Overview for wet swale (wetland channel)
Wet swales occur when the water table is located very close to the surface or water does not readily drain out of the swale. A wet swale acts as a very long and linear shallow biofiltration or linear wetland treatment system. Wet swales do not provide volume reduction and have limited treatment capability. Incorporation of check dams into the design allows treatment of a portion or all of the water quality volume within a series of cells created by the check dams. Wet swales planted with emergent wetland plant species provide improved pollutant removal. Wet swales may be used as pretreatment practices. Wet swales are commonly used for drainage areas less than 5 acres in size.
Contents
Function within the treatment train
Wet swales provide limited water quality treatment and no volume control and are not recommended practices unless options for other BMPs are limited. Wet swales are designed primarily as in-line systems for stormwater quality and typically are used in conjunction with other structural controls in the stormwater treatment train. Wet swales may be used at various locations within a treatment train and can be used for pretreatment, conveyance, and/or primary treatment.
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 1 acre of new impervious area is being created, and the permit stipulates certain standards for various categories of stormwater management practices.
When volume control is constrained at a site and other BMP options (e.g. constructed pond, media filter) are not feasible, a wet swale with check dams provides treatment for a portion or all of the water quality volume stored behind the check dams. For regulatory purposes, wet swales that incorporate check dams into their design fall under the “Infiltration / Filtration" category described in Part III.D.1. of the MPCA CGP. 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. Although it is expected that in many cases the wet swale 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 wet swale 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 wet swale, but not specifically required by the MPCA CGP.
- RECOMMENDED: Indicates design guidance that is helpful for wet swale performance but not critical to the design.
There are situations, particularly retrofit projects, in which a wet swale is constructed without being subject to the conditions of the MPCA 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 important to note that additional and potentially more stringent design requirements may apply for a particular wet swale, depending on where it is situated both jurisdictionally and within the surrounding landscape.
Retrofit suitability
The use of wet swales as a retrofit practice primarily depends on existing infrastructure and whether the invert or flowline of the wet swale outlet allow meeting design requirements.
Special receiving waters suitability
The following table provides guidance regarding the use of wet swales 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.
Design restrictions for special waters
Cold climate suitability
In cold climates, some special considerations are HIGHLY RECOMMENDED for surface systems like wet swales to ensure sustained functionality and limit the damage that freezing temperatures and snow and ice removal may cause.
For all BMPs it is HIGHLY RECOMMENDED that snow and ice removal plans including predetermined locations for stockpiling be determined prior to or during the design process. Wet swales cannot be used for significant snow storage areas as debris build-up and plant damage are likely to occur. Some snow storage is unavoidable when BMPs are adjacent to areas where snow removal is required. It is critical that the property owner and snow and ice removal contractor have identified other areas for large scale snow storage.
Plant selection is critical to ensure that the damaging effects of snow and ice removal do not severely impact plantings or seedings. Even a small amount of snow storage can break and uproot plants requiring additional maintenance in the spring. Woody trees and shrubs should be selected that can tolerate some salt spray from plowing operations.
Water quantity treatment
Wet swales are not typically a primary practice for providing water quantity control. They are normally either designed off-line using a flow diversion or configured to safely pass large storm flows. In limited cases, wet swales may be able to accommodate the channel protection volume, Vcp, in either an off- or on-line configuration, and in general they can provide some (albeit limited) storage volume. Wet swales can help reduce detention requirements for a site by providing elongated flow paths and longer times of concentration, and provide very limited volumetric losses from infiltration and evapotranspiration. Generally, to meet site water quantity or peak discharge criteria, it is HIGHLY RECOMMENDED that another structural control (e.g., detention) be used.
Water quality treatment
Wet swales provide some removal of sediment and associated pollutants through filtering and settling. Less significant processes can include evaporation, infiltration, transpiration, biological and microbiological uptake, and soil adsorption. Pollutant removal data for select parameters are provided here.
Limitations
The following general limitations should be recognized when considering installation of wet swales.
- Nitrification of water may occur where aerobic conditions exist
- Wet swales offer limited water quantity control
- The potential for nuisance insects or odors exists
- Water quality performance can change seasonally
- Wet swales are impractical in steep areas, because maintaining a constant water surface elevation or pool becomes too difficult
- Wet swales are impractical in extremely flat areas, because the lack of gradient may cause excessive ponding and prevent positive drainage
- Vegetation must be periodically trimmed to keep woody vegetation in check
- A wet swale can erode during peak rainfall when water volume and velocity are high
- Standing water in wet swales may foster mosquitoes, so vector control is recommended
- Resuspension of sediment can occur during peak storm events
- Standing water causes water temperature to rise, which reduces oxygen in the water and negatively impacts nutrient removal