This section provides an overview of stormwater wetlands. 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 wetlands are typically installed at the downstream end of the treatment train (they are considered an end-of-pipe BMP). Stormwater wetland size and outflow regulation requirements can be significantly reduced with the use of additional upstream BMPs. However, when a stormwater wetland is constructed, it is likely to be 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 is being created, and the permit stipulates certain standards for various categories of stormwater management practices.
For regulatory purposes, stormwater wetlands currently fall under the “Wet Sedimentation Basin” category described in Section 18 of the permit. If used in combination with other practices, credit for combined stormwater treatment can be given. Due to the statewide prevalence of the MPCA permit, 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 wetland will be used in combination with other practices, standards are described for the case in which it is a stand-alone practice. Of note, the MPCA will evaluate the need to keep stormwater wetlands under the “wet sedimentation basin” category in future CGP revisions and consider it as a bioretention system instead.
The following terms are used throughout this Manual to distinguish various levels of stormwater wetland design guidance:
Required:Indicates design standards stipulated by the MPCA Permit (or other consistently applicable regulations).
Highly recommended:Indicates design guidance that is extremely beneficial or necessary for proper functioning of the wetland, but not specifically required by the MPCA permit.
Recommended:Indicates design guidance that is helpful for stormwater wetland 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 permit. 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 stormwater wetland, depending on where it is situated both jurisdictionally and within the surrounding landscape.
As a retrofit, stormwater wetlands have the advantage of providing both educational and habitat value. One disadvantage of wetlands, however, is the difficulty in storing large amounts of runoff without consuming a large amount of land. Therefore, the most common type of wetland retrofit involves the modification of an existing dry or wet pond.
The following table provides guidance regarding the use of wetlands in areas upstream of special receiving waters.
Design restrictions for special water or other sensitive receiving watersheds.
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|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
*Applies to groundwater drinking source areas only; use the sensitive lakes category to define BMP Design restrictions for surface water drinking supplies
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.
Wetland performance can be diminished in spring months when large volumes of runoff occur in a relatively short time and carries the accumulated pollutant load from the winter months. Because stormwater wetlands are relatively shallow, freezing of the shallow pool can occur. Also, freezing of inlet and outlet structures can occur, which will reduce performance of the stormwater wetland. To avoid these problems, the Center for Watershed Protection (Caraco and Claytor, 1997) made some general design suggestions, which are adapted as follows:
Stormwater wetlands are well-suited to provide channel protection and overbank flood protection. As in ponds, this is accomplished with live storage (extended detention) above the permanent pool.
Pollutants are removed from stormwater runoff in a wetland through uptake by wetland vegetation and biota (algae, bacterial), vegetative filtering, soil adsorption, and gravitational settling in the slow moving marsh flow. Volatilization and chemical activity can also occur, breaking down and assimilating a number of other stormwater contaminants such as hydrocarbons.
Pollutant removal efficiencies and optimum effluent concentrations for selected parameters are provided in following two tables.
Percent removal of key pollutants for stormwater wetlands. Removals represent median values from Winer (2000) and are rounded.
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1. Average of zinc and copper.
2. Based on fewer than five data points (i.e., independent monitoring studies)
Typical BMP best attainable effluent concentrations. Values from ASCE BMP database and Winer 2000
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|Practice||TSS (mg/l)||TP (mg/l)||TN (mg/l)||Cu (ug/l)||Zn (ug/l)|
The following general limitations should be recognized when considering installation of stormwater wetlands: