Proper sediment control is one of the most important SWPPP requirements for any construction zone. Not only does sediment erosion pose a risk to surrounding wildlife and ecosystems, but it could leave your construction zone liable to fines and significant financial damages.
According to the EPA and Clean Water Act, construction zones that disturb more than one acre of land are responsible for proper permitting and erecting sentiment erosion controls.
Generally, construction managers have a few options for sediment control, including:
- Silt Fences
- Straw Bales
- Sand Bags
- Gravel Bag Berms
- Fiber Rolls
Silt fences are considered the most effective barrier for trapping coarse sediment erosion from disturbed areas.
In fact, laboratory work from the Virginia Highway and Transportation Research Council found that silt fences work more effectively at trapping sediment materials than straw bales and even slow the flow of stormwater runoff.
Unfortunately, one of the biggest obstacles to proper sediment control is the proper installation of silt fences, which determine their efficacy. For this reason, we’ve prepared this guide to discuss the use of silt fences and proper Best Managment Practices (BMPs) to set up and use silt fences in your SWPPP.
What is a Silt Fence?
A silt fence is a temporary sediment control barrier constructed of geotextile fabric entrenched in the ground and attached to wooden or metal poles.
The goal of silt fences is to control the flow of sheet and overland flows and trap coarse sediment material so that they don’t exit the construction site and end up in rivers, streams, or sewage systems.
Once construction has finished, construction managers can implement plans to stabilize the previously disturbed area and take down their silt fencing.
According to some estimates, a 100ft. stretch of silt fencing is estimated to hold up to 50 tons of sediment in place.
Silt fencing is typically located downstream or downhill from active workzones with about 2’ high of exposed silt fencing to gather materials. Silt fences are also entrenched anywhere from 8-12” in the ground and compacted using soil to ensure they remain sturdy and prevent washouts.
Below, we’ve listed a few appropriate and inappropriate uses of silt fences to control sediment erosion and runoff.
Silt Fence Uses
- Along the perimeter of an active construction zone
- Around material stockpiles and soil area
- Below exposed slopes to prevent overflow
- Around streams and other vulnerable ecological systems
Silt Fence Restrictions
- Overland or sheet flows of over 0.5cfs
- Areas subject to flooding
- Uneven or contoured terrain
- Slopes greater than 4H:1V
With this in mind, let’s outline a few silt fence BMPs to help you develop your SWPPP and ensure proper compliance with the law.
SWPPP BMPs for Silt fences
Installing silt fences at the right location is important because it will help avoid flooding and sediment erosion loss. In addition, a silt fence should pool water and sediment erosion inside the barricades, preventing water from flowing around the sides or over the top of the barricaded area.
Some BMPs to properly locate and install your silt fence include:
- Running the right length of fence: Running too long of a fence could end up concentrating distrubed materials at the lowest point of your fence and cause flooding.
- Give your fence enough room. The drainage area above a silt fence shouldn’t exceed more than a quarter of an area. Consider using multiple silt fences throughout to concentrate sediment runoff and silt into manageable areas.
- Use J-hook fences to concentrate sediment runoff in small areas. Instead of running a long stretch of fence around your entire perimeter, use J-fences to trap runoff and erosion in small areas to prevent spread throughout and beyond the site. This strategy also helps prevent sediment flow from going around the fence.
- You can trap sediment runoff and water flow in heavy rain storms by running a short fence along the top ends of a hill and placing the middle beneath the ends at the toe. This will be easier to clean up and restore since only heavy materials should flow over the top.
- Install fences before paved areas, conduits, and inlet basins. Paved areas can be difficult to treat and control if flooded by sediment runoff.
- Direct heavy sediment flows to a sediment pond. Use silt fences to divert excessive stormwater runoff to a pond that can be easily stored during construction and stabilized afterward.
Using the right silt fence materials is important for adequately controlling sediment erosion and keeping your silt fence within budget. Generally, we recommend a heavy porous fabric that won’t rip and tear. Avoid any lightweight fabric that doesn’t stand up to heavy materials, including woven or mono-filament.
Next, use metal posts instead of wooden posts for proper stability and to save money on chainlinks and structural wiring. Metal posts are also easier to drive into the ground (up to 2’ feet) than wooden posts and tend to stand up to excessive water flows better without sagging or tipping.
Space metal posts about 6-7 feet in areas with light horizontal loads and 3-4 feet in areas where there is excessive risk of flooding over the top of the fence.
There are generally two methods to properly install a silt fence, including the trenching and static slicing method. Trenching is a traditional method that involves digging a 6” deep and 6” wide trench with curved edges to install silt fabric and drive posts into the ground. The fabric is then held in place by a heavy material like concrete or stones that help prevent washout and direct runoff to a proper discharge area.
The static slicing method is often more straightforward, featuring a machine that cuts 12” slits in the ground while pushing soil vertically next to the slit. Once the mesh material is installed, a tractor wheel is rolled over the disturbed soil about two times to compact the soil and achieve nearly perfect compaction. This method is especially useful because it reduces infiltration and spaces in the soil to prevent water from flowing underneath the fence.
Finally, routine inspection and maintenance of silt fencing should be implemented to ensure proper function. One problem you’ll run into is that silt fencing typically clogs after use and can be nearly impossible to clean back to new again.
Generally, when sediment has risen to about half the level of your fence, it’s wise to install a second fence above or below the original fence. Since your original fence will not be able to trap materials and flow like it once did, you’ll need to routinely improve or erect new fencing, following these BMPs, to ensure proper compliance.
Removing sediment can be difficult, so leaving the sediment in place and erecting new fences is always your most efficient option.
Whether you’re developing a stormwater management plan or an SWPPP, proper silt fence BMPs are crucial to ensuring your construction zone complies with federal and state law and doesn’t disturb surrounding environments.
In addition, following proper BMPs for silt fencing is important to ensure your silt fences work properly.