Flow-Through Rates And Evaluation Of Solids Separation Of Compost Filter Media Vs. Silt Fence In Sediment Control Applications

Abstract

Soil loss rates from construction sites can be 1,000 times the average of natural soil erosion rates and 20 times that from agricultural lands. Silt fence (SF) is the current industry standard used to control sediment originating from construction activities. Silt fences are designed to act as miniature detention ponds. Research has indicated that SF sediment filtering efficiency is related to its ability to detain and pond water, not necessarily the filtration ability of the fabric. Design capacity and spacing is based on flow through rate and design height. In addition, increased detention of runoff and pressure from ponding may increase the likelihood of overtopping or failure of SF in field application. Testing was conducted on compost silt socks (SS) and silt fence to determine sediment filtering efficiency, flow through rate, ponding depth, overtopping point, design height, and design capacity. Results indicate flow through rate changes with time, as does pond depth, due to the accumulation of solids on/in the sediment filters. Changes in depth with time were a linear function of flow rate after ten minutes of flow, up to the time the sediment filter is topped. Predicting the capacity of SF and SS to handle runoff without the filter being topped requires consideration of both runoff rate and length of runoff time. Data showed SS 1/2 the height of SF are less likely to overflow than SF when sediment-laden runoff flow is less than 5 gpm/ft. Depth behind a 24 in. SF increased more rapidly than behind a 12 in. SS, and at the end of the 30 minutes, the depth behind the SF was 75% greater than that behind the SS. Removal of solids by the SF and the SS were not shown to be statistically different. Results were used to create an MS ExcelTM based interactive design tool to assist engineers and erosion and sediment control planners on how to specify compost SS relative to SF in perimeter sediment control applications.