What Is Silt Fence and How Does It Work?

Silt fence is a temporary sediment barrier constructed of a woven or non-woven geotextile fabric supported by wood or steel stakes. It's installed along the down-slope perimeter of disturbed soil — typically the boundary of an active construction site — to intercept sheet-flow stormwater runoff before it leaves the site.

Silt fence doesn't block water; it filters it. As sediment-laden runoff reaches the barrier, the fence slows the flow velocity, which causes suspended sediment to settle out and pond on the up-slope side. Filtered water then passes through the fabric at a reduced rate. This is why silt fence has to be installed along the contour of the slope rather than straight across it — fence run perpendicular to flow concentrates water at low points and can be overtopped or undermined.

Because silt fence works by ponding and filtering rather than impounding large volumes, it is only effective for sheet flow from limited drainage areas — it is not designed to handle concentrated channel flow, which requires a different BMP such as a check dam or sediment basin.

Two sites can use identical fabric and get completely different results based on installation alone. Fabric that isn't trenched and backfilled allows runoff to flow underneath the barrier entirely, bypassing the filtration mechanism. Stakes spaced too far apart let the fabric bow and eventually fail under hydrostatic load. And fence that isn't tied into returns at the ends allows flow to simply go around the barrier rather than through it.