Extended Dry Detention Basin

From the Massachusetts Stormwater Handbook

Image of Extended Dry Detention Basin


Extended dry detention basins are modified conventional dry detention basins, designed to hold stormwater for at least 24 hours to allow solids to settle and to reduce local and downstream flooding. Extended dry detention basins may be designed with either a fixed or adjustable outflow device. Pretreatment is a fundamental design component of an extended dry detention basin to reduce the potential for clogging. Other components such as a micropool or shallow marsh may be added to enhance pollutant removal.

Ability to Meet Massachusetts Stormwater Management Standards

2 - Peak FlowWith proper design can provide peak flow attenuation.
3 - RechargeProvides no groundwater recharge.
4 - TSS RemovalWhen combined with sediment forebay provides 50% TSS removal.
5 - Higher Pollutant LoadingMay be used as treatment BMP provided basin bottom is lined and sealed. For some land uses with higher potential pollutant loads, may also need oil grit separator, sand filter, lined bioretention area, or equivalent prior to discharge to extended dry detention basin.
6 - Discharges near or to Critical AreasShall not be used for discharges near or to critical areas
7 - RedevelopmentExisting dry detention basins may be retrofitted to become extended dry detention basins


  • Least costly BMP that controls both stormwater quantity and quality.
  • Good retrofitting option for existing basins.
  • Can remove significant levels of sediment and absorbed pollutants.
  • Potential for beneficial terrestrial and aquatic habitat.
  • Less potential for hazards than deeper permanent pools.


  • Infiltration and groundwater recharge is negligible, resulting in minimal runoff volume reduction.
  • Removal of soluble pollutants is minimal.
  • Requires relatively large land area.
  • Moderate to high maintenance requirements.
  • Potential contributor to downstream warming.
  • Sediment can be resuspended after large storms if not removed.

Pollutant Removal Efficiencies

  • Total Suspended Solids (TSS) 50% provided it is combined with sediment forebay or equivalent
  • Total Nitrogen 15% to 50%
  • Total Phosphorus 10% to 30%
  • Metals (copper, lead, zinc, cadmium) 30% to 50%
  • Pathogens (coliform, e coli) Less than 10%


Inspect extended dry detention basinsAt least twice a year and during and after major storms.
Examine the outlet structure for evidence of clogging or outflow release velocities that are greater than design flow.At least twice a year.
Mow the upper-stage, side slopes, embankment, and emergency spillway.At least twice a year.
Remove trash and debris.At least twice a year.
Remove sediment from the basin.At least once every 5 years.

Special Features

Design extended dry detention basins with two distinct stages; stage one should have the capacity to regulate peak flow rates of large, infrequent storms (10, 25, or 100-year recurrence intervals). Design the lower stages of the basin to detain the 2-year storm for at least 24 hours to remove pollutants from the runoff

LID Alternatives

Bioretention Areas

Decentralized stormwater management system that directs stormwater runoff from different sections of the site to small bioretention areas distributed throughout the site.


Generally, extended dry detention basins are not practical if the contributing watershed area is less than ten acres. MassDEP recommends four acres of drainage area for each acre-foot of storage in the basin. Extended dry detention basins can be used at residential, commercial and industrial sites.

Because they have a limited capability for removing soluble pollutants, extended dry detention basins are more suitable for commercial applications where there are high loadings of sediment, metals and hydrocarbons. Do not use extended dry detention basins by themselves in low-density residential areas, where soluble nutrients from pesticides and fertilizers may be a concern. Combine extended dry detention basins with a shallow marsh system or other BMPs for more efficient pollutant removal.

Existing dry detention basins can be retrofitted as extended dry detention basins at a relatively low cost by simply modifying the outlet structure. Because of the land requirements, extended dry detention basins are not feasible at sites where land costs or space is at a premium. Investigate soils, depth to bedrock, and depth to water table before designing an extended dry detention basin for a site.

Sites where bedrock is close to the surface can significantly increase excavation costs and make extended dry detention basins infeasible. If on-site soils are relatively impermeable, such as soil group D (as defined by the NRCS), problems with standing water may arise. In this case, using a wet basin may be more appropriate. A water table within two feet of the bottom of the extended dry detention basin can also create problems with standing water. On the other hand, if soils are highly permeable, such as well-drained sandy and gravelly soils (NRCS Soil Group A), it will be difficult to establish the shallow marsh component in the basin.


