BMP Blast: Silt Fence

BMP Blast: Silt Fence
 
What’s the first BMP that comes to mind when you think of construction site sediment control?  Chances are you thought of silt fence (SF)! This month, we dig into how this relatively inexpensive practice is used and some common installation pitfalls to avoid.
 
SF is a sediment-trapping barrier comprised of geotextile fabric and support posts. The fence is installed downslope of areas where sheet and rill erosion are expected to occur. Sediment-laden runoff ponds behind the fence and allows for deposition. Although it is a very popular and effective BMP, the Ohio Department of Natural Resources finds that about 75% of SF does not function due to poor installation. Here are some key points to follow:

• Follow the contour. Install SF on a level contour at least five feet away from the toe of a slope to promote sediment deposition. Each end of the SF should be “j-hooked” or curved slightly up the slope to prevent water from flowing around it. If the fence is installed up and down slopes, it will concentrate and channelize the runoff. Simply running a line of SF around the entire site may not be the most effective option. Consider installing discreet segments of silt fence along the topography as to not overwhelm the system.
• Trenching. Install SF with at least 8 inches of the geotextile fabric below ground. An un-trenched silt fence is useless because runoff will flow right under it.
• Pulled taut with strong posts. Install SF with the geotextile fabric pulled taut between strong wooden or metal posts spaced 5 to 10 feet apart. The posts should be located on the downslope side of the fence. Seams in the SF should occur at a post and overlap at least 6 inches. For a stronger fence, consider using wire backing.
• Backfill and compact. Fill the trench back in with dirt and compact it to secure the fence in place. Consider using a silt-fence installation machine which slices, installs, and backfills with ease.

Do not use SF…

• on steep slopes.
  
• as a check dam.
  
• below high velocity/high flow/concentrated flow areas.
  
• to capture drainage from areas larger than what is recommended.

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What kind of maintenance does silt fence require?

• Routine inspections after rainfall events
• Removal of sediment when it reaches one-half the height of the SF
• Replace damaged/torn fabric
• Repair downed SF

 
Assess what caused the failure and make changes to prevent it from happening again. Is there a concentrated flow pointing at the fence? Did someone drive over it? Is the fence sufficiently trenched and compacted? Is the drainage area too big?
 
For more details on the minimum criteria for SF fabric/posts and maximum slope length above SF, please refer to the Ohio Environmental Protection Agency’s (OEPA) Rainwater and Land Development Manual, Chapter 6.3.