Modeling Sediment Trapping in a Vegetative Filter Accounting for Converging Overland Flow

Vegetative filters (VF) are used to remove sediment and other pollutants from overland flow. When modeling thehydrology of VF, it is often assumed that overland flow is planar, but our research indicates that it can be two-dimensionalwith converging and diverging pathways. Our hypothesis is that flow convergence will negatively influence the sedimenttrapping capability of VF. The objectives were to develop a two-dimensional modeling approach for estimating sedimenttrapping in VF and to investigate the impact of converging overland flow on sediment trapping by VF. In this study, theperformance of a VF that has field-scale flow path lengths with uncontrolled flow direction was quantified using fieldexperiments and hydrologic modeling. Simulations of water flow processes were performed using the physically based,distributed model MIKE SHE. A modeling approach that predicts sediment trapping and accounts for converging anddiverging flow was developed based on the University of Kentucky sediment filtration model. The results revealed that as flowconvergence increases, filter performance decreases, and the impacts are greater at higher flow rates and shorter filterlengths. Convergence that occurs in the contributing field (in-field) upstream of the buffer had a slightly greater impact thanconvergence that occurred in the filter (in-filter). An area-based convergence ratio was defined that relates the actual flowarea in a VF to the theoretical flow area without flow convergence. When the convergence ratio was 0.70, in-filterconvergence caused the sediment trapping efficiency to be reduced from 80% for the planar flow condition to 64% for theconverging flow condition. When an equivalent convergence occurred in-field, the sediment trapping efficiency was reducedto 57%. Thus, not only is convergence important but the location where convergence occurs can also be important.