Elizabeth Olliver, Doug Edmonds, John B Shaw

Sediment is the most valuable natural resource for deltaic environments, and to build new land sediment must be retained in the delta instead of being transported offshore. Despite this, we do not know what controls sediment retention within a delta. Here we use a calibrated numerical model of Wax Lake Delta, LA, USA to analyze sediment retention for different flood-wave magnitudes, tidal amplitudes, and vegetation extents. We only model transport of silt since it comprises most of the incoming sediment load. Our results show that as flood size increases, areally-averaged vertical accretion increases from 0.33 cm to 2 cm, but this comes at a cost because delta-scale sediment retention decreases from 72% to 34%. On a fully vegetated delta, we show that the buffering effect of vegetation reduces island-directed sediment flux by 14 to 22% because sediment takes the less resistive path in the channel. When sediment gets onto the islands, the trapping effect of vegetation increases retention by ~10%. But, this is not enough to offset the buffering effect, and vegetation decreases vertical accretion and sediment retention across the delta reduces by up to ~0.5 cm and 6%, respectively. We suggest that vegetation will increase sedimentation only when trapping compensates for buffering. Finally, greater tidal amplitude at higher discharges enhances vertical accretion by ~0.5 cm per flood as compared to a minimum tidal amplitude condition. These results inform how coastal deltaic systems grow and suggest how to operate sediment diversions more efficiently in deltas with reduced sediment supply.