A sediment trap is a small, temporary ponding area designed to catch and remove sediment from runoff. Runoff enters the trap and is impounded in a basin behind a stone weeper, reducing the velocity of the runoff and allowing suspended sediments to settle out.
Sediment traps are applicable on sites with drainage areas of less than 5 acres and are typically placed downstream of a Vegetated Swale or other conveyance. To maximize the effectiveness of this practice, they should be located on the lowest point, near the edge of the site, to maximize the area served by the trap. Because sediment traps are, at best, 70-80 percent efficient and are ineffective for smaller sized sediments, they are best used in conjunction with other BMPs.
Advantages
Disadvantages
Sediment traps must be designed for water quality control for storms up to the 1-year, 24-hour storm event. In addition, traps must also be capable of safely passing the 10-year, 24-hour storm event. Lengthening the basin, which increases the volume of the practice and detention time, may increase trapping efficiency. However, trapping efficiency is a function of particle size rather than of basin size, and, as a result, larger basins may or may not increase efficiency.
The ponding area of a sediment trap shall have, at a minimum, a 2:1 length to width ratio. The banks of the basin shall be compacted during construction and must possess a maximum height of 5 feet, with a minimum top width of 4 feet and slopes 2:1 or flatter.
The basin should be seeded, mulched, and lined with a geotextile filter fabric, whose opening size will vary depending upon the soil type that is present on site.
The crest of the outlet must be 1 foot below the top of the embankment, with weir length and stone size dependent upon the area drained by the practice. All other outlet criteria for this practice should follow the design specifications discussed on the Stone Weeper page.
Sediment traps reduce the flow velocity and allow sediment to settle out. The efficiency for this practice is dependent upon the proper design, installation, and maintenance of the structure. However, in general, sediment trap efficiency follows the ensuing graph.