Overview for stormwater ponds

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.

Function within stormwater treatment train

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

MPCA Permit applicability

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:

• “Required:” Indicates design standards stipulated by the MPCA CGP (or other consistently applicable regulations).
• “Highly recommended:” Indicates design guidance that is extremely beneficial or necessary for proper functioning of the pond, but not specifically required by the MPCA CGP.
• “Recommended:” Indicates design guidance that is helpful for pond performance but not critical to the design.

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.

Retrofit suitability

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.

Special receiving waters suitability

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.

Table 12.8.1: Guidance regarding the use of stormwater ponds in areas upstream of special receiving waters.

BMP Group	A - Lakes	B - Trout Waters	C - Drinking water1	D - Wetlands</center>	E - Impaired waters
Wet extended detention pond	Preferred	Some variations restricted due to pool and stream warming concerns	Preferred	Preferred (alteration of natural wetlands as stormwater wetlands not allowed	Preferred

¹ applies to ground water drinking source areas only

Cold climate suitability

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:

• Inlet pipes should not be submerged, since this can result in freezing and upstream damage or flooding.
• Burying all pipes below the frost line can prevent frost heave and pipe freezing. Wind protection can also be an important consideration for pipes above the frost line. In these cases, designs modifications that have pipes “turn the corner” are helpful.
• Incorporating winter operating levels as part of the design to introduce available storage for melt events (see figure at right and Cold climate impact on runoff management).
• Increase the slope of inlet pipes to a minimum of 1% to prevent standing water in the pipe, reducing the potential for ice formation. This design may be difficult to achieve at sites with flat local slopes.
• If perforated riser pipes are used, the minimum opening diameter should be ½”. In addition, the pipe should have a minimum 8” diameter.
• When a standard weir is used, the minimum slot width should be 3", especially when the slot is tall.
• Baffle weirs can prevent ice reformation during the spring melt near the outlet by preventing surface ice from blocking the outlet structure.
• In cold climates, riser hoods should be oversized and reverse slope pipes should draw from at least 6" below the typical ice layer.
• Alternative outlet designs that have been successful include using a pipe encased in a gravel jacket set at the elevation of the aquatic bench as the control for water-quality events. This practice both avoids stream warming and serves as a non-freezing outlet.
• Trash racks should be installed at a shallow angle to prevent ice formation.

Water quantity treatment

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.

Water quality

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.

Practice	TSS (%)	Total phosphorus (%)	Total nitrogen (%)	Metals1 (%)</center>	Bacteria (%)	Hydrocarbons (%)
Wet Extended detention pond	80	40	30	60	70	802

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).

Practice	TSS (mg/L)	Total phosphorus (mg/L)	Total nitrogen (mg/L)	Metals (ug/L)</center>	Bacteria (ug/L)
Wet Extended detention pond	10	0.2	1.3	5.0	30

Limitations

The following general limitations should be recognized when considering installation of stormwater ponds. Ponds generally:

• Consume a large amount of space
• Tend to increase water temperature and may cause downstream thermal impact
• Have the potential for nuisance insects or odor
• Are problematic for areas of low relief, high water table, or near-surface bedrock
• Pose safety concerns