If properly designed, constructed and maintained, wet basins can provide substantial wildlife and wetlands habitat. Due to the presence of the permanent wet pool, properly designed and maintained wet basins can provide significant water quality improvement across a relatively broad spectrum of constituents including dissolved nutrients. Widespread application with sufficient capture volume can provide significant control of channel erosion and enlargement caused by changes to flow frequency relationships resulting from the increase of impervious cover in a watershed.
There may be some aesthetic concerns about a facility that looks swampy. Some concern about safety when constructed where there is public access. Mosquito and midge breeding is likely to occur in wetlands. It cannot be placed on steep unstable slopes. Need for base flow or supplemental water if the water level is to be maintained. Require a relatively large footprint Depending on volume and depth, pond designs may require approval from the State Division of Safety of Dams Design and Sizing Guidelines Capture volume determined by local requirements or sized to treat 85% of the annual runoff volume. Outlet designed to discharge the capture volume over a period of 24 hours. Permanent pool volume equal to twice the water quality volume. Water depth not to exceed about 4 feet. Wetland vegetation occupying no more than 50% of surface area. Include energy dissipation in the inlet design and a sediment forebay to reduce resuspension of accumulated sediment and facilitate maintenance.
(1) A maintenance ramp should be included in the design to facilitate access to the forebay for maintenance activities and for vector surveillance and control. To facilitate vector surveillance and control activities, road access should be provided along at least one side of BMPs that are seven meters or less in width. Those BMPs that have shoreline-to-shoreline distances in excess of seven meters should have perimeter road access on both sides or be designed such that no parcel of water is greater than seven meters from the road.
(2) Pond Configuration – The wet basin should be configured as a two-stage facility with a sediment forebay and the main pool. The basins should be wedge-shaped, narrowest at the inlet and widest at the outlet. The minimum length to width ratio should be 1.5 where feasible. The depth in the center of the basin should be about 4 feet deep to prevent vegetation from encroaching on the pond open water surface.
(3) Pond Side Slopes – Side slopes of the basin should be 3:1 (H: V) or flatter for grass stabilized slopes. Slopes steeper than 3:1 should be stabilized with an appropriate slope stabilization practice.
(4) Sediment Forebay – A sediment forebay should be used to isolate gross sediments as they enter the facility and to simplify sediment removal.
Typical maintenance activities and frequencies include:
Schedule semiannual inspections for burrows, sediment accumulation, the structural integrity of the outlet, and litter accumulation.
Remove accumulated trash and debris in the basin at the middle and end of the wet season. The frequency of this activity may be altered to meet specific site conditions and aesthetic considerations.
Where permitted by the Department of Fish and Game or other agency regulations, stock wet ponds/constructed wetlands regularly with mosquitofish (Gambusia spp.) to enhance natural mosquito and midge control.
Introduce mosquito fish and maintain vegetation to assist their movements to control mosquitoes, as well as to provide access for vector inspectors. An annual vegetation harvest in summer appears to be optimum, in that it is after the bird breeding season, mosquito fish can provide the needed control until vegetation reaches late summer density, and there is
Maintain emergent and perimeter shoreline vegetation as well as site and road access to facilitate vector surveillance and control activities. Remove accumulated sediment in the forebay and regrade about every 5-7 years or when the accumulated sediment volume exceeds 10 percent of the basin volume. Sediment removal may not be required in the main pool area for as long as 20 years.