Flow Pathways and Sediment Trapping in a Field-Scale Vegetative Filter

Vegetative filters (VF) are a best management practice installed in many areas to control sediment movement towater bodies. It is commonly assumed that runoff proceeds perpendicularly across a VF as sheet flow. However, there is littleresearch information on natural pathways of water movement and performance of field-scale VF. The objectives of this studywere: (1) to quantify the performance of a VF where the flow path is not controlled by artificial borders and flow path lengthsare field-scale, and (2) to develop methods to detect and quantify overland flow convergence and divergence in a VF. Ourhypothesis is that flow converges and diverges in field-scale VF and that flow pathways that define flow convergence anddivergence areas can be predicted using high-resolution topography (i.e., maps). Overland flow and sediment mass flow weremonitored in two 13 15 m subareas of a 13 225 m grass buffer located in Polk County in east-central Nebraska.Monitoring included a high-resolution survey to 3 cm resolution, dye tracer studies to identify flow pathways, andmeasurement of maximum flow depths at 51 points in each subarea. Despite relatively planar topography (a result of gradingfor surface irrigation), there were converging and diverging areas of overland flow in the buffer subareas. Convergence ratiosranged from -1.55 to 0.34. Predicted flow pathways using the high-resolution topography (i.e., map) closely followed actualflow paths. Overland flow was not uniformly distributed, and flow depths were not uniform across the subareas. Despiteconverging and diverging flow, the field-scale VF trapped approximately 80% of the incoming sediment.