Erosion and Sediment Control
Innovative Small-Scale Performance Evaluations of Geotextile Fabrics Used in Silt Fence Applications
Silt fence installations have been used as a means for intercepting and treating construction site stormwater runoff prior to offsite discharge for well over three decades. Determining the performance capabilities and effectiveness of various geotextile fabrics is challenging when evaluating in-field installations on construction sites. The challenge lies within uncontrollable weather patterns and inconsistent site conditions, making replicated field experiments difficult to perform. To achieve replicable performance evaluations of geotextiles, several parameters need to be continuously monitored during installation and throughout the experimental process. Standard small-scale testing methodologies for evaluating the filtering component of silt fence installations have failed to mimic realistic flows and sediment loadings commonly seen in field applications. For example, ASTM D5141 Standard Test Method for Determining Filtering Efficiency and Flow Rate of the Filtration Component of a Sediment Retention Device specifies that 75 L (20 gal.) of sediment-laden water be introduced during testing in less than 10 seconds. This method does not mimic realistic runoff conditions for all regions of the U.S. resulting from a 2-yr, 24-hr storm event, which is stipulated in the U.S. Environmental Protection Agency’s Construction General Permit as the minimum capacity requirement for all erosion and sediment control (ESC) practices. To address these issues, this study evaluated the performance capabilities of two nonwoven and three woven silt fence geotextiles using an innovative testing methodology and a newly developed small-scale testing apparatus located at the Auburn University – Erosion and Sediment Control Testing Facility. The overall intent for conducting these small-scale evaluations was to develop a deeper understanding of effluent flow rates, sediment retention capabilities, and water quality impacts associated with geotextile fabrics. Results suggest that effluent flow rates of nonwoven geotextiles are on average 43% lower than woven materials, which results in extensive upstream retention times of impounded stormwater for nonwoven materials. Measured effluent flow rates also suggest that standardized bench-scale flow tests fail to accurately predict in-field flow through capabilities. These finding thus question the implementation strategy of standardized flow through data. Sediment retention results indicate that nonwoven geotextiles have an average sediment retention rate of 97% while woven geotextiles average 91%. Finally, water quality analyses suggest that the primary means for turbidity reductions rely on the process of sedimentation during storm events (i.e., 46% reduction) and filtration during post storm dewatering (i.e., 19% reduction). This suggests that having adequate stormwater storage upstream of an installation is important to dissipate inflow energy, promote sedimentation, and minimize resuspension of particles.