Wet Basins

From the Massachusetts Storm Water Handbook

Image of a wet basin


Wet basins use a permanent pool of water as the primary mechanism to treat stormwater. The pool allows sediments to settle (including fine sediments) and removes soluble pollutants. Wet basins must have additional dry storage capacity to control peak discharge rates. Wet basins have a moderate to high capacity to remove most urban pollutants, depending on how large the volume of the permanent pool is in relation to the runoff from the surrounding watershed.

Ability to Meet Massachusetts Stormwater Management Standards

2 - Peak FlowCan be designed to provide peak flow attenuation.
3 - RechargeProvides no groundwater recharge.
4 - TSS Removal80% TSS removal credit when combined with sediment forebay as pretreatment.
5 - Higher Pollutant LoadingMay be used as treatment BMP provided basin bottom is lined and sealed. For some land uses with higher potential pollutant load, may require pretreatment by oil grit separator, sand filter or equivalent prior to discharge to wet basin
6 - Discharges near or to Critical AreasDo not use for discharges to cold- water fisheries
7 - RedevelopmentNot usually suitable.


  • Capable of removing both solid and soluble pollutants
  • Capable of removing nutrients and metals
  • Aesthetically pleasing BMP.
  • Can increase adjacent property values when properly planned and sited.
  • Sediment generally needs to be removed less frequently than for other BMPs.
  • Can be used in retrofits


  • More costly than extended dry detention basins.
  • Larger storage volumes for the permanent pool and flood control require more land area.
  • Infiltration and groundwater recharge is minimal, so runoff volume control is negligible.
  • Moderate to high maintenance requirements.
  • Can be used to treat runoff from land uses with higher potential pollutant loads if bottom is lined and sealed.
  • Invasive species control required.

Pollutant Removal Efficiencies

  • Total Suspended Solids (TSS) 80% with sediment forebay
  • Total Nitrogen 10% to 50%
  • Total Phosphorus 30% to 70%
  • Metals (copper, lead, zinc, cadmium) 30% to 75%
  • Pathogens (coliform, e coli) 40% to 90%


Inspect wet basins to ensure they are operating as designedAt least once a year.
Mow the upper-stage, side slopes, embankment and emergency spillway.At least twice a year.
Check the sediment forebay for accumulated sediment, trash, and debris and remove it.At least twice a year.
Remove sediment from the basin.As necessary, and at least once every 10 years.

Special Features

MassDEP requires a sediment forebay as pretreatment to a wet basin.

LID Alternative

1. Design measures to reduce impervious areas, shrinking the size of the wet basin

2. Use if LID site design credits for the water quality volume requirement (Stormwater Standard 4)

3. Decentralized Stormwater Management System that uses vegetative filter strips to direct stormwater runoff to BMPs located throughout the site

Wet Basins

A wet basin may be created by constructing an embankment or excavating a pit. The primary component of a wet basin is the deep, permanent pool, but other components, such as a shallow marsh, may be added to the design (see basin/ wetland design in constructed wetlands section). MassDEP requires a sediment forebay as pretreatment to a wet basin. The sediment forebay plus the wet basin collectively are credited with an 80% TSS removal rate.

The basic operation of a wet basin allows incoming stormwater to displace the water present in the pool. This stormwater remains until displaced by runoff from another storm event. Increased retention time allows particulates, including fine sediments, to settle out of the water column. The permanent pool also serves to protect deposited sediments from resuspending during large storm events. Another advantage of wet basins is the biological activity of algae and fringe wetland vegetation, which reduces the concentration of soluble pollutants. Wet basins may be designed with a multi-stage outlet structure to control peak rate discharges from different design storms. When properly designed and maintained, wet basins can add recreation, open space, fire protection, wildlife habitat, and aesthetic values to a property.


Generally, dry weather base flow and/or large contributing drainage areas are required to maintain pool elevations. The minimum contributing drainage area must be at least 20 acres, but not more than one square mile. Sites with less than 20 acres of contributing drainage area may be suitable only if sufficient groundwater flow is available. Use wet basins at residential, commercial and industrial sites. Because wet basins remove soluble pollutants, they are ideal for sites where nutrient loadings are expected to be high. In such instances, source controls must also be implemented to further reduce nutrient loadings.

Investigate soils, depth to bedrock, and depth to water table before designing a wet basin. At sites where bedrock is close to the surface, high excavation costs may make wet ponds infeasible. If the soils on site are relatively permeable or well drained, such as a soil type in Hydrologic Group A (as defined by the Natural Resource Conservation Service), it will be difficult to maintain a permanent pool. In this situation, it may be necessary to line the bottom of the wet pond to reduce infiltration. Designing wet basins for multiple storms will provide peak rate control. In such instances, design the upper stages of wet basins to provide temporary storage of larger storms (i.e., 10, 25, and 100-year 24-hr. storms). Wet basins are generally ineffective in controlling the post-development increase in runoff volume, although some infiltration does occur, as well as evaporation in summer months.

