Numerical Simulations of Wind Effects on Wave Nonlinearity and Hurricane-Induced Sediment Transport on Louisiana Coast

Abstract

The objective of this study is to model wind effects on wave nonlinearity and the sediment suspension, transport and redistribution caused by hurricanes. The following questions are addressed through numerical simulations: (1) How do winds affect wave triad interactions and wave shape in the shallow water? (2) What is the role of hurricanes in coastal landscape evolution? Do they create more erosion or deposition? (3) Where does the observed post-hurricane deposition on coastal wetlands come from? First, wind effects were incorporated into a Boussinesq-type wave model, and evolution equations were derived for triad interactions with winds. Second, a coupled modeling system for hurricane waves, storm surge, and sediment transport was developed for the Louisiana coast. Third, the modeling system was extended to three dimensions (3D), and the impact of barrier islands on hurricane-induced sediment redistribution was evaluated using the 3D model. The Boussinesq model and the evolution equations together illustrated why following (opposing) winds can enhance (suppress) triad interactions and how the wave shape varies due to the nonlinear wave-wave interactions. The process-based modeling system for coastal Louisiana demonstrated that a major hurricane event has the ability to deliver a considerable amount of sediment to the coastal wetlands, and estimated that Hurricane Gustav (2008) delivered 25.6 million metric tons of sediment to the wetlands in the Terrebonne and Barataria Basins, and most of the observed sediment accretion (97.3% for Terrebonne and 99.8% for Barataria) came from the estuaries. The net deposition on wetlands was 21% smaller in the 3D model than the results from the 2D model using the same sediment properties, while the finding that the hurricane-induced deposition came from erosion in the coastal bays held true regardless of the dimensionality of the model. The deterioration of barrier islands affected the maximum surge level, wave heights and sediment transport in the protected estuaries, but the net effect on sediment fluxes from the continental shelf to the bays and from the bays to wetlands varied by location. Numerical experiments suggested that the contribution from marine sediment to wetland deposition would still be very small even when the barrier islands were severely degraded.