Dry Detention Basin
Description
A dry detention basin is an impoundment or excavated basin for the short-term detention of stormwater runoff from a completed development that allows a controlled release from the structure at downstream, pre-development flow rates. Conventional dry detention basins typically control peak runoff for 2-year and 10-year 24-hour storms. They are not specifically designed to provide extended dewatering times, wet pools, or groundwater recharge. Sometimes flows can be controlled using an outlet pipe of the appropriate size but this approach typically cannot control multiple design storms.
Ability to Meet Massachusetts Stormwater Management Standards
| Standard | Description |
| 2 - Peak Flow | Provides peak flow attenuation |
| 3 - Recharge | Provides no groundwater recharge |
| 4 - TSS Removal | Does not receive any TSS removal credit |
| 5 - Higher Pollutant Loading | May be used if bottom is lined and sealed |
| 6 - Discharges near or to Critical Areas | Do not use for discharges near or to critical areas |
| 7 - Redevelopment | Not usually suitable |
Advantages/Benefits
- Controls peak runoff flows for 2-year and 10-year storms
- Low cost BMP
Disadvantages/Limitations
- Provides negligible removal of TSS compared to extended dry detention basins and wet basins.
- Provides negligible groundwater recharge.
- Frequently clogs at inlets and outlets, dramatically affecting retention times and pollutant removal efficiency.
- Cannot be used to control multiple storm events
- Susceptible to resuspension of settled materials by subsequent storms
- Requires large land area
- Cannot be used in watersheds with cold- water fisheries.
Pollutant Removal Efficiencies
- Total Suspended Solids (TSS) Does not remove TSS.
- Bacteria (coliform, e coli) Less than 10%
- Total Phosphorus 10% to 30%
- Total Nitrogen 5% to 50%
- Metals copper, lead, zinc, cadmium) 30% to 50%
Maintenance
| Activity | Frequency |
| Inspect wet basins to ensure they are operating as designed | At 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
Include a multiple stage outlet structure to control peak discharges for the 2-year and 10-year storms.
LID Alternative
Consider using a treatment train that includes vegetated filter strips or dry water quality swales and bioretention areas.
Consider decentralized stormwater management systems that direct stormwater runoff from various portions of the site to bioretention areas selectively located across the site.
Applicability
Because they have a limited capability for removing soluble pollutants, dry detention basins are used solely for water quantity control to attenuate peak flows and limit downstream flooding. Generally, dry detention basins are not practical if the contributing watershed area is less than ten acres. MassDEP recommends at least four acres of drainage area for each acre-foot of storage in the basin.
Dry detention basins may be used as part of a stormwater treatment train in combination with other treatment practices that are effective at removing TSS and providing recharge. The size of a dry detention basin can be substantially reduced if it is placed at the end of a treatment train to take advance of reduced runoff volume resulting from upstream practices that provide infiltration.
Effectiveness
Compared to extended dry detention basins or wet basins, dry detention basins have an extremely limited ability to remove TSS. A dry detention basin is designed to empty out completely in less than 24 hours, resulting in limited settling of sediments and the potential for resuspension of sediments in subsequent storms. Extended dry detention basins provide a minimum 24-hour detention time and incorporate in their design additional features aimed at enhancing pollutant removal, such as a sediment forebay, micropool, or shallow marsh.
Planning Considerations
Consider the following setback requirements when designing a detention basin:
- Distance from a septic system leach field - 50 feet.
- Distance from a septic system tank - 25 feet.
- Distance from a private well - 50 feet
- Distance from the property line -10 feet.
Investigate soils, depth to bedrock, and depth to water table at a site before designing a dry detention basin. At sites where bedrock is close to the surface, high excavation costs may make dry detention basins infeasible. If soils on site are relatively impermeable (such as Soil Group D), a dry detention basin may experience problems with standing water. In this case, building a wet basin may be more appropriate. On the other hand, if the soils are highly permeable, such as well-drained sandy and gravely soils (Soil Group A), it will be difficult to establish a shallow marsh component in the basin.