Planning Considerations

Evaluate soils and depth to bedrock before designing a wet basin. At sites where bedrock is close to the surface, high excavation costs may make wet basins infeasible. If the soils are permeable (A and B soils), heavy drawdown of the basin may occur during dry periods. In these situations, compact the basin soils or install a liner at the bottom of the basin to minimize the potential for drawdown. Specifications for basin materials include (in order of decreasing costs):
  • 6-inch clay
  • Polyvinyl liner
  • Bentonite
  • 6 inches of silt loam or finer material
To be effective in reducing peak runoff rates, locate the basin where it can intercept most of the runoff from the site, typically a low elevation that is near freshwater wetlands. Like all stormwater best management practices, wet basins must not be constructed in wetland resource areas other than isolated land subject to flooding, bordering land subject to flooding, land subject to coastal storm flowage, and riverfront area. Select a location that can accommodate the need to attenuate peak discharge rates without adversely impacting nearby wetland resources.

It is preferable to create the wet basin by excavating a pit below the grade of land. When this is not feasible, an earthern embankment can be created. Embankments or dams created to store more than 15 acre-feet, or that are more than 6 feet high, are under the jurisdiction of the Massachusetts Department of Conservation and Recreation (DCR) Office of Dam Safety and must be constructed, inspected, and maintained according to DCR guidelines.


See the following for complete design references:

Wet Extended Detention Pond Design: Step by Step Design. 1995. Claytor.

Volume and geometry are the critical parameters in a wet basin design; the relationship of the volume in the permanent pool to the contributing runoff volume directly affects pollutant removal rates. Generally, bigger is better; however, after a certain threshold level, increasing the pool size results in only marginal increases in pollutant removal. The permanent pool must be sized at a minimum to hold twice the water quality volume (this is equivalent to a VB/VR of 2) when a wet basin is designed to provide peak rate attenuation in addition to water quality treatment.

The peak rate volume is an additional volume above the permanent pool. The permanent pool volume must not be counted as part of the volume devoted to storage associated with peak rate attenuation. When designing a wet basin to also accommodate peak rate attenuation, a multiple stage outlet must be included as part of the design.

Make the minimum contributing drainage area at least 20 acres, but no more than one square mile. Sites with less than ten acres of contributing drainage area may be suitable if sufficient groundwater flow is available to maintain the permanent wet pool.

Pool depth is an important design factor, especially for sediment deposition. Use an average pool depth of 3 to 6 feet. Settling column studies and modeling analyses show that shallow basins remove more solids than deeper ones. However, resuspension of settled materials by wind action might be a problem in shallow basins that are less than two feet deep.

Wet Basin Design Criteria

Maximum Drainage area>20 acres unless sufficient groundwater flow
Permanent Pool Volume>2 x WQv (equivalent to Vb/Vr ratio of 2)
Minimum Pool Surface Area>0.25 acres
Minimum Length to Width Ratio>3:1
Mean Permanent Pool Depth3 to 6 feet
Maximum Permanent Pool Depth8 feet
Maximum Pool Slopes<3H:1V
Maximum Safety & Aquatic Bench Slopes<2H:1V
Perimeter Accessway Width>15 feet
Perimeter Vegetative Buffer>25 feet
Sediment ForebayRequired (not included in wet basin sizing)
Pool Drain (for maintenance purposes)Required maximum pool drain time: 40 hours

Depths greater than eight feet may cause thermal stratification. Stratified pools tend to become anoxic (low or no oxygen) more often than shallower ponds. If possible, vary depths throughout the basin.

Providing deeper pools can provide fish habitat. It may be advantageous to introduce fish to the wet basins to reduce mosquito breeding. When designing wet basins to support fish, a fisheries biologist should be consulted. Fish habitat features may include trees to provide shading over the deeper depths. Selection of trees should be done carefully to avoid embankment or sidewall failure.

Use intermittent benches around the perimeter of the basin for safety and to promote vegetation. Design the safety bench to be at least ten feet wide and above normal pool elevations. Make the aquatic bench at least ten feet wide and maintain depths of 12 to 18 inches at normal elevations to support aquatic vegetation. Shallow depths near the inlet will concentrate sediment deposition in a smaller, more accessible area. Deeper depths near the outlet will yield cooler bottom water discharges that may mitigate downstream thermal effects.

