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Instream Sediment Control Techniques

Construction in or near watercourses can cause sediment in the stream bed to become dislodged. Sediment that is allowed to travel downstream may be damaging to the habitats of fish and other creatures living in the stream. Several techniques have been developed to control the distribution of sediment within stream water, including silt barriers and temporary dam structures.

  1. Silt Barriers

    • Silt barriers, also called silt curtains, are temporary, semicircular structures made of a flexible fabric that filters debris and sediment out of flowing water. Floating silt barriers are attached to the sides of a stream and allow some water to flow around and under the curtain, making them a good choice for streams with a higher flow velocity. Staked barriers can be anchored to the sides and bottom of the stream so that the curtain conforms to the stream bed. These types of barriers works best for calmer streams with low flow velocities and currents.

    Cofferdams

    • Cofferdams are temporary structures made of materials such as rocks, sandbags, concrete blocks, hay bails or wood. These dams encircle a construction site to filter sediment out of the water before it escapes into the rest of the stream. The inside of the cofferdam is pumped dry so that construction crews can work below the water level. Cofferdams are a good choice for watercourses with a high flow velocity and strong currents, which could dislodge less sturdy alternatives such as silt curtains. Some cofferdams are lined with a silt barrier fabric to increase the amount sediment they catch.

    Instream Weirs

    • Instream weirs are shallow dams that sit below the water level and stretch across the entire width of the stream. These dams are usually made of large rocks and slope down the sides of the riverbank so that they form a "V" shape, letting only the water from the middle of the stream pass through uninhibited. Weirs are very sturdy structures and work best for filtering out large debris and coarse materials suspended in the water of the stream.

    Water Structures

    • Water structures are made of polyethylene pipes that are pumped full of water to create a temporary dam structure. These dams work best in environments with low flow velocities as they may need to be anchored to the stream bed in the presence of strong currents. Their construction makes them more susceptible to damage from large debris, so they should only be used in watercourses where damaging objects such as rocks and tree limbs are scarce.

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