WHAT CONTROLS SEDIMENT RETENTION IN A PROGRADING RIVER DELTA?

Building of deltaic land requires sediment deposition nearshore, yet most sediment bypasses the delta. To account for this most models of land building specify an ad hoc sediment retention fraction. This underscores an incomplete understanding of deltaic sedimentation processes. For instance, without a better understanding of what drives sediment retention we cannot make accurate predictions of how much deltaic land is built per unit volume of sediment. Towards this end, we conduct a 2D numerical modeling study of unsteady surface water flow and sediment transport with vegetation on Wax Lake Delta, LA to determine what controls sediment retention. We use a seamless digital elevation model (DEM) of the Wax Lake Delta (WLD) for the initial topography. The deltaic island elevation data for the DEM is derived from LiDAR data, while the channel and delta front bathymetry is derived from single and multi-beam data. The upstream boundary condition is based on discharge data from the USGS gauge located in the Wax Lake Outlet at Calumet, LA and the downstream water level boundary conditions on tidal data from the NOAA gauge located in the Atchafalaya Delta at Amerada Pass, LA. The deltaic islands in our DEM are populated by vegetation, the areal extent of which varies throughout the year. We calibrate the model against 72 measured water depths with a root mean square error of 0.08. Surprisingly, our results suggest the microtidal forces play an important role in sediment retention. For example, when steady state discharge and microtidal conditions were applied, up to ~2x more sediment was deposited on the deltaic islands than when a discharge flood wave was applied, with or without tidal fluctuations. Given that sediment is one of the most valuable resources in the Mississippi River Delta, our study suggests that estimates of sediment retention should account for tides and that land building may be enhanced during certain combinations of discharge and tidal conditions.