Use a minimum pool surface area of 0.25 acres. Enhance the performance of the wet basin by enlarging the surface area to increase volume, instead of deepening the pool, although this increases water temperatures and evaporation rates. The original design of wet basin depths and volumes should take into account the gradual accumulation of sediment. Accumulating sediment in the pool will decrease storage volume and reduce pollutant removal efficiency.

MassDEP requires a sediment forebay to pretreat stormwater before it enters the wet basin. Forebays trap sediment before the runoff enters the primary pool, effectively enhancing removal rates and minimizing long-term operation and maintenance problems. Removing sediment from the forebay is easier and less costly than from the wet basin pool, so design sediment forebays for ease of maintenance. Hard bottom forebays make sediment removal easier. Make forebays accessible by heavy machinery to faciltate maintenance.

To avoid reducing the pollutant removal capability and to maximize travel distance, locate the inflow points as far from the outlet structure as possible. To maximize stormwater contact and retention time in the pool, use a length to width ratio of at least 3:1.

Set the invert elevation of the inlet pipe at or below the surface of the permanent pool, preferably within one foot of the pool. Pipes discharging above the pool can erode the banks and side slopes. Design all inflow points with riprap or other energy dissipators to reduce inflow velocities.

Establish wetland vegetation on the aquatic bench to enhance the removal of soluble nutrients, facilitate sediment trapping, prevent sediment resuspension, provide wildlife and waterfowl habitat, and conceal trash and debris that may accumulate near the outlet. Six to eighteen inches of water depth are needed for wetland vegetation growth.

Make the slopes of the pools no steeper than 3:1. Flatter slopes help to prevent bank erosion during larger storms and facilitate routine bank maintenance tasks, such as mowing. Flat slopes also provide for public safety, and allow easier access. In addition, design the sides of the pool that extend below the safety and aquatic benches to the bottom of the pool at a slope that will remain stable, usually no steeper than 2:1 (horizontal to vertical).

Design the invert of the wet basin outlet pipe to convey stormwater from approximately one foot below the pool surface and to discharge into the riser in the pond embankment. To prevent clogging, install trash racks or hoods on the riser.

To facilitate access for maintenance, install the riser within the embankment. Place anti-seep collars or filter and drainage diaphragms on the outlet barrel to prevent seepage and pipe failure. Make the vital parts of the structure accessible to maintenance personnel during normal and emergency conditions. Install a bottom drainpipe to allow complete draining of the wet basin in case of emergencies or for routine maintenance.

Fit both the outlet pipe and the bottom drain pipe with adjustable valves at the outer end of the outlet to permit adjustment of the detention time, if necessary.

To prevent scour at the outlet, install a flow transition structure, such as a lined apron or plunge pad, to absorb the initial impact of the flow and reduce the velocity to a level that will not erode the receiving channel or area.

Design embankments and spillways to conform with DCR Dam Safety regulations, if applicable. All wet basins must have an emergency spillway capable of bypassing runoff from large storms without damaging the impounding structure.

Provide an access way for maintenance, with a minimum width of 15 feet and a ma ximum slope of 15%, by public or private right-of-way. Equipment that will be used for maintenance must be capable of using this access-way. This access should extend to the forebay, safety bench, and outf low structure and should never cross the emergency spillway, unless the spillway has been designed for that purpose. Place vegetative buffers around the perimeter of the wet basin to control erosion and remove additional sediment and nutrients. The vegetative buffer must be at least 33 feet (10 meters). Vegetation must be designed to prevent the introduction of invasive species.


Inspect wet basins at least once per year to ensure they are operating as designed. Inspect the outlet structure for evidence of clogging or excessive outflow releases. Potential problems to check include: subsidence, erosion, cracking or tree growth on the embankment, damage to the emergency spillway, sediment accumulation around the outlet, inadequacy of the inlet/outlet channel erosion control measures, changes in the condition of the pilot channel, erosion within the basin and banks, and the emergence of invasive species. Make any necessary repairs immediately. During inspections, note any changes to the wet basin or the contributing watershed area because these may affect basin performance. At least twice a year, mow the upper-stage, side slopes, embankment and emergency spillway. At this time, also check the sediment forebay for accumulated material, sediment, trash, and debris and remove it. Remove sediment from the basin as necessary, and at least once every 10 years. Providing an on on-site sediment disposal area will reduce the overall sediment removal costs.


Galli, J. 1990, Thermal Impacts Associated with Urbanization and Stormwater Best Management Practices. Prepared for the

Maryland Department of Environment, Baltimore, MD, by the Metropolitan Council of Governments, Washington, D.C.