Sediment Control Practices
What sediment control does and does not do
Sediment control BMPs capture soil that has already detached. They do not stop detachment, and they do not remove dissolved pollutants, fine clays, or nutrients effectively. A well-designed basin removes the coarse fraction at roughly 70–90% efficiency; the rest leaves the site.
Silt fence - the most misapplied BMP
Silt fence is a permeability barrier that ponds water so sediment settles; it is not a high-flow filter. Install it on the contour, keyed into the ground, with posts on the downstream side, and use J-hooks or returns at the ends. Never place it across a concentrated flow path, in a swale, or as a substitute for a check dam or basin. Common trap: a design that routes channelized flow to a silt fence line.
Traps vs basins
Sediment traps are small, temporary, and typically drain only a few acres with no engineered dewatering device. Sediment basins have a designed outlet (riser, skimmer, or perforated riser) and serve larger areas. Know the difference; exam items often hinge on whether a dewatering structure is required.
Other practices
- Check dams: small swales only, typically ≤2 ft high, not in live streams; space them so the toe of the upstream dam meets the crest elevation of the next one downstream.
- Inlet protection: must not flood the roadway or cause ponding that creates a hazard.
- Construction entrance: 50 ft minimum of coarse aggregate over geotextile, with a stabilized exit.
Rule of thumb
Many state general permits allow about 0.25 acre of drainage per 100 linear feet of silt fence, and require on the order of 3,600 ft³ of sediment storage per disturbed acre in a basin. Always verify against the governing state criteria before citing a number.