The primary pollutant removal mechanism in extended dry detention basins is settling; therefore, the degree of pollutant removal depends on whether the pollutant is in the particulate or dissolved form. Expect limited removal for soluble pollutants, but high removal rates for particulate pollutants. Enhanced removal of soluble pollutants in the lower stage of the basin can occur by natural biological removal processes if it is maintained as a shallow wetland. The degree of removal by such wetlands depends on the wetland’s size in relation to its loading. When designed properly, extended dry detention basins are effective in reducing pollutant loads and controlling post-development peak discharge rates. Extended dry detention basins may be used to meet Stormwater Management Standards 2 and 4. However extended dry detention basins do little to reduce post-development increases in runoff volume or maintain recharge.

Planning Considerations

Check the soils, depth to bedrock and depth to water table before designing an extended dry detention basin. Where bedrock is close to the surface, high excavation costs may make extended dry detention basins infeasible. If soils on-site are relatively impermeable, or the water table is within two feet of the bottom of the basin, the basin may experience problems with standing water. If soils are highly permeable, it will be difficult to establish a shallow marsh component in the basin, unless a liner is used. Maximum depth of the extended dry detention basin may range from 3 to 12 feet. The depth of the basin may be limited by groundwater conditions or by soils.

Construct extended dry detention basins above the normal groundwater elevation (i.e. the bottom of the basin should not intercept groundwater). If runoff is from a land use with a higher potential pollutant load, provide adequate pretreatment and a greater separation between the bottom of the basin and the seasonal high groundwater table.

To be effective in reducing peak runoff rates, the extended dry detention basin is ordinarily located where it can intercept most of the runoff from the site, usually at the lowest elevation of the site where freshwater wetlands are frequently found. Like all other best management practices, extended dry detention basins may 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 areas. Select a location that will not adversely affect wetland resource areas but will still provide the peak rate attenuation required by Standard 2. 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 state Office of Dam Safety and are subject to regulation.


[See the following document for complete design references: Design of Stormwater Pond Systems. 1996. Schueler. Center for Watershed Protection.]

Extended dry detention basin design must accommodate large, infrequent storm events for runoff quantity control, as well as small, frequent storm events for runoff quality control. Typically, the first flush of runoff contains the highest concentrations of pollutants. Consequently, design the extended dry detention basin to maximize the detention time for the most frequent storms. Routing calculations for a range of storms should provide the designer with the optimal basin size.

Generally, most particulates settle within the first 12 hours of detention; however, finer particulates may require additional time to settle. The minimum detention time for the Water Quality Volume is 24-hours. The most traditional and easiest method for extended detention routing is the 24 hour brimfull draw down (Required Water Quality Volume/24 hours = Qavg). This sets the average discharge rate. An orifice is then sized based on a max Q = 2*Qavg, using the brimfull head (Qmax = (CA(2gh)1/2) where h is the head when the basin is full to the Required Water Quality Volume (WQv) elevation, g is acceleration due to gravity, A is the net opening area, and C is the orifice coefficient. The orifice coefficient is determined by consulting tables in standard references such as the Civil Engineering Reference Manual for the PE Exam, 10th Edition, by Michael R. Lindeburg, P.E., 2006.

The critical parameters in sizing an extended dry detention basin are storage capacity and the maximum rate of runoff released from the basin. To meet the requirements of Standard 2, design the storage volume to hold the pre-development peak flow.

To maximize sedimentation, design the extended dry detention basin to lengthen the flow path, thereby increasing detention time. To maximize the detention time, locate the inflow points as far from the outlet structure as possible. Long, narrow configurations with length to width ratios of 2:1 provide better removal efficiencies than small deep basins. Consider using internal berms and other baffles to minimize short-circuiting of flows and increase detention times.

Reducing inflow velocities lengthens detention times, enhances sedimentation of solids in incoming runoff, and minimizes the potential for resuspension of settled pollutants. Design all inflow points with riprap or other energy dissipators, such as a baffle below the inflow structure. MassDEP requires a sediment forebay to enhance the removal rates of particulates, decrease the velocity of incoming runoff, and reduce the potential for failure due to clogging.

