This section provides an overview of stormwater wet ponds. It includes a discussion of permit applicability, function within the treatment train, cold climate and retrofit suitability, and role in water quality and quantity treatment.
Stormwater ponds are typically installed as an end-of-pipe BMP at the downstream end of the treatment train. Stormwater pond size and outflow regulation requirements can be significantly reduced with the use of additional upstream BMPs. However, due to their size and versatility, stormwater ponds are often the only management practice employed at a site and therefore must be designed to provide adequate water quality and water quantity treatment for all regulated storms.
One of the goals of this manual is to facilitate understanding of and compliance with the MPCA Construction General Permit(CGP), which includes design and performance standards for permanent stormwater management systems. These standards must be applied in all projects in which at least one acre of new impervious area (or common area of development) is being created, and the permit stipulates certain standards for various categories of stormwater management practices.
For regulatory purposes, stormwater ponds fall under the category Wet Sedimentation Basin described in the CGP. If used in combination with other practices, credit (stormwater credit) for combined stormwater treatment can be given. Due to the statewide prevalence of the MPCA CGP, design guidance in this section is presented with the assumption that the permit does apply. Also, although it is expected that in many cases the pond will be used in combination with other practices, standards are described for the case in which it is a stand-alone practice.
Of course, there are situations, particularly retrofit projects, in which a stormwater pond is constructed without being subject to the conditions of the MPCA CGP. While compliance with the permit is not required in these cases, the standards it establishes can provide valuable design guidance to the user. It is also important to note that additional and potentially more stringent design requirements may apply for a particular pond, depending on where it is situated both jurisdictionally and within the surrounding landscape.
Ponds are widely used for stormwater retrofits and have two primary applications as a retrofit design. In communities where dry detention ponds (see Types of stormwater ponds) were designed for flood control in the past, these facilities can be modified by adding a permanent pool for water quality treatment and adapting the outlet structure for channel protection. This is desirable because dry ponds have limited effectiveness for pollutant removal. Alternatively, new ponds can be installed in available open areas as a part of a comprehensive watershed retrofit inventory.
Note that the MPCA CGP permanent pool specifications do not apply to retrofit ponds that serve an existing developed area unless new impervious acreage occurs as part of the retrofit project. Therefore, any of the aforementioned pond variants may be considered, along with other alternative approaches to treatment basin design.
The table below provides guidance regarding the use of stormwater ponds in areas upstream of special special receiving waters.
Design restrictions for special waters - constructed ponds and wetlands
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|Constructed wetlands||Some variations NOT RECOMMENDED due to poor P removal, combined with other treatments.||NOT RECOMMENDED
except for wooded wetlands
but no use of natural wetlands
|Wet Extended Detention Pond||RECOMMENDED||Some variations NOT RECOMMENDED due to pool and stream warming concerns||RECOMMENDED||RECOMMENDED (alteration of natural wetlands as stormwater wetlands not allowed)||RECOMMENDED|
*Applies to groundwater drinking source areas only; use the sensitive lakes category to define BMP Design restrictions for surface water drinking supplies
One of the biggest problems associated with proper pond operation during cold weather is the freezing and clogging of inlet and outlet pipes. To avoid these problems, the Center for Watershed Protection (Caraco and Claytor, 1997) made some general design suggestions, which are adapted as follows.
Ponds are one of the best and most cost-effective stormwater treatment practices for providing runoff detention storage for channel protection and overbank flood control (see Unified sizing criteria). These goals are achieved with the use of extended detention storage, where runoff is stored above the permanent pool and released at a specified rate through a control structure. Wherever an embankment is constructed to store water at a level higher than the surrounding landscape, dam safety regulations must be followed to ensure that downstream property and structures are adequately protected.
Ponds are primarily detention practices and therefore do not retain significant amounts of water. There is some loss to evapotranspiration and seepage through the bottom of the pond.
Ponds rely on physical, biological, and chemical processes to remove pollutants from incoming stormwater runoff. The primary treatment mechanism is gravitational settling of particulates and their associated pollutants as stormwater runoff resides in the pond. Another mechanism for the removal of pollutants (particularly nutrients) is uptake by algae and aquatic vegetation. Volatilization and chemical activity can also occur, breaking down and assimilating a number of other stormwater contaminants such as hydrocarbons.
The longer the runoff remains in the pond, the more settling (and associated pollutant removal) and other treatment can occur, and after the particulates reach the bottom of the pond, the permanent pool protects them from resuspension when additional runoff enters the basin. For these reasons, because they lack the crucial permanent pool, dry extended detention ponds are not considered an acceptable option for meeting water quality treatment goals; however, they may be appropriate to meet water quantity criteria (Vcp, Vp10, Vp100; see Unified sizing criteria). It should again be noted that the only type of pond complying with the MPCA CGP is the wet extended detention pond (or wet sedimentation basin) constructed according to the minimum standards outlined in the permit.
The long detention or retention time associated with stormwater ponds can be problematic in coldwater fisheries due to the potential increase in water temperature. In these situations, detention times should be limited to a maximum of 12 hours or other treatment alternatives ( infiltration) should be explored.
Removal efficiencies and typical stormwater pond effluent concentrations for key pollutants for wet extended detention ponds are provided in the following two tables.
Median pollutant removal percentages for several stormwater BMPs. Sources. More detailed information and ranges of values can be found in other locations in this manual, as indicated in the table. NSD - not sufficient data. NOTE: Some filtration bmps, such as biofiltration, provide some infiltration. The values for filtration practices in this table are for filtered water.
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|Biofiltration and Tree trench/tree box with underdrain||80||link to table||link to table||link to table||50||35||95||80|
|Iron enhanced sand filter||85||65 or 746||85||40 or 606||35||80||50||80|
|Dry swale (no check dams)||68||link to table||link to table||link to table||35||80||0||80|
|Wet swale (no check dams)||35||0||0||0||15||35||35||NSD|
|Constructed wet ponds4, 5||84||50 or 685||84||8 or 485||30||60||70||80|
|Permeable pavement (with underdrain)||74||41||74||0||NSD||NSD||NSD||NSD|
|Vegetated (grass) filter||68||0||0||0||NSD||NSD||NSD||NSD|
|Harvest and reuse||Removal is 100% for captured water that is infiltrated. For water captured and routed to another practice, use the removal values for that practice.|
TSS=Total suspended solids, TP=Total phosphorus, PP=Particulate phosphorus, DP=Dissolved phosphorus, TN=Total nitrogen
1Data for metals is based on the average of data for zinc and copper
2BMPs designed to infiltrate stormwater runoff, such as infiltration basin/trench, bioinfiltration, permeable pavement with no underdrain, tree trenches with no underdrain, and BMPs with raised underdrains.
3Pollutant removal is 100 percent for the volume infiltrated, 0 for water bypassing the BMP. For filtered water, see values for other BMPs in the table.
4Dry ponds do not receive credit for volume or pollutant removal
5Removal is for Design Level 2. If an iron-enhanced pond bench is included, an additional 40 percent credit is given for dissolved phosphorus. Use the lower values if no iron bench exists and the higher value if an iron bench exists.
6Lower values are for Tier 1 design. Higher values are for Tier 2 design.
Typical pollutant concentrations leaving stormwater pond BMPs. Concentrations are in milligrams per liter (ppm). Note that a range of values, from low to high, is provided for TSS and TP
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|Practice||TSS Low-Med-High||TP Low-Med-High||TN||Cu||Zn|
The following general limitations should be recognized when considering installation of stormwater ponds. Ponds generally