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Protecting Our Waters -- Detention and Retention Basins

Wet Basins | Design Considerations | Dry Basins | Infiltration Basins
Costs | Detention & Infiltration Regulations

Image of a wet detention basinBoth detention basins and infiltration basins are best used in situations where source-area controls will not be enough to accomplish established peak runoff or water-quality goals. As such, they have been shown to be highly effective in urban areas.

Because both water quality and flood control can be achieved using infiltration and detention basins, local stormwater ordinances should require use of infiltration and detention basins unless prohibited by site constraints. The ordinance should require wet detention facilities be designed in accordance with the WDNR Wet Detention Basin Technical Standard. Basin sites should be restored with native vegetation appropriate for the soil saturation conditions and level of maintenance desired. WDNR will develop a technical standard for infiltration basins.

Image of a wet detention basin near residential  homes.Long-term maintenance of detention and retention facilities has been the responsibility of the municipality, the property owner (commercial and industrial development), or the homeowner’s association (residential development) depending on the use of the basin. Many regional basins are constructed by a municipality, who then retains ownership and maintenance responsibilities.

However, some municipalities negotiate with the developer to construct a regional system, and the developer and taxpayers each pay a fair proportional share of the basin, and then give ownership to the municipality, who is then responsible for maintenance. The decision on maintenance should be made according to the annual funds available to the municipality for stormwater management and on staff availability to perform the work.

Image of a wet detention basin.Many techniques can be used to reduce the impact of increased runoff generated by urbanization including limiting the amount of impervious area and utilizing grassed swales to convey and treat runoff. There are some situations, as in the case of existing urban areas, where they don’t apply or are not effective enough. For example, it is difficult to require property owners to remove existing pavement or to find space to construct grass swale in highly urbanized areas. In these cases, detention and infiltration basins have been used to reduce the impacts of increased runoff rates and remove pollutants contained in stormwater runoff.

The WDNR has created several administrative rules that apply to stormwater management. Administrative Code Chapter NR 216 deals with stormwater discharge permits for municipalities as well as from private industry. Chapter NR 151 deals with water quality performance standards from transportation facilities and agricultural operations.

Properly designed and maintained wet detention basins can remove 80 percent of the suspended solids in urban  runoff.Wet Detention Basins -- Back to Top
Wet detention basins have a permanent pool of water year-round. The permanent pool allows pollutant particles in stormwater runoff to settle out over an extended period of time, and nutrient uptake also occurs through biological activity. (1) Wet detention basins can be used to treat runoff from a single property or can be incorporated into a regional stormwater management plan where runoff from a large area discharges into a single basin or series of basins.

Design Considerations -- Back to Top
The WDNR has developed a technical standard for the design of wet detention basins (No. 1001), which you can download in pdf format here:

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The goal of these basins is to achieve at least an 80 percent reduction in suspended solids.

The size of the wet detention basin depends on the land use and amount of impervious area draining to it (See Table 1 in Wet Detention Basin Technical Standard, page 3). A low-density residential development would require a smaller basin than a commercial area because it typically has less impervious surface and generates fewer pollutants.

Wet detention basins may be designed with a forebay at the upstream end to trap large particles such as road sand (2). Utilizing a forebay allows the majority of sediment to accumulate in a confined location, making periodic cleanout easier.

Runoff is retained in the basin by an outlet structure that controls the rate at which runoff leaves the basin. A common design of an outlet structure has a series of orifices or weirs that allow increased discharge as basin water levels increase.

The basin should have a wide, flat shelf located no more than 1 foot below the permanent water level as a safety measure in case a person or animal falls into the water. There should also be a grass access drive around the basin above the permanent water level to allow maintenance vehicles access to the basin.

Wet detention basins also have an aesthetic quality depending on the extent of landscaping done on the site. It is not always obvious that the water feature is for stormwater runoff, especially if the outlet structure is hidden.

Image of a wet basin with abundant plant growth surrounding it.In some cases, wet detention basins may have the potential to provide wildlife habitat for many species including fish, waterfowl, amphibians, and small mammals. It is extremely important, however, that accumulated sediment be removed from the basin periodically. Fifteen ponds in southern Ontario were studied in 1997 and 1998 because there were concerns that the wildlife using the ponds might be exposed to high level of contaminants. The study found that some stormwater contaminants can remain in the water column of the pond and may be toxic to wildlife living in the water. Other contaminants such as trace metals and organic compounds bind with solids that settle to the bottom of the pond as sediment. As sediment accumulates, the concentration of metal and organic contaminants could exceed levels that have toxic effects on the organisms that live or feed in the sediment. The contaminants may also accumulate in the tissues of animals living in the water or sediment and predators that consume these animals. It is therefore necessary to clean out the ponds periodically and dispose of the sediment properly. (3)

There are also concerns about geese congregating at wet detention basins. Planting tall grasses around the perimeter of the water and not mowing that area can help control geese. They can also be controlled by application of chemicals to the grass around the basin that is offensive, but not harmful, to geese.

