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Sediment Trap

General

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 in swales and other conveyance channels. 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

  • Cost-effective
  • Relatively easy to construct
  • Low maintenance

Disadvantages

  • Low trapping efficiency for fine particles
  • Ineffective with large storm events
  • Maximum life span of 18 months
  • Maximum drainage area of 5 acres

Design

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.

Basin

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.

Outlets

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.

Construction

  • Sediment traps should be operational before site grading begins
  • Sediment traps should be removed after the site has been permanently stabilized

Maintenance

  • Accumulated sediment shall be removed when it reaches ½ of the outlet
  • Sediment traps shall be inspected for damage and repaired after each rainfall event
  • If the sediment trap does not drain completely within 24 hours of a storm event, the geotextile and stone outlet should be cleaned

Method to Determine Practice Efficiency

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.

References