Definition: A silt fence is a temporary sediment barrier consisting of filter fabric entrenched into the soil and attached to supporting posts.  Silt fence installed with a trencher or by slicing is the most effective installation method to ensure against common silt fence failures.

The slicing method for silt fence installation utilizes an implement towed behind a tractor to “plow” or slice the silt fence material into the soil.  The slicing method requires the “Tommy” silt fence machine or equivalent.  Silt fence machines install the silt fence by slicing through the soil, rather than excavating it.  Slicing minimally disrupts the soil upward and slightly displaces the soil, maintaining the soil’s profile and creating an optimal condition for future mechanical compaction.  Compacted soil resists water infiltration and moisture saturation, thus nearly eliminating washouts.

Purpose: Silt fence is a sediment control practice.  Silt fence is intended to be installed where sediment-laden water can pond, thus allowing the sediment to fall out of suspension and separate from the runoff.  It is not intended to be an erosion control practice.  Improperly applied or installed silt fence will increase erosion.  Reasons for the high failure rate of improperly designed and installed silt fence include:
  • Improper placement on the site;
  • Allowing excessive drainage area to the silt fence structure;
  • Shallow trenches with little or no soil compaction;
  • Inadequate attachment to posts;
  • Failure to maintain the silt fence after installation;
  • Installing silt fence along property boundaries producing "concentrated" runoff.

Improper installation on slopes causes gullies

Silt fence must only be installed where water can pond.  When placed off contour, silt fence will effectively divert runoff if that is desired.

Silt fence can be used where:
  • sheet and rill erosion would occur;
  • protection of adjacent property or areas beyond the limits of grading is needed (perimeter control);
  • the size of the drainage area is no more than 1/4 acre per 100 linear feet of silt fence;
  • the maximum flow path length above the barrier is 100 feet (30.5 m);
  • the maximum slope gradient above the barrier is 2:1;
  • small swales are carrying silt, the slope is less than 2%, and the drainage area is less than 2 acres (0.8 ha);
  • no practice other than a silt fence is feasible.

This silt fence was not keyed in and is improperly placed.

Design Considerations:

Silt fences have a useful life of one season.  Their principal mode of action is to slow and pond the water and allow soil particles to settle.  Silt fences are not designed to withstand high heads of water, and therefore should be located where only shallow pools can form.  Their use is limited to situations in which sheet or overland flows are expected.  Install silt fences with smiles.

The slicing method has the capability to turn in a short distance, thus properly installing silt fence where needed.  Turning enables upturns on the ends of silt fence runs, maneuvering around obstacles on construction sites, protection along property lines, and following contours.

Silt fences normally cannot filter the volumes generated by channel flows. When installed across a concentrated flow path, undercutting of the fence often occurs.  Silt fences should not be designed to impound sediment or water more than 18 inches (0.5 m) high.  Sediment shall be cleaned from behind the fence when it reaches 50% of the designed impoundment height (9 inch (0.2 m)).

Some design considerations include:

These silt fences were not properly keyed into the soil.

Construction Specifications:

Typical silt fence specifications were written 25 years ago and have changed little since.  Some states have recognized some of the inherent problems, such as inadequate trench depth, and implemented minor changes to improve efficacy.  The 25 year-old specifications, referred to as the trenching method, have never been tested for efficacy and proven worthwhile.  A trencher was simply the only piece of equipment available at the time capable of securing the fabric into the soil, regardless of efficacy.

Proper installation of silt fence using the "slicing" method.

Today, many contractors just open a furrow with a blade and backfill onto the fabric with the crumbs.  Loose soil, either from the trencher or the blade, absorbs water quickly and becomes saturated easily, washing out under the fabric.

Installation of Silt Fence with Smiles

Page 2 Silt Fence Part 2
(Installation and Maintenance)
(Typical Drawings)

Source: John McCullah - CPESC; North Carolina Erosion and Sediment Control Planning and Design Manual; California BMP Handbook; EPA Stormwater Management for Construction Activity.

Silt Fence That Works, Thomas Carpenter, CPESC, 2000.  Tommy Silt Fence Machine, 3718 S.W. Court Ave., Ankeny, Iowa, 50021 (800) 965-4665
Installation of Silt Fence Using the Tommy Static Slicing Method, Environmental Technology Verification Report, Washington, DC, 2000

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