The Influence of Vegetation on the Retention of Sediment Within a Marsh Platform: Comparing Supply Restriction and Trapping Efficiency

Abstract:

Sediment supply and retention play an important role in delta land building and marsh restoration, and both are influenced by the interaction between flow, vegetation and sediment transport. This study examined two competing effects of vegetation on sediment accumulation on a marsh platform: supply restriction and enhanced retention efficiency. When vegetation grows on a marsh platform, the velocity in the marsh decreases as vegetated density increases. The reduction in velocity creates conditions more conducive to deposition by reducing or eliminating resuspension. This process enhances the fraction of sediment entering the marsh that is retained, which is called retention efficiency. However, as the velocity entering the marsh decreases, the supply of sediment to the marsh also decreases. This is called supply restriction. The net accumulation of sediment on the marsh platform depends on the interaction of both processes. To explore this interaction, a random displacement model (RDM) was employed to simulate sediment transport over a marsh platform with and without vegetation based on stochastic methodologies and physical mechanisms. The deposition and resuspension probability were constructed based on previous studies. The RDM method was validated against experimental data of suspended sediment concentration. The RDM was used to investigate how the vegetation characteristics (e.g. density, height, and stem diameter) affect sediment restriction and trapping efficiency and the net impact on sediment accumulation on the marsh platform. Cases without vegetation were also run for comparison. This work may provide insight into productive ways to manage vegetation to enhance land-building and marsh restoration.