Sediment and the Use of Silt Fences

Sediment is the single biggest pollutant of the waters in the Upper Broad watershed. Not only does it have environmental costs, but economical costs as well. If we will take the time to consider our properties and the natural dynamics affecting that property that contribute to erosion and sedimentation, we can make preparations to reduce the amount of material leaving the site.

 

If you have ever gotten a land disturbance permit in Lake Lure, there is a section on the application that asks what measures will be taken to prevent erosion. The most common thing that is written down in this section is silt fence. Silt fence is required for most land-disturbing activities in Lake Lure due to our steep slopes and high runoff rates.   The primary purpose for silt fence is to serve as a temporary containment of sediment around the perimeter of a construction site. Silt fence typically is nothing more than a filter cloth that is held up by wooden or metal stakes. The cloth captures sediment by backing up sediment-laden water, allowing the soil particles to settle out. Any water that makes it through the fence is relatively clear.

 

Silt fence is most often used as a perimeter shield, sort of a last line of defense if you will. However, in most cases, silt fence is almost never properly installed and no other erosion control measures have been put in place, rendering the silt fence ineffective. Most people try to use silt fence as a diversion and this is simply an ineffective use of the measure.

 

Here are some things to consider about silt fence:

 

- Silt fences should not be installed as a primary erosion control measure. They should be used in conjunction with other erosion control measures.

 

- Silt fences must be maintained, because their primary purpose is to settle out material, they are naturally going to fill up. This material must be periodically removed in order to maintain the efficiency of the measure.

 

- Silt fences must be installed properly. The most common mistake made with silt fence is failure to bury the bottom portion of it. When this does not happen, everything runs underneath it and will often blow the fence out.

 

- Silt fence will not be adequate if the drainage area is too large or the slope too steep. This will cause the fences to be overwhelmed because of the sediment load.

 

- DO NOT install silt fence across streams, channels, or any location where concentrated water flow is anticipated.

 

Silt fences are most effective when used in conjunction with a diversion of some kind (usually an earthen berm). To install silt fence properly you should dig a trench approximately 8 inches deep and 4 inches wide, or a V-trench along the line of the fence, then you need to drive in your posts to which the fence will be attached. If you buy pre-assembled fence from Lowe 's or Home Depot it typically comes with the stakes already fitted into the fence. Unroll the silt fence and begin driving the posts into the ground along the length of your trench. When the fence is up, there should not be any slack (any droop) in the fabric. It should be relatively tight. The lower foot of cloth should go into the trench, with 8 inches going down, and 4 inches lapping toward the up slope side of the trench. Bury the cloth and backfill with compacted earth or gravel.   If that cloth is not buried, it is not installed correctly. To reduce maintenance, dig out a shallow sediment storage area on the up slope side of the fence where sedimentation is expected and provide good access for cleanout and maintenance. After the site has been stabilized, silt fence can be removed.

 

The silt fence is like the last line of defense for sediment control. It is used primarily as perimeter defense to prevent sediment from encroaching on adjoining properties, roads, rights-of-way, streams, etc. The North Carolina Department of Environment and Natural Resource Land Quality Section offers classes for contractors and developers that show how to properly install erosion control measures and offer what is called a Clean Water Contractor Certification. Encourage all contractors and developers that you know to participate in this class. The benefits are good for everyone. Clean Water Contractors get recommendations from DENR as reliable protectors of water quality and are preferred among landowners wanting to improve or develop their property. Go online to find out more about this workshop at www.dlr.enr.state.nc.us.  

 

Erosion control measures are only affective if they are implemented correctly. Some online resources to consider on the subject are found at the following addresses:

 

www.dfr.state.nc.us/stewardship/pdf/laymans_road_guide.pdf

 

     www.tetratech-ffx.com/wstraining/pdf/esc_guide.pdf

 

If you have questions about erosion control measures, please call 828-625-9983 ext. 123 or send an email to ECO@TownofLakeLure.com.