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Bioretention Basin


Bioretention basins are infiltration devices used for the treatment and infiltration of stormwater runoff. A bioretention basin is made up of several layers, which treat stormwater as it is filtered. These basins remove pollutants and reduce runoff volume and temperature. Bioretention basins can be used as a stand-alone method of stormwater treatment or in conjunction with other stormwater management practices.


  • Promotes infiltration of stormwater
  • Reduces pollutants in runoff
  • Decreases peak flow rates and volumes of runoff
  • Helps preserve base flow in streams
  • Reduces temperature impacts of runoff


  • Not suitable for construction site erosion control
  • Susceptible to clogging
  • Damaged by runoff with large amounts of salt-based deicers
  • Not suitable for drainage areas larger than 2 acres

Bioretention basins are best suited to treating small drainage areas adjacent to runoff source areas, such as parking lots or streets. Appropriate placement of bioretention basins is important because of the need for proper maintenance. For example, basins located in open, visible areas are more likely to be properly maintained and, in turn, provide aesthetic value. Also, bioretention basins should not be used near foundations, basements, roads, or on sites with high water tables or steep slopes. Bioretention basins are susceptible to clogging with sediment and, therefore, should not be used for erosion control during construction.


For complete design of bioretention basins, please refer to DNR Standard 1004. Bioretention basins should be designed with careful consideration given to each of the following key components: drainage area and pretreatment, ponding zone, vegetation and mulch layer, engineered soil layer, storage layer, underdrain, and sand/native soil interface layer (refer to Figure 1).

Drainage Area And Pretreatment

The maximum drainage area allowed for a bioretention basin is 2 acres and the drainage area should not contribute significant sources of sediment. To maintain flow towards the basin, slopes should not be less than 0.5% for paved areas and 1% for vegetated areas. In any case, the slopes toward the basin should not be greater than 20%.

Although not required, pretreatment options should be explored. Pretreatment is intended to reduce the initial amount of pollutants in the runoff going to the basin. Several options for pretreatment are available, including settling basins, vegetated swales, and filter strips. Pretreatment should be chosen based upon site conditions and constraints.

Ponding Zone

The ponding zone receives and holds runoff until it has an opportunity to infiltrate. The maximum ponding depth, as regulated by a weir or standpipe, may not exceed 12 inches. The drawdown time must be a maximum of 24 hours. The side slopes of the ponding area should not be steeper than a 2:1 horizontal to vertical ratio.

Vegetation and Mulch Layer

The vegetation and mulch layer is the first layer to be infiltrated by the runoff. When establishing the vegetation layer, plants or plant plugs should be used rather than seed. When choosing what type of vegetation to use, native species that are able to handle the different environmental conditions of a basin should be selected. Permeable, biodegradable, matting with pre-germinated native plants (i.e. native vegetated mat) may be used in place of plugs and mulch. Turf grass or invasive plants shall not be used to vegetate the basin.

The mulch used shall be hardwood to prevent excessive floating and be free from any foreign material. The mulch should be spread in a uniform layer 2-3 inches thick.

Engineered Soil Layer

The engineered soil layer is composed of sand, compost and topsoil. The compost must meet the Wisconsin Department of Natural Resources Specification S100 for compost. The sand must meet the specifications found in the DNR technical standard 1004 and the topsoil must be classified as a sandy loam, loamy sand or loam texture soil according to the USDA classification system. The engineered soil layer must have a minimum depth of 24 inches. If gravel is used for the storage layer, a layer of pea gravel with a 4-inch maximum depth may be used between the engineered soil layer and the gravel storage layer. The pea gravel layer can be considered part of the 24-inch soil layer and prevents the engineered soil from settling down into the storage layer.

Storage Layer

The storage layer promotes infiltration. Since infiltration is the only way water is able to exit the storage layer, it is an important component of the bioretention facility. A storage layer is necessary when the native soil has an infiltration rate of less than 3.6 inches per hour. The maximum depth of the storage area is 48 inches and must be such that the total device drain time is 72 hours or less. The storage layer may be composed of sand or clear double-washed stone of uniform size (i.e. WisDOT coarse aggregate #2).


An underdrain pipe should be placed at the top of the storage layer as a stable outlet for runoff that cannot be infiltrated as quickly as needed. The pipe must be a minimum of 6 inches in diameter and made with materials that can withstand large loads. The perforations in the underdrain should allow the pipe to drain at full capacity, while maintaining the integrity of the pipe.

In order to prevent clogging in the underdrain pipe, the pipe must be protected with either filter fabric or a filter sock. If the storage layer is sand, filter socks must be used. When a filter sock is used, the openings in the sock must be small enough to keep out sand particles, but must not restrict the flow through the perforated pipe. Another acceptable option of pipe protection is a layer of pea gravel. If pea gravel is used, it must be a layer 4 inches thick to be adequate. The pea gravel must be washed and large enough that it will not fall through the perforations in the pipe.

The underdrain must have a clean-out port that can be accessed as needed for maintenance. The underdrain must discharge to a stable outlet, such as swales or storm sewers. If it is possible for backflow to occur, a check valve should be installed.

Sand/Native Soil Interface Layer

An interface layer is necessary when the infiltration rate of the native soil is less than 3.6 inches per hour. The interface layer shall be formed by a layer of sand three inches deep, which is vertically mixed with the native soil to a depth of 2 to 4 inches.

Other Considerations

To regulate the maximum ponding depth of the basin, overflow devices such as a weir or standpipe should be installed. The discharge from these overflow devices must be directed to a stable outlet.

If the basin does not include an underdrain, observation wells must be installed to monitor the basin function. Observation wells must be positioned in the center of the area to be monitored. The maximum area served by one well is 1,000 square feet.


  • Runoff shall not be allowed in the basin until after the tributary area is stabilized
  • Construction of the basin should only occur during suitable site conditions - if construction of the basin occurs during saturated soil conditions, the soil in the device could be unnecessarily compacted
  • Compaction of the soils used for the bioretention device must be avoided - heavy equipment may not be used in the construction of the basin
  • The engineered soil shall be premixed before placement and be dry enough to prevent clumping and compaction
  • The engineered soil should be placed in several 12-inch deep lifts
  • The basin should be mulched before the planting of the vegetation in order to prevent compaction


  • Accumulated sediment in pretreatment devices should be removed as needed
  • Bioretention basins should be inspected semi-annually
  • Additional mulch should be added at least once a year and as needed to maintain 2-3 inches of cover
  • Bioretention basins should be inspected monthly for signs of erosion and sediment accumulation - all necessary repairs should be performed immediately

Method to Determine Practice Efficiency

A properly designed bioretention basin that has been sized to meet the applicable infiltration performance standard is assumed to have a sediment reduction efficiency of 80% and oil and grease removal that meets county treatment standards.

In order to determine the infiltration performance of this practice WinSLAMM, RECARGA or other approved models may be used. Additional information regarding acceptable modeling of infiltration practices is found on the Infiltration Guidance page.