| Vegetative filters are installed to remove sediment and other pollutants from overland flow. It is often assumed that the overland flow is uniformly distributed, but this research has indicated otherwise. In this study, I quantified the performance of a vegetative filter (Clear Creek Buffer) that had field-scale flow path lengths with uncontrolled flow direction. This included monitoring overland flow and sediment mass flow in two areas of an in-place vegetative filter, modeling two-dimensional overland flow accounting for non-planar topography and spatial variability of vegetation density and infiltration, and modeling sediment trapping considering converging and diverging overland flow in the vegetative filter. Simulations of water flow processes were performed using the physically-based, distributed model, MIKE SHE. A sediment-trapping modeling approach that accounts for converging and diverging flow was developed based on the University of Kentucky sediment filtration model for grassed areas. The non-planar topography had a significant effect on the distribution and depth of overland flow in the vegetative filter. Additionally, accounting for spatially variable roughness impacted the two-dimensional overland flow, but variable soil hydraulic properties had a lesser impact. The vegetative filter trapped approximately 80% of the incoming sediment. The simulation results of various convergence ratio scenarios showed that as convergence increases filter performance decreases, and the impact is greater at greater flow rates and shorter filter lengths. Convergence that occurs in the contributing field (in-field) upstream of the buffer had a slightly greater impact than convergence in the filter itself. When the convergence ratio was 0.70, the sediment trapping efficiency was reduced from 80% to 64% and from 80% to 57% for in-filter and in-field convergence, respectively. Thus, not only is convergence important but the location where convergence occurs can also be important. Therefore, both in-filter and in-field convergence should be considered when evaluating the effectiveness of vegetative filters. |