Design sediment forebays for ease of maintenance. Hard bottom forebays make sediment removal easier. All forebays must be accessible for maintenance by heavy machinery, if necessary.

A low flow channel routes the last remaining runoff, dry weather flow and groundwater to the outlet, which should be installed in the upper stage of the basin to ensure that the extended dry detention basin dries out completely. The maximum flow velocity (which should be set at the 2-year peak discharge rate) depends on the nature of the material used to line the channel. Consider whether a pervious or impervious channel lining is most appropriate.

Pervious linings allow runoff to interact with soil and grass, thereby increasing the sorption of pollutants. Make design velocities in pervious low flow channels high enough to prevent sedimentation but low enough to prevent scouring and erosion.

Impervious channels are simple to construct, easy to maintain, and empty completely after a storm event. Runoff flows and differential settling can undermine impervious channels unless constructed and maintained properly. Locate the top of the impervious channel lining at or below the level of the adjacent grassed areas to ensure thorough drainage of these areas. When designing impervious channels, take into account settlement of the lining and the adjacent areas as well as the potential for frost impacts on the lining. Provide impervious lining with broken stone foundations and weep holes. Consider the potential for erosion or scour along the edges of the lining caused by bank-full velocities. Maintain a low outflow discharge rate at the downstream end of the impervious channel to ensure sufficient treatment of runoff, which backs up and overflows onto the grassed basin bottom.

Use low flow underdrains connected to the principal outlet structure or other downstream discharge point to promote thorough drying of the channel and the basin bottom. Take into account the depth of the low flow channel when preparing the final bottom grading plan. Establish wetland vegetation in a shallow marsh component or on an aquatic bench in the lower stage of the extended dry detention basin to enhance removal of soluble nutrients, increase sediment trapping, prevent sediment resuspension, and provide wildlife and waterfowl habitat. Proper soils and surface depth or groundwater depth are needed to maintain wetland vegetation.

Make the side slopes of the extended dry detention basin no steeper than 3:1, and use intermittent benches to foster vegetative growth and provide for safety. Flatter slopes help to prevent bank erosion during larger storms, make routine bank maintenance tasks (such as mowing) easier, prevent animals from getting trapped, and allow easier access to the basin. Include a multi-stage outlet structure to provide an adequate level of water quality and flood control. To meet the water quantity control standards, use the required design storm runoff rates as the outlet release rates. For water quality control, the release rate will vary with the design storm selected. For extended dry detention basins with shallow marshes or permanent pools, place the lowest stage outlet at an elevation that will create a permanent pool of water.

The type of outlet structure needed will depend on factors such as the type of spillway, basin configuration and extended detention outflow rate. Design the outlet to control the outflow rate without clogging. Locate the outlet structure in the embankment for maintenance, access, safety and aesthetics. Design the outlet to facilitate maintenance; the vital parts of the structure must be accessible during normal maintenance and emergency situations. It also must contain a draw- down valve for complete detention basin draining within 24 hours.

To prevent scour at the outlet, use 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 in accordance with the state regulations for Dam Safety (302 CMR 10.00). All extended dry detention basins must have an emergency spillway capable of bypassing runoff from large storms without damaging the impounding structure.

Provide a public or private right-of-way access for maintenance that is at least 15 feet wide with a maximum slope of 5:1. Make sure this access extends to the forebay, safety bench, and outflow structure, and never crosses the emergency spillway, unless the spillway has been designed for that purpose. Use vegetative buffers around the perimeter of the basin for erosion control and additional sediment and nutrient removal.


Inspect extended dry detention basins at least once per year to ensure that the basins are operating as intended. Inspect extended dry detention basins during and after major storms to determine if the basin is meeting the expected detention times. Examine the outlet structure for evidence of clogging or outflow release velocities that are greater than design flow. Potential problems that should be checked 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; and erosion within the basin and banks. Make any necessary repairs immediately. During inspections, note any changes to the extended dry detention basin or the contributing watershed, because these could affect basin performance.

Mow the upper-stage, side slopes, embankment, and emergency spillway at least twice per year. Also remove trash and debris at this time. Remove sediment from the extended dry detention basin as necessary, but at least once every 5 years. Providing an on-site sediment disposal area will reduce the overall sediment removal costs.