The maximum depth of dry detention basins typically ranges from 3 to 12 feet. The depth of the basin may be limited by groundwater conditions or by soils. Locate dry detention basins above the normal groundwater elevation (i.e., the basin bottom should not intercept groundwater). Investigate the effects of seepage on the basin if the basin intercepts the groundwater table.
Investigate the effects of a dry detention basin on wetland resources. Mitigate altered wetland resources according to local, state and federal regulations. Like all stormwater BMPs, dry detention basins may not be constructed in wetland resource areas except for bordering land subject to flooding, isolated land subject to flooding, land subject to coastal storm flowage, and riverfront areas. Embankments or dams that store more than 15 acre- feet or that are more than 6 feet high are regulated by the state Office of Dam Safety.
Design
The critical parameters in determining the size of the basin are the storage capacity and the maximum rate of runoff released from the basin. Design dry detention basins to store the volume required to meet the peak rate attenuation requirements of Standard 2 for the 2-year and 10-year 24-hour storms. In some cases, compliance with Standard 2 may require flood storage volume to prevent an increase in off-site flooding from the 100-year 24-hour storm.
Design a multiple stage outlet structure to control peak discharges for the 2-year and 10-year 24-hour storms. Provide an emergency spillway. Build the spillway in the existing ground--not in the embankment. Make the interior embankment slopes no greater than 3:1. To provide drainage, make the minimum slope of the bottom 2%. Provide access for maintenance. Design embankments to meet safety standards. Stabilize the earthern slopes and the bottom of the basins using seed mixes recommended by the NRCS.
[Note: for complete design references, see: Design of Stormwater Pond Systems. 1996. Schueler. Center for Watershed Protection.]
MassDEP recommends using impervious channels because they are simple to construct and easy to maintain. They can be designed to empty completely after a storm. Impervious channels can be undermined by runoff and differential settling if they are not 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 the channels, consider settlement of the lining and the adjacent areas, the potential for frost impacts on the lining and the potential for erosion or scour along the edges of the lining caused by bank-full velocities. Provide impervious linings with broken stone foundations and weep holes. Design the channel to maintain a low outflow discharge rate at the downstream end of the channel.
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. Consider the depth of the low flow channel when preparing the final bottom-grading plan.
Design dry detention basin side slopes to be no steeper than 3:1. Flatter slopes help to prevent erosion of the banks during larger storms, make routine bank maintenance tasks (such as mowing) easier, and allow access to the basin. Include a multi-stage outlet structure to provide an adequate level of flood control. To meet the water quantity control standards, use the required design storm runoff rates as outlet release rates.
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 structures should be accessible during normal maintenance and emergency situations. Include a draw-down valve to allow the dry detention basin to completely drain within 24 hours. To prevent scour at the outlet, include 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 conformance with the state regulations for Dam Safety (302 CMR 10.00). All dry detention basins must have an emergency spillway capable of bypassing runoff from large storms without damaging the impounding structure. Provide an access for maintenance by public or private right-of-way, using a minimum width of 15 feet and a maximum slope of 5:1. This access should extend to the forebay, safety bench and outflow structure, and should never cross 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.
Maintenance
It is critical to provide access for maintenance, especially to the interior of the basin. Inspect dry detention basins at least once per year to ensure that they are operating as intended. Inspect basins during and after storms to determine if the basin is meeting the expected detention times. Inspect 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 changes to the detention basin or the contributing watershed because these changes could affect basin performance. Mow the side slopes, embankment, and emergency spillway at least twice per year. Remove trash and debris at this time. Remove sediment from the basin as necessary, and at least once every 10 years or when the basin is 50% full. Provide for an on-site sediment disposal area to reduce the overall sediment removal costs.
Resources
MassHighway. Stormwater handbook for Highways and Bridges. May 2004.
T.R. Schueler. Center for Watershed Protection. Design of Stormwater Pond Systems. 1996.