Wet detention basins can also be constructed to provide additional storage volume above the permanent pool to reduce the quantity of runoff from a site. Many local stormwater ordinances require reduction of runoff rates and volumes from a site. Use of wet detention basins can be an effective way of meeting local quantity requirements as well as state water quality requirements.

Wet detention basins, because of a pernament pool, tend to result in higher stormwater temperatures because the water is held for a period of time before it is discharged into local waterbodies. This warm water can be detrimental to certain cold-water aquatic species such as trout.

Image of a dry detention basin near a residential area.Dry Detention Basins -- Back to Top
A dry detention basin is typically designed to store runoff volume and discharge it slowly to reduce the peak discharge downstream. As normally designed, these basins typically have little effect on the volume of stormwater released to the receiving water. The peak flow reduction is often accomplished through use of a multistage outlet structure that allows increased discharge as water levels in the basin increase, with the individual outlets being culverts or spillways. The utilization of these basins alone is typically not considered effective in removing pollutants because there is not a permanent pool of water for solids to permanently settle out and accumulate. It is common to see some sediment accumulated in the bottom of a basin after a storm; however, much of it resuspends during the next storm if it is not removed.

Image of a dry detention basinInfiltration Basins -- Back to Top
Infiltration basins have the added benefits of providing some pollutant removal and reducing the volume of stormwater released from the basin. Pollutants contained in the infiltrated runoff are absorbed by the soil particles and are considered permanently removed from the water. Infiltration basins also provide for some groundwater recharge if the soil conditions permit. In portions of the Milwaukee River Basin, soils are predominantly clay, which does not allow much infiltration. Infiltration basins should include provisions for pretreating the water to prevent premature clogging of the basin. The combination of pretreatment and infiltration removes the greatest amount of pollutants.

Pollutants contained in the infiltrated runoff are absorbed by the soil particles and are considered permanently removed from the water. Infiltration basins also provide for some groundwater recharge if the soil conditions permit.

For infiltration and dry detention basins, accumulated sediment must be periodically removed to avoid resuspension of the sediment and the resulting release of pollutants back into the discharge from the basin.

Depending on the size and configuration, dry detention basins can also be used for recreational purposes during dry weather. A dry detention area with gentle slopes can be used as a play area or incorporated into general open space.

An access road and a forebay will ease the maintenance of a wet detention basin, and a maintenance agreement will help ensure that the wet detention basin functions properly over the long term.Costs -- Back to Top
The decision to provide wet detention basin, dry detention, or infiltration basin should be based on the goal to be achieved and on-site constraints.

Wet detention basins, and sometimes infiltration basins, can be cost effective in highly urban areas if a single basin can treat a large area where using source controls are difficult or impossible, based on stormwater management analyses. The primary costs of wet detention basins and infiltration basins are the construction and land acquisition costs. Maintenance costs vary depending on the extent of landscaping and frequency of sediment removal. A guideline for sediment removal is for five-year intervals, but will vary depending on the contributing land use.

Wet detention basins are among the most cost effective and most widely used stormwater treatment practice. (2) They are used to treat stormwater runoff in existing urban areas and in newly developing areas alike.

As with wet detention basins and infiltration basins, the main costs associated with dry detention basins are construction and land acquisition or loss of development costs. Dry detention is most economical when it is incorporated into planned open space. Maintenance costs are limited to periodic mowing and sediment removal.

In some situations, there are currently matching grants available to help pay for the design and construction of stormwater BMPs that will improve runoff quality. Low interest loans are also available to pay for the local share.

Source for text in oval:
First Oval: DNR, Wet Detention Basin Technical Standard No. 1001
Second Oval: DNR, UW-Extension Publication #GWQ017, "Stormwater Ponds"

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Photos by Dave Fowler, Milwaukee Metropolitan Sewage District and Eddee Daniel, Friends of Milwaukee's Rivers

This page was created on March 26, 2003.
This page was last updated on August 26, 2003.