Influence of Floods, Tides, and Vegetation on Sediment Retention in Wax Lake Delta, Louisiana, USA

Sediment is the most valuable natural resource for deltaic environments because it is required to build new land. For land building to occur, 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, Louisiana, USA to analyze sediment retention for different riverine flood magnitudes, tidal amplitudes, and vegetation extents. Our results show that as riverine flood magnitude increases, areally averaged vertical accretion increases from 0.33 to 2 cm per 60‐day flood, but sediment retention decreases from 72% to 34%. For the uniform vegetation characteristics considered, the buffering effect, defined as the reduction of sediment flux onto the islands in the presence of vegetation, reduces the sediment flux onto the islands 14 to 22% on a fully vegetated delta. When sediment is transported onto the islands, vegetation enhances retention, which we refer to as the trapping effect, by ~10%. But, this does not offset the buffering effect, and vegetation decreases vertical accretion and retention in the delta up to 6% (or ~0.5 cm per 60‐day flood). 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 60‐day flood compared to smaller tidal amplitudes. These results provide insight on the mechanisms behind coastal systems growth, and suggest how sediment diversions might be operated more efficiently in deltas with reduced sediment supply. Key Points Sediment retention analysis on a delta varying riverine flood magnitude, tidal amplitude, and vegetation cover using numerical modeling Vertical accretion increases with flood size as input normalized retention decreases, while tides increase retention during large floods Vegetation as parameterized reduced accretion and retention due to the greater effect of the buffering effect versus the trapping effect