Developing Sediment Traps for runoff
The purpose of a trap or basin is to provide an area where muddy runoff is allowed to pool, so sediment will settle out. Sediment traps and basins are installed in natural drainage areas before excavation or fill work begins. Do not depend on sediment traps and basins alone to control sediment loss from your construction site. Other uphill controls on bare areas, slopes, and in ditches and channels are needed to prevent overloading traps and basins. Containment for the pooling area can be an excavated hole or a dike made of earth or stone. Straw bales and silt fencing are not approved for use as containment structures for concentrated runoff flows.
Low-lying sites on the downhill side of bare soil areas where flows converge are ideal places to install temporary sediment traps and basins. In general, sediment traps are designed to treat run off from about 1 upto 5 acres. Sediment basins are larger, and serve larger areas of about 5 to 10 acres. Basins draining areas larger than 10 acres require an engineered design, and often function as permanent storm water treatment ponds after construction is complete.
Any depression, or low-lying place that receives muddy flows from exposed soil areas can serve as a sediment trap. Installing several small traps at strategic locations is often better than building one large basin. The simplest approach is to dig a hole or build a dike (berm) of earth or stone where concentrated flows are present. This will help to detain runoff so sediment can settle out. The outlet can be a rock lined depression in the containment berm.
Some design considerations must be allowed for. Do not put sediment traps or basins in or next to flowing streams or other waterways. Make sure pooled water does not flood buildings, roadways, or other structures.