Detention Basin fact sheet - Dauphin County

1 Best Management Practices Fact SheetDetention PondsPURPOSE: Detention ponds temporarily store stormwater runoff, thereby reducingthe peak rate of runoff to a stream or storm sewer. They help to prevent localizedflooding and, if designed to do so, provide some water quality benefits and reducestreambank erosion a storm, runoff drains from impervious surfaces directly to storm sewers orwaterways. Large storm events contribute a significant volume of runoff moving at anincreased rate, which raises the potential for erosion and flooding ponds are basins that receive and hold runoff for release at a predeterminedrate, thereby reducing the peak runoff delivered to storm sewers and streams.

2 Theponds generally are earthen structures constructed either by impoundment of anatural depression or excavation of existing soil. Detention ponds are designed torelease all captured runoff over time, and do not allow for permanent pooling of runoff is released through multi-level outlet structures consisting of weirs,risers, orifices or pipes, which provide for increased discharge as water levels in thebasin increase (see reverse side). Detention ponds are generally ineffective at removing pollutants in runoff because theydo not provide adequate holding time for solids to settle before water is released intoa stream or storm sewer system. However, extending the Detention time of the basinand/or including a forebay to the Basin in the design when space allows will enhancewater quality and quantity benefits.

3 Extended Detention will require a larger trap sediment to pretreat runoff prior to release to the main pond, and alsoprovide additional temporary storage of areas where thermal impacts to receiving waters, such as trout streams, are aconsideration, extended Detention times should be shortened. Typical extendeddetention time is 24 hours but may be longer. Time should be reduced to 12 hours ifthermal impacts are an to their ability to contain a substantial volume of runoff, Detention ponds aresuited for placement at all sites, including large sites. Detention basins are mostfrequently used at sites where other stormwater BMPs do not apply or are not effective.

4 Suitable for capturing runoff from a drainagearea of at least five acres Inflow and discharge hydrographs should becalculated for each selected design storm Location of Basin should be down gradient ofdisturbed/developed areas on site Construction on or near steep slopes ormodifying existing slope is not recommended Planting of native vegetation on floor of basinand embankments is recommended Floor of Basin should be at least two feetabove high water table Design for maximum water depth of 10 feet Design for length to width ratio of 2:1, minimumwidth of 10 feet; side slope ratio no greater than3:1, maximum height of side embankments lessthan 15 feet Site placement should be at least 10 feet fromproperty line and 50 feet from private well orseptic system to address water quality concerns Forebay for should contain 10% to 15% of totalpool volume Compaction of Basin bottom should be avoided Outlet structures must be resistant to corrosion andclogging by debris.

5 Sediment and plant materialGeneral Design ConsiderationsAdditional ResourcesPA Department of Environmental Protection- Pennsylvania Stormwater Best ManagementPractices ManualUS Environmental Protection for Watershed Manager s Resource Council of Minnesota EnvironmentalServices - Urban Small Sites BMP and click on EnvironmentalServices to find the link to the manualBenefits Reduces peak rate of runoff Alleviates flooding Cost effective Can be designed to address water quality Space surrounding pond can be landscaped toenhance aesthetics and provide habitat forwildlifeA riser is an outflow structure used in conjunction with adetention Basin for the purpose of maintaining a givenamount of flow released from the Detention Basin to astream or storm sewer system.

6 Drainage holes placedalong the height of the riser connect to a drain under thedetention Basin that has an outlet to a stream or stormdrain. When the water level in the Detention Basin reachesthe height of the entry holes of the riser, the water entersthe holes and flows into the drainage pipe for release fromthe stone placed near the outlet point absorbs theenergy of the water being released from the riser, whichhelps decrease runoff velocity and maintain an acceptablerate of runoff to the stream or storm RETAINEDINFLOWRIP-RAPPILOT CHANNELAQUATICBENCHOUTFALLMICROPOOLMAINT ENANCE ACCESS TO MICROPOOLSEDIMENTFOREBAYEMERGENCYSPILLWA YMAXIMUM ELEVATIONOF EXTENDED DETENTIONPOOLMAXIMUM ELEVATIONOF SAFETY STORMHow a Riser Outflow Structure WorksHow a Riser Outflow Structure WorksHow a Riser Outflow Structure WorksHow a Riser Outflow Structure WorksHow a Riser Outflow Structure WorksDetention Basin Overhead ProfileEMERGENCYSPILLWAYSTABLEOUTFALLANT I-SEEP COLLAROR FILTER DIAPHRAGMINFLOWRISEREMBANKMENTHOODBARREL PIPE

7 FOREBAYMICROPOOLPONDING AREAD etention Basin Side ProfileDetention Basins Featured in the BMP TourTwo types of Detention basins are featured in DCCD s stormwaterBMP tour. Each is designed to illustrate how a Detention Basin canbe incorporated with site first Basin (#7 on the tour) uses a riser structure to aid withregulating the volume of runoff released from the site. The secondbasin (#10) takes advantage of the site s natural that the contour of the land makes a natural depressionsuitable for detaining water. Surrounding existing vegetation will aidwith absorption of water through plants roots.

8 A rock outfall hasbeen added near the Basin inlets to reduce the energy of runoffentering the at DCCD Tour Site #7 Basin at DCCD Tour Site #10 Diagram courtesy of Maryland BMP ManualDiagram courtesy of Maryland BMP ManualDetention vs. RetentionWhat s the Difference?At times, the terms Detention and retentionseem to be used interchangeably - bothare designed to capture and store runoffand they appear to be similar in , they differ in is defined as maintainingposession of; holding in a particularplace ; Detention refers to delaying anaction. For stormwater objectives, thisrefers to the amount of time water is leftto stand in the ponds, whose primary functionis to delay the release of runoff tostreams, are designed for the eventualrelease of water.

9 Retention basins aredesigned to allow sediment and pollutantsto settle out of the water after it iscaptured through use of a permanentpool. Additional water received from astorm remains in the Basin until it infiltratesinto the ground or it reaches a depth thatflos into a pipe or overflow structure. As itinfiltrates, the water is filtered through thesoil for removal of pollutants.