Low Impact Development Benefits of Level Spreader - Vegetative Filter Strip Systems

摘要

Vegetative filter strips (VFS) have been employed to reduce pollutant export from agricultural watersheds for years. In order to enhance the effectiveness of VFSs, level spreaders have been employed to distribute flow evenly across the length of the upslope end of the buffer. During the past decade, level spreaders have been required in nutrient-sensitive watersheds in NC. to reduce erosion in riparian buffers. An assessment of the performance of four level spreader - vegetative filter strip (LS-VFS) systems was conducted in the Piedmont of North Carolina. At each site, one 7.6 m (25 ft) wide grassed VFS and one 15.2 m (50 ft) wide, half grassed, half forested VFS drained highly impervious watersheds. Monitored parameters included rainfall, inflow to, and outflow from each LS- VFS system. The VFSs promoted infiltration, which resulted in a substantial decrease inflow volume and peak flow rate between the inlet and outlet of the system. To date, 58 storm events have been monitored for hydrology in Louis burg, NC. Mean flow volume was reduced by greater than 40% for both the 7.6 m and 15.2 m VFSs. Reconcentration of surface flow in the VFS was shown to substantially impair filter strip performance. These results show that a LS- VFS system can effectively reduce the hydrologic impacts of impervious surfaces.Twenty-one and twenty-two flow-proportional water quality samples were collected and analyzed for the Apex and Louisburg sites, respectively. Constituents monitored included TKN, NO_3+NO_2 TN, NH_4 Org-N, TP, Ortho-P, PBP, and TSS. All LS-VFS systems studied significantly reduced mean TSS concentrations (p<0.05), with the 7.6 m buffers reducing TSS by at more than 50% and the 15.2 m buffers reducing TSS by more than 65%. Concentrations of TKN, TN, Org-N and NH4-N were significantly reduced (p<0.05) by both 15.2 m VFSs, while results were mixed for the 7.6 m VFSs. Significant pollutant mass reduction was observed (p<0.05) for all nine pollutant species analyzed at the Louisburg site due to infiltration in the VFSs. The effects of VFS length and/or vegetation type are very important for pollutant removal, as effluent pollutant concentrations were lower (with one exception) for the 15.2 m VFSs. Median effluent concentrations for TN and TP for the four LS- VFSs were better than fair water quality benchmarks for the Piedmont of North Carolina, but only met good water quality metrics in one-half of the studied storm events.