A design procedure for vegetative filter strips using VFSMOD-W

Although vegetative filter strips (VFS) are a common BMP used for runoff sediment control, there is currently no widely accepted objective design criteria available to select optimal construction characteristics (filter length, width, slope, vegetation) needed to achieve a desired sediment reduction. A design procedure for VFS using VFSMOD-W is presented. VFSMOD, the main component of VFSMOD-W, is a field-scale, mechanistic, storm-based model developed to route the incoming hydrograph and sedigraph from an adjacent field through a VFS and to calculate the resulting outflow, infiltration, and sediment trapping efficiency. A front-end model, UH, was developed and added to VFSMOD-W to generate the necessary source area design inputs for VFSMOD. For each design storm, UH generates a rainfall hyetograph, a runoff hydrograph, and sediment loss from the source area using a combination of the NRCS curve number method, the unit hydrograph, and the modified Universal Soil Loss Equation based on topography, land use, and soil type. With these inputs, a set of response curves, i.e., sediment and runoff reduction vs. filter construction characteristics, can be developed from VFSMOD-W outputs for a given design scenario. To illustrate this procedure, a design case was presented where the goal was to obtain a 75% runoff sediment reduction for conditions similar to those of the North Carolina Piedmont region. In addition to two soil types present in the area, the range of conditions used in the analysis included two design alternatives (one concentrating field runoff in a narrower filter), four design storms with 1 to 10 year return periods, and buffer lengths ranging from 1 to 100 m. For the range of design storms considered, the optimal filter lengths obtained were 1 to 4 m for the sandy clay soil and 8 to 44 m for the clay. The results show that in some cases current environmental regulations pertaining to filter lengths in the area will not be sufficient. This application case clearly illustrates the importance of using an objective design procedure based on the specific location characteristics when implementing VFS as an effective off-site BMP.