Silt Fence
What is a Silt Fence?
This is a temporary barrier made of woven wire and fabric filter cloth (geotextile)
that is used to catch sediment-laden runoff from small areas of disturbed soil
to keep silt from getting in streams and homes.
When is a Silt Fence Used?
Silt fences are used for specific situations. Major considerations are slope,
slope length, and the amount of drainage area from which the fence will catch
runoff. Silt fences should be installed on the contour of a slope. Silt fences
should not be installed across drainage ways, swales, gullies, ditches or other
areas of concentrated water flow. Silt fences should be installed near homes,
roads, ponds, and streams to divert debris flows away from these sites. Here are
some design considerations:
| Slope Steepness |
Maximum Slope Length |
| 2:1—(50%) |
50 feet |
| 3:1—(33%) |
75 feet |
| 4:1—(25%) |
125 feet |
| 5:1—(20%) |
175 feet |
| <5:1—(<20%) |
200 feet |
What Materials are Needed?
Fence Posts
Posts should be at least 36 inches long. Wood posts should be of
hardwood with a minimum cross-section area of three inches. Steel posts should
be standard “T” section and should weigh no less than one pound per linear foot.
Wire
Wire fence should be at least 14-gauge with openings no larger than six
inches square.
Geotextile Fabric
Geotextile fabric should have the following minimum
material properties to be suitable for this type of use.
Availability
These materials can be obtained from fencing contractors, ranch
equipment suppliers, and home improvement stores.
| Geotextile Property |
Minimum Acceptable Value |
Test Method |
| Grab Tensile Strength (lb) |
90 |
ASTM D1682 |
| Elongation at Failure (%) |
50 |
ASTM D1682 |
| Mullen Burst Strength (PSI) |
190 |
ASTM D3786 |
| Puncture Strength (lb) |
40 |
ASTM D751 (mod) |
| Equivalent Opening Size |
40-80 |
US standard sieve sizes |
| Ultraviolet Radiation Stability |
90 |
ASTM G26 |
How is a Silt Fence Installed?
Prefabricated silt fence should be used whenever possible to minimize
installation labor requirements. An eight-inch deep trench is dug along the silt
fence alignment. The silt fence is unrolled and stretched tight while the posts
are driven at least 16 inches below the ground surface. Sections of silt fence
shall be joined at a post by overlapping the geotextile six inches and wrapping
it around the post before the post is driven. The wire shall be overlapped and
wired to the post. After the silt fence is erected, the trench is backfilled and
the backfill is tamped by wheel rolling with small equipment or foot traffic.
What Maintenance is Needed?
Inspect the silt fence after every runoff event. Repair any damage
immediately. Remove sediment and other debris from the upstream side of the
fence when it accumulates to the extent that visible bulges develop in the silt
fence. Remove the silt fence after vegetation or other permanent erosion control
measures are installed and functional.
Silt Fence Illustrations
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