Monitoring Data on Effectiveness of Sediment Control Techniques

Abstract

Silt fences are among the most common BMPs implemented for sediment control at construction sites. Field evaluation of the efficiency of silt fences in removing sediment in highway construction runoff showed that sediment removal was not attributable to filtration by the fabric, but was the result of particle settling in the pool of water behind the fence. The median concentration of solids discharged from the silt fence controls was approximately 500 mg/L. This concentration is significantly lower than the 3000 mg/L of TSS often reported in construction runoff and suggests that approximately 75% of the particles in the runoff were removed by sedimentation in the water impounded by the fences. Geotextile silt fences proved to be ineffective in reducing turbidity. The particles responsible for creating turbidity are mainly silt and clay and the particle diameter normally is much smaller than the size of the openings in the fabric. The observed data indicated that silt and clay size particles c omprised 92% of the total suspended solids that reach the face of the silt fence. High sediment removal efficiencies were achieved with silt fences in flume studies. The geometry of the flume created a large ponded area behind the controls resulting in long detention times and significant particle settling even with the fine grained sediment used in the tests. Mean efficiency for sediment removal in the flume ranged from 68 to 90% and was highly correlated with the detention time . These data indicate that silt fences should be sit ed in the field to maximize the volume of water impounded behind the fence. Flow rates through the silt fence fabric of sediment-laden runoff were two orders of magnitude less than those typically specified by many regulatory a gencies. The flow rates of a sediment slurry through geotextile fences are a function of apparent opening size as well as permittivity (or other measures of clean water flow rates). The fabric that resulted in the longest detention times in this series o f flume tests had the highest reported permittivity, but the smallest apparent opening size, suggesting that clogging of the fabric with sediment affected the hydraulic performance.