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Wet Basin

General

A wet basin (wet pond) is a constructed stormwater basin that retains a permanent supply of water while also temporarily accumulating stormwater runoff. Its purpose is to reduce stormwater flow velocity and trap sediment and other associated pollutants. Stormwater enters the basin and is temporarily retained, allowing pollutants to settle out. Pollutants, such as metals, nutrients, sediment, and organic substances, are removed in wet detention basins by the settling of particulates, biological uptake, consumption, and decomposition.

Wet basins can be used in residential, commercial, and industrial areas if the contributing watershed is large enough. Generally, the watershed should be at least 10 acres to ensure a constant supply of water to maintain water depth in the basin. Due to the permanent ponding of water, wet basins can remove large amounts of pollutants and are more effective in removing plant nutrients than most other management practices. The large volume of storage in the basin helps to reduce peak discharges from storm events, which, in turn, reduces down-stream flooding while limiting streambank erosion.

Advantages

  • Capable of removing both solid and soluble pollutants
  • One of the most effective and reliable devices for removing pollutants from the stormwater
  • Wildlife habitat is created when the ponds are properly designed and maintained
  • Can be aesthetically pleasing if designed properly, which can increase adjacent property values
  • Pond sediment does not have to be manually removed as often as other management practices

Disadvantages

  • Pond requires a relatively large land area
  • Generally not practical in areas where the drainage area is less than 10 acres
  • Pond discharge usually consist of warm water, so their use may be limited in areas with temperature sensitive fisheries
  • Improperly maintained ponds may result in nuisance odors, algae blooms, and rotting debris
  • Provides a potential breeding ground for mosquitoes if not designed properly
  • Safety

Design

Forebays/Inlets

Forebays, which are separated from the rest of the basin by a wall or berm, receive runoff and prevent concentrated flow from entering the basin, allowing sediment to settle out before it reaches the basin. They also simplify maintenance by concentrating sediments and extending the holding capacity and life of the basin. 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. Forebays should be located opposite of the basin’s outlet to increase detention time.

Volume and Surface Area

Each basin should be modeled and built according to the individual characteristics of the watershed in which it is being placed, as each watershed will have a different hydrologic makeup. There is a direct correlation between the size of the basin and the pollution removal rate, the larger the basin, the greater the removal rate of sediment and pollutants from the runoff. Each basin must be designed to handle the runoff produced from the 2 and 10 year, 24-hour storm event for its watershed, and also be able to safely pass the 100-year storm event. The volume of the basin should also be sufficient to provide for the storage of accumulated sediment as well as the runoff from these storm events.

The surface area of the basin will vary as well based on location; however, a minimum area of 0.25 acres is recommended to sustain the permanent ponding of water.

Shape

Wet basins should be designed so that the previously held stormwater is replaced by the newer stormwater, a process referred to as plug flow. Plug flow allows the water to remain in the basin long enough to facilitate the settling of sediment and the adsorption of pollutants and sediment. Failure of the basin to do so is called short-circuiting. To avoid short-circuiting, the basin should be designed with a length to width ratio of 3:1 in either a long, narrow shape or a teardrop shape. These shapes encourage proper mixing of the water column and increase the amount of time the stormwater remains in the basin. In addition, these shapes lessen the amount of sediment stirring caused by wind, allowing pollutants to remain settled in the sediment. If these shapes are not feasible, structures that lengthen the flow path, such as gabions and baffles, should be installed.

Basin Slopes

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, while providing enough slope to provide adequate drainage. Submerged slopes should consist of two types: an aquatic bench and the pool slopes. Aquatic benches should extend at least ten feet from the water’s edge and have a slope with a 10:1 ratio. This shallow area promotes the growth of aquatic vegetation and also increases the safety of the basin, allowing someone who has fallen in to quickly regain his or her footing. Pool slopes are located toward the center of the basin beyond the aquatic shelf. These slopes will depend on the soil stability of the site, but in general should not exceed a 2:1 ratio, as banks are likely to become unstable at higher ratios.

Depth

Basins should be constructed with an average depth of 3-6 feet, with depths varying throughout. Shallow basins tend to remove a greater percentage of solids than deep ones. Depths greater than 10 feet may encounter low oxygen levels and thermal stratification, and are generally not recommended unless measures are taken to ensure proper oxygen content. The aquatic shelf of the basin should include shallow areas with depths ranging from 6-18 inches to promote the growth of aquatic vegetation and to improve the safety of the basin.

Sediment Control

Wet basins should remove, at a minimum, 80% of the total suspended solids from the runoff generated from the site. To increase the effectiveness and the life of wet basins, sediment forebays or other pretreatment devices may be used and are recommended. Forebays and other pretreatment devices act to slow runoff before it enters the basin, allowing pollutants and larger sediments to settle out before reaching the basin. In addition, the basin design should incorporate sediment accumulation over a period of at least 25 years to ensure the proper holding capacity over the life of the basin.

Outlets

Each basin should have 2 outlet structures, a principle outlet and a de-watering outlet. The principle outlet slowly releases the water to the receiving waters, while the dewatering outlet, which is used on a limited basis, allows the basin to be drained quickly for maintenance purposes. Both types of outlets should allow access to maintenance personnel while restricting access to the general public.

Outlets should be designed with stability in mind and should be able to endure frost heave and settling. In addition, all outlets should be designed to resist obstruction, which may be accomplished in several ways, including the use of skimmers and trash racks.

Construction

  • Wet basins may be used during construction to treat site runoff
  • Once construction is complete, the basin must be returned to its original design volume and depth

Maintenance

Maintenance costs are estimated at 3-5% of construction cost per year

  • Inspect basin at least twice a year to ensure it's operating properly and to check for any potential problems, such as: subsidence, erosion, tree growth on the embankment, sediment accumulation around the outlet, and damage to the emergency spillway. Repairs must restore the basin to the specifications of the approved plan.
  • Sediment removal is required once the average depth of the permanent pool is less than 3.5 ft, usually between every 5 and 25 years. The frequency of this event depends on the design of the basin, forebay and the occurrence of any large loading events.
  • Excavation is prohibited below the original design depth.
  • If a liner is present, it must be protected from damage during sediment removal or when the liner is undergoing repair.
  • To maximize filtration, mowing in buffer areas around stormwater ponds should be minimized. If occasional mowing is necessary, the mowing height is recommended to be no shorter than 6 inches. Applications of fertilizer, herbicide, pesticide or other chemicals are discouraged.

Method to Determine Practice Efficiency

Wet basins reduce the flow velocity of runoff, allowing suspended particles to settle out. In addition, these devices remove nutrients, heavy metals, and other pollutants from stormwater by utilizing several biological processes. The efficiency for this practice is dependant 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, refer to Appendix IV, Basin Efficiency, on page IV.1.


References