The Effect of Baffle Weave Opening on Reducing TSS and Turbidity from Sediment Basin Discharge

Abstract. Construction activities can result in adverse effects on the surrounding environment with excessive sediment loads. Stormwater runoff can result in sustained environmental damage through accelerated erosion and sedimentation. These processes may be exacerbated by construction activities through removal of vegetation and mass grading. Sediment basins mitigate potential impacts by capturing and detaining site runoff and transported sediment before reaching receiving waters. One common sediment reduction BMP for construction site disturbance is the sediment basin. Sediment basins installed at construction sites, agricultural operations, and other disturbed areas provide temporary ponding for runoff that allows some sediment to settle before water is discharged. Sediment basins, while effective on coarse materials, are less effective on finer particles such as dispersed clays. Utilizing baffles may enhance basin performance. Baffles reduce the average flow velocity, distribute the flow across the entire basin width, reduce volume of dead storage, help minimize sediment resuspension and provides additional time for sediment particles to settle and thus decrease TSS discharging into receiving waters. Various materials have been used as baffles within sediment ponds ranging from plywood to silt fence fabric to coir blankets. The objectives of this study were to determine the effect that light penetration ratings of coir blankets have on reducing sediment discharge when used as baffles within a sediment basin. Various light penetration ratings of coir baffles were evaluated to determine their effect on total suspended solids and turbidity reductions. Based on the SCDOT design standard, a 1:5 scale sediment basin was constructed at the Clemson University erosion and sediment control research facility in Pickens County, South Carolina. Coir baffles were then installed within the sediment basin and simulated sediment laden stormwater was discharged into the basin. Influent samples were taken during the time period that the basin was filled. Effluent samples were taken once discharge occurred from the basin and then over time as the basin drained. A floating skimmer was used for surface water withdrawal from the sediment basin. Both total suspended solids (mg/l) and turbidity (NTUs) were measured for both influent and effluent samples. Based on influent and effluent data, percent reductions for both TSS and turbidity were determined for each of the baffle configurations. Data suggest there was not a significant statistical difference between baffles tested. Numerically, Baffle 3 provided the best TSS percent reduction and Baffle 1 provided the best turbidity percent reduction. As tested, baffles removed at least 93% of TSS and 90% of turbidity. The lack of a significant difference between baffles configurations indicates the rated light penetration percentage, or openness of the baffle weave pattern, does not necessarily predict optimal performance. Utilizing an energy reducing partial barrier across the width of the basin producing more of a plug-flow appears to be the critical physical process in reducing TSS/turbidity. These results should benefit the construction industry by providing options regarding LPR specifications and thus more flexibility in products used as baffle material in sediment ponds.