Design of Sediment basins
Sediment basins are somewhat larger than traps, but the construction approach is the same. Sediment basins usually have more spillway protection due to their larger flows. Most have risers and outlet pipes rather than rock spillways to handle the larger flows.
Sediment basins are often designed to serve later as storm water treatment ponds. If this is the case, agreements are required for long-term sediment removal and general maintenance. Construction of a permanent, stable outlet is key to long-term performance.
A minimum storage volume of over 130 cubic yards per acre of exposed soil drained is required for basins and traps. Traps and basins are designed so that flow paths through the trap or basin are as long as possible, to promote greater settling of soil particles.
An overflow outlet can be made by making a notch in the containment area and lining it with rock. Rock in the notch must be large enough to handle overflows, and the downhill outlet should be stabilised with rock or other flow dissipaters similar to a culvert outlet. Overflow should be at an elevation so dam will not over top. Allow at least one foot of free board. Outlets must be designed to promote sheet flow of discharges onto vegetated areas if possible. If the discharge will enter a ditch or channel, make sure it is stabilized with vegetation or lined.
Side slopes for the excavation or earthen containment berms are 2:1 or flatter. Berms are made of well-compacted clayey soil, with a height of 5 feet or less. Well mixed rock can also be used as a containment berm for traps. Place soil fill for the berm or dam in six inch layers and compact. The entire trap or basin, including the ponding area, berms, outlet, and discharge area, must be seeded and mulched immediately after construction.
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