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Sediment Basin

General

A sediment basin is a temporary ponding area, designed to catch and remove sediment from runoff while controlling the rate stormwater is released. Stormwater enters the basin and is impounded behind an embankment structure, temporarily ponding the water and allowing suspended sediments to settle out. As the water level in the basin rises and reaches its design depth, the water is drained by a riser, which releases only the relatively sediment free upper portion of the water column. As a result, the majority of the sediment is contained in the basin.

Sediment basins, which consist of an embankment, a riser, and an outlet pipe, are generally temporary practices with a maximum life span of 18 months. If a permanent practice is desired, the sediment basin should be designed following the criteria set forth in the wet basin section of this manual. Sediment basins, which have a maximum efficiency of 70-80%, are applicable on sites with a maximum drainage area of 100 acres and are one of the most widely used practices for on-site erosion control. However, sediment basins should not be used on sites where the failure of the practice may endanger lives or property.

Advantages

  • Cost-effective
  • Capable of draining a large area, up to 100 acres
  • Maximum efficiency of 70-80%

Disadvantages

  • Requires a large land area
  • Ineffective with large storm events
  • May require frequent maintenance
  • Maximum lifespan of 18 months
  • Ineffective for small sediment particles

Design

Basin

Sediment basins should be designed according to the individual site characteristics and must be capable of handling the runoff from the 10-year, 24-hour storm event. Basins, which may be designed with or without a permanent pool of water, are required to have a surface area at least 1.2 times the area needed to settle a 0.0005 mm particle. However, the surface area, as well as the rest of the features of the practice, should be designed with the individual site characteristics in mind.

To increase trapping efficiency, the basin may be lengthened, thus increasing the volume and the detention time of the practice. 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.

Basins should be teardrop shaped, with the inlet at the narrow end, and have a length to width ratio of at least 2:1 to ensure proper treatment of runoff. However, where site constraints prevent this design, baffles may be implemented to maximize the detention time and to prevent short-circuiting of the practice.

The banks of the sediment basin shall be constructed of clean mineral soil, free of roots, debris, and oversized stones. They shall be constructed at least 10% higher than the design height to allow for any settlement that may occur. To encourage safety and stability, the banks shall have a maximum height of 15 feet. Those with heights between 10 and 15 feet must be at least 10 feet wide, while heights less than 10 feet require a minimum width of 8 feet. However, regardless of the height and width of the banks, slopes shall be designed with a horizontal to vertical ratio of 2:1 or flatter.

Example of a Sediment Basin

Source: Adapted from Virginia Soil and Water Conservation Commission

Spillways and Outlets

Sediment basins shall be designed with a principal and an overflow spillway, whose size and capacity is dependent upon the drainage area served by the practice. At a minimum, however, principal spillways must be capable of handling the runoff from the 10-year, 24-hour storm event, while overflow spillways must be able to safely pass the 100-year, 24-hour storm event.

Principal spillways, which provide the main outlet for the basin, consist of a riser and an outlet pipe. Risers control the volume of water present in the basin with a dewatering outlet, which can be designed in many ways to meet the needs of each site. The riser empties into the outlet pipe, which carries the water through the embankment structure to a stable outlet, where it is discharged. Risers and outlet pipes may be constructed from a variety of materials, such as PVC, corrugated metal, or concrete. Regardless of the material selected, all seams and joints should be sealed and watertight.

Outlets must be held in place with a stable base, constructed of either steel or reinforced concrete. For sediment basins with risers less than 10 feet tall, the base must be at least twice as wide as the riser. Steel bases shall be at least ¼ inch thick and be overlain with at least 2 feet of compacted earth or stone. Concrete bases must be at least 18 inches thick and have the riser set in at least 9 inches. Sediment basins with risers that exceed heights of 10 feet require specific flotation calculations be performed using a safety factor of at least 1.2.

Overflow spillways provide a safe outlet during very large storm events and prevent possible damage to the structure. They shall be designed trapezoidal in shape with a minimum bottom width of 8 feet. In addition, overflow spillways shall be at least 25 feet long and discharge at non-erosive velocities.

Spillways must discharge to a stable outlet at non-erosive velocities. Outlets used for sediment basins shall follow the specifications discussed in the Stone Outlet Protection page.

Construction

  • Sediment basins must be constructed and functional before any land upslope of the practice is disturbed
  • Sediment basins should be constructed near the lowest point, near the edge of the site, to maximize the area served by the practice
  • Sediment basins should be constructed with access for maintenance activities (such as clean out and disposal of accumulated sediment)
  • The embankments must be constructed upon scarified ground (to a depth of 6 inches) and may not be built on frozen ground
  • Sediment basins should be removed after the site has been stabilized and permanent BMPs have been established

Maintenance

  • The entire structure should be inspected weekly for damage and signs of erosion, with necessary repairs made immediately
  • Accumulated sediment must be removed when it reaches ½ of the basin storage capacity

Method to Determine Practice Efficiency

Sediment basins reduce the flow velocity of runoff, allowing suspended particles to settle out. The efficiency for this practice is dependent upon the size of the basin, the size of the drainage area, and other site characteristics. As a result, the efficiency for this practice must be calculated using factors unique to each site. For more information, please refer to the the Basin Efficiency page.