This section provides an overview of stormwater 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.
Using the treatment train approach to BMP selection, 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 Part III.C.1 of the CGP. If used in combination with other practices, 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.
The following terms are thus used in the text to distinguish various levels of stormwater pond design guidance:
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 were designed for flood control in the past, these facilities can be modified by adding a permanent wet pool for water quality treatment and adapting the outlet structure for channel protection. 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.
Table 12.8.1 provides guidance regarding the use of stormwater ponds in areas upstream of special receiving waters. This table is an abbreviated version of a larger table in which other BMP groups are similarly evaluated. The corresponding information about other BMPs is presented in the respective sections of this Manual.
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 Uniform 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 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 Uniform 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 (e.g., infiltration) should be explored.
Removal efficiencies for key pollutants for wet extended detention ponds are provided in Table 12.8.2. Typical effluent concentrations for selected water quality parameters are provided in Table 12.8.3.
Table 12.8.2: Removal efficiencies for key pollutants for wet extended detention ponds. Data in the table are from Winer (2000) and have been rounded.
1 average of zinc and copper; 2 based on fewer than five data points (i.e. independent monitoring studies)
Table 12.8.3: Typical effluent concentrations for selected water quality parameters. Values are from the ASCE BMP database and from Winer (2000).
The following general limitations should be recognized when considering installation of stormwater ponds. Ponds generally: