A silt fence is a temporary sediment barrier designed to manage soil erosion in areas where the ground has been disturbed. Its primary function is to intercept runoff water containing loose soil and create a ponding area, allowing the suspended sediment to settle out before the water passes through the specialized fabric. This control measure helps protect nearby waterways and storm drain systems from contamination, fulfilling regulatory requirements for site management.
Preparation and Site Layout
Successful silt fence installation begins with a thorough site assessment to determine the path and material needs based on local topography and soil type. The fence line must follow the contour of the land, running perpendicular to the direction of water flow to maximize water detention time. Placing the fence directly down a hill or in an area of concentrated water flow, such as a ditch, will likely cause it to fail or be washed out quickly.
The layout should incorporate “J-hooks” or a slight upturn at the ends of the fence run, which prevents water from flowing around the barrier. Planning for a maximum drainage area of roughly 10,000 square feet per 100 feet of fence is a useful guideline for determining the total length needed. Material selection involves choosing a geotextile fabric and support posts made of wood or steel, which must be durable enough to withstand the load of retained water and sediment.
Securing the Silt Barrier
The physical installation process centers on ensuring the bottom edge of the fabric is properly anchored into the ground to prevent water from undercutting the barrier. The two primary methods for achieving this secure embedment are the trenching method and the slicing method, each offering different advantages depending on the site conditions.
The trenching method involves excavating a shallow trench along the planned fence line. The geotextile fabric is then placed into this trench, with the bottom portion of the material laid flat and extended uphill. The posts are driven into the ground on the downhill side of the fabric, providing structural support against the pressure of the retained water and sediment. Finally, the excavated soil is backfilled into the trench over the embedded fabric and compacted to create a robust seal, which prevents water from seeping underneath the fence line.
The slicing or plowing method uses specialized machinery to insert the fabric into the ground. A narrow blade is pulled through the soil, slicing a slit while simultaneously feeding the geotextile fabric into the opening to an effective depth of about eight to twelve inches. This mechanical process is faster and provides more consistent compaction because the machinery compacts the soil vertically on both sides of the fabric immediately after insertion. Regardless of the method used, support posts should be spaced between six and ten feet apart, with closer spacing required in areas expecting higher runoff velocity or steeper slopes.
When multiple sections of fence fabric are needed for long runs, they must be securely joined by overlapping the ends at a support post and folding the fabric around the post. This overlap should be generous, wrapping the ends around each other to prevent gaps that could serve as a path for sediment-laden water. Posts must be situated on the downstream side of the fabric, ensuring the structural support is positioned to withstand the weight of the material and the accumulated sediment.
Ongoing Monitoring and Removal
The effectiveness of a silt fence relies on consistent inspection and maintenance after installation is complete. Inspections should be performed at least once a week and immediately following any significant rainfall event to check for damage, sagging, or signs of water bypassing the barrier. Any tears in the fabric or leaning posts must be repaired promptly, and sections where water has eroded the soil underneath the fence must be re-trenched and compacted.
Removal of accumulated sediment must be performed when the soil reaches approximately one-third to one-half the height of the exposed fence fabric. Allowing the sediment to build up higher than this range reduces the fence’s capacity and increases the risk of failure or overflow. The removed sediment should be deposited in an area where it will not be washed back toward the fence or into a natural waterway.
The silt fence is a temporary measure and should only be removed once the disturbed area has achieved permanent stabilization. This stabilization is typically defined as the establishment of a minimum of 70% vegetative cover or the application of other permanent stabilization materials. Upon removal, the fabric and posts should be taken down, and the accumulated sediment should be spread out and stabilized with vegetation.