 of sediments on the WLD area. Including waves in the analysis resulted in doubling the amount of erosion in the study area, comparing to the wave-excluding scenario. The exclusion of fluvial input caused minor changes in deltaic morphology during the event. Vegetation cover was represented as rigid rods in the model which add extra source terms for drag and turbulence to influence the momentum and turbulence equations. Vegetation slowed down the floodwater propagation and decreased flow velocity on the islands, leading to a 47% reduction in the total amount of erosion. Morphodynamic impact of the hurricane track relative to the delta was explored. Simulations indicate that the original track of Hurricane Rita (landfall 120 km west of the WLD) produced twice as much erosion and deposition at the delta compared to a hurricane of a similar intensity that made landfall directly on the delta. This demonstrates that the wetlands located on the right side of a hurricane track experience more significant morphological changes than areas located directly on the hurricane track.