GeneralA dry basin temporarily retains stormwater and gradually releases it to a conveyance structure (refer to Grassed Swale, Lined Waterway or Outlet, or Subsurface Drain). Its purpose is to reduce stormwater peak flow rates and trap sediment particles. By trapping sediment, associated pollutants are also removed. Dry basins are designed to drain completely within 48 hours of the storm event.
Dry basins are often utilized in thermally sensitive watersheds, as they do not increase the temperature of the runoff. These structures also provide only limited pollutant removal and accumulated sediment is often resuspended by subsequent storm events. As a result, dry basins should be used in conjunction with other management practices.
- Reduces peak flows
- Able to be used in thermally sensitive areas
- Requires a relatively large land area
- Generally not practical in areas where the drainage area is less than 10 acres
- Accumulated sediments may be resuspended if not removed between storm events
ForebaysForebays are small sediment traps required at all basin inlets. Forebays, which are separated from the rest of the basin by a berm, receive runoff and prevent concentrated flow from entering the basin, allowing sediment to settle out before reaching the main basin. They simplify maintenance by concentrating sediments and extending the holding capacity and life of the basin.
VolumeEach basin should be designed according to the individual characteristics of the site. The basins should be sized to achieve the sediment reduction goal and safely pass the 100-year storm event. Refer to the NRCS website (www.nrcs.usda.gov) for the most up-to-date NRCS standards.
Shape and SlopeDry basins should be designed with a length to width ratio of at least 3:1 in a shape that increases detention time, such as a long, narrow shape or a teardrop shape. If these shapes are not feasible, baffles should be installed to increase the flow path. The bottom of the basin should be sloped towards the outlet to ensure proper drainage and prevent standing water. Forebays should have an area equal to 10-25% of the basin’s surface area, with a length to width ratio of at least 2:1 to provide proper flow.
The side slopes of the basin should not exceed a 4:1 ratio and should not be less than 10:1. Slopes in this range prevent excessive erosion and makes maintenance tasks both easier and safer and provide adequate drainage. The banks of the basin should not exceed a height of 20 feet and should be overbuilt by at least 10 percent to allow for settling and subsidence (should be consistent with the NRCS standards). To prevent the erosion of the structure, the banks and the bottom of the basin should be seeded with vegetation that is tolerant of inundation or should be lined with stone (refer to Temporary Seeding, Native Plants, or Lined Waterway or Outlet).
OutletsThe basin outfall must be designed to handle the structure’s peak flow rate and must discharge to a stable outlet (refer to Stone Outlet Protection). Outlet structures may incorporate a perforated riser or gabion basket and should be resistant to clogging. They may incorporate trash racks, skimmers, or other devices. Outlets should be designed with stability in mind and should be able to endure frost heave and settling.
Dry basins must also incorporate an emergency spillway into the design of the structure to safely pass flows that exceed the design capacity of the basin. Emergency spillways prevent large flows from overwhelming the capacity of the basin without causing damage to the outfall structure and should discharge to a stable outlet (refer to Stone Outlet Protection, or Lined Waterway or Outlet).
- A dry basin may be used during construction to treat site runoff, but must be restored to its original design volume and depth after construction is complete
- Trash and other debris should be removed regularly to prevent clogging
- Dry basins should be inspected at least twice a year to ensure they are operating properly and to check for any potential problems, such as: sediment accumulation, subsidence, erosion, damage to the emergency spillway, and woody vegetation
- Accumulated sediment should be removed from the basin as necessary