You should install a silt fence along the perimeter of a disturbed area, particularly at the bottom of slopes where sheet flow runoff is expected to occur. It is crucial to place the barrier perpendicular to the direction of the water flow to maximize its filtering capabilities and prevent water from riding along the edge of the fabric. It should never be installed across a concentrated flow path, such as a ditch, stream, or channel, because the sheer volume and velocity of the water will easily wash it away.
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