Silt Fence Calculations
FAQ: How can I calculate the efficiency of silt fence on site?
Question: I am trying to figure out some calculations on the silt fence. I am required to calculate silt fence efficiency on my site and in the silt fence ratio, we need peak outflow. I have my Coefficient and Rainfall intensity numbers. I need an area to finish off my peak outflow equation.Answer: Thank you for your inquiry. For caclulating Silt Fence efficiency, the following physical properties should help:
- Filtering Efficiency of 75 - 85 % (minimum) which is highly dependent on local conditions
- Tensile Strength at 20 % (maximum) should allow for Standard Strength of 30 lbs/linear inch (minimum)
- Elongation effects should consider Extra Strength of 50 lbs/linear inch (minimum)
- Ultraviolet Radiation allowance of 90 % (minimum)
- Slurry Flow Rate should be 0.3 gal/ft2/min (minimum)
The size of the drainage area is no more than 1/4 acre per 100' of perimeter control. The max slope length behind the barrier is 100' and the max gradient behind the barrier is 50%.
USEPA (1993) reports the following effectiveness ranges for silt fences constructed of filter fabric that are properly installed and well maintained: Average total suspended solids removal of 70 percent, sand removal of 80 to 90 percent, silt-loam removal of 50 to 80 percent, and silt-clay-loam removal of 0 to 20 percent. Removal rates are highly dependent on local conditions and installation.