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:

• determine what kind of runoff, and how much, is coming onto the site; too much volume of water per silt fence area means failure will happen;
• determine where and how the total volume is going to exit; total drainage area is the prime consideration of silt fence quantity, not necessarily slope;
• soil type can play a role in the placement and quantity requirements; sandy soils might require more silt fence per area to contain the volume of potential sediment; clay soils might need fewer fences because the volume of potential sediment loss is less, although the volume of water might be greater because clay soils allow less rainfall infiltration;
• type, size and spacing of fence posts;  wood posts are inadequate and should not be used; steel t-posts weighing at least 1.25 lbs per ft. are required, as they can be driven 24 inches into compacted soil, which is necessary to hold a horizontal load 18 inches high, and they can also be recycled and used repetitively; improper spacing of posts causes failures;
• type of filter cloth; if all the elements of the silt fence installation are properly adhered to, the fabric does not make much difference; even lightweight non-woven fabric will hold 18 inches of sediment; wire supported fence is costly and ineffective.