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Minnesota Stormwater Manual

Overview for dry swale (grass swale)
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photo of a dry swale
Photo of a dry swale. Courtesy of Limnotech.
Green Infrastructure: Swales can be an important tool for retention and detention of stormwater runoff. Because they utilize vegetation, swales provide additional benefits, including cleaner air, carbon sequestration, improved biological habitat, and aesthetic value.

Dry swales, sometimes called grass swales, are similar to bioretention cells but are configured as shallow, linear channels. Dry swales function primarily as a conveyance BMP, but provide treatment of stormwater runoff, particularly when used in tandem with check dams that temporarily retain water in a series of cells. Dry swales are designed to prevent standing water. The filter bed consists of a sandy loam subsoil or engineered soil media mix, which allows the water to filter through and remove some pollutants. Dry swale design may also include an underdrain that carries away water in the drain pipe after it has filtered through the base media. Dry swales may thus be constructed as filtration or infiltration practices, depending on soils. If soils are highly permeable (A or B soils), runoff infiltrates into underlying soils. In less permeable soils, runoff is treated by engineered soil media and flows into an underdrain. Dry swales typically have vegetative cover such as turf or native perennial grasses.

In the following discussion, swales are identified as filtration/infiltration practices. Swales are considered filtration practices when an underdrain is employed and infiltration practices when an underdrain is absent. Check dams may be utilized for either configuration. Engineered media is typically utilized in filtration swales and not utilized for infiltration swales.

Function within the treatment train

Dry swales may be located at the end of the treatment train, the main form of conveyance between or out of BMPs, or designed as off-line configurations where the water quality volume is diverted to the filtration/infiltration practice. In any case, the practice may be applied as part of a stormwater management system to achieve one or more of the following objectives:

  • reduce stormwater pollutants (filtration/infiltration practices)
  • increase groundwater recharge (infiltration practices)
  • decrease runoff peak flow rates (filtration/infiltration practices)
  • decrease the volume of stormwater runoff (infiltration practices)
  • preserve base flow in streams (infiltration practices)
  • reduce thermal impacts of runoff (filtration/infiltration practices)

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 land is disturbed and1 acre of new impervious area is being created, and the permit stipulates certain standards for various categories of stormwater management practices.

For regulatory purposes, dry swales 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 dry 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 dry swale (grass 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 dry swale, but not specifically required by the MPCA CGP. RECOMMENDED: Indicates design guidance that is helpful for dry swale performance but not critical to the design.

There are situations, particularly retrofit projects, in which a dry 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 dry swale, depending on where it is situated both jurisdictionally and within the surrounding landscape.

Retrofit suitability

The use of dry swales as a retrofit practice primarily depends on existing infrastructure and the compatibility of existing storm drain inverts that need to connect to the dry swale outflow. In general, four to six feet of elevation above the existing collection system invert is needed for dry swale retrofits (2 to 3 feet is needed for perimeter filters).

Special receiving waters suitability

The following table provides guidance regarding the use of dry 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. Note that the suitability of a dry swale depends on whether the practice has an underdrain (i.e. filtration vs. infiltration practice).

The following tables provide guidance regarding the use of dry 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. Note that the filtration table applies to dry swales with underdrains and the infiltration table to swales with no underdrain.

Design restrictions for special waters Infiltration BMP design restrictions for special watersheds

Cold climate suitability

Dry swales should remain effective water quality improvement systems for many years, even during winter conditions, if designed and constructed properly. It has been shown that hydraulic efficiency and infiltration rates can remain at levels used for design sizing. However, in cold climates, some special considerations are HIGHLY RECOMMENDED for surface systems like dry swales to ensure sustained functionality and limit the damage freezing temperatures and snow and ice removal may cause. One concern with dry swales in cold weather is the ice that forms both over the top of the facility and within the soil interstices. To avoid these problems to the extent possible, it is HIGHLY RECOMMENDED that the facility be actively managed to keep it dry before it freezes in the late fall. This can be done by various methods, including limiting inflow, under-drainage, and surface disking.

Even if the infiltration properties of a dry swale are marginal for snowmelt runoff during the period of deep frost in the winter, the storage available in the facility will provide water quality benefit if the facility is dry entering the melt season. Routing the first highly-soluble portions of snowmelt (first flush) to a dry swale provides the opportunity for soil treatment (such as filtration, adsorption, microbial activity) of these soluble pollutants. However, flow originating in an industrial area, a high traffic area where large amounts of salt are added, or another potential stormwater hotspot (PSH) should be diverted away from dry swales if pretreatment features have not been properly designed to handle such an increase in loading.

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. Dry swales cannot be used for significant snow storage areas as debris build-up, plant damage, and lower infiltration rates 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.

Excessive deicing agents have the potential could lead to reduced soil infiltration rates (from excess sodium) or concentrations that exceed surface water or groundwater standards (from excess chloride). Locations such as busy intersections on slopes, parking garage ramps, or walkways near the entrances of commercial buildings are likely to be heavily treated with deicing agents. This should be taken into consideration when siting any dry swale.

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

Where a project’s ultimate development replaces vegetation and/or other pervious surfaces with one (1) or more acres of cumulative impervious surface, the Permittee(s) must design the project so that the water quality volume (Vwq) of one (1) inch of runoff from the new impervious surfaces created by the project is retained on site (i.e. infiltration or other volume reduction practices) and not discharged to a surface water. If the water quality volume cannot be retained due to site constraints, a portion of the water quality volume should be retained on site to the extent that site conditions allow.

The amount of stormwater volume infiltrated depends on the design variant selected. Smaller swales should either be designed off-line using a flow diversion, or designed to safely pass large storm flows while still protecting the infiltration area. In limited cases (e.g. extremely permeable soils), these dry swales can accommodate the channel protection volume, Vcp, in either an off- or on-line configuration.

In general, supplemental stormwater practices will be necessary to satisfy channel and flood protection requirements when dry swales are used. However, these practices can help reduce detention requirements for a site through volume reduction.

Water quality treatment

Dry swales can remove a wide variety of stormwater pollutants through chemical and bacterial degradation, sorption, and filtering. Surface water load reductions are also realized by virtue of the reduction in runoff volume. Properly designed infiltration systems discussed later in this section will accommodate a design volume based on the required water quality volume. Excess water must be by-passed and diverted to another BMP so that the design infiltration occurs within 48 hours. In no case should the by-passed volume be included in the pollutant removal calculation. No pollutant removal occurs for runoff water that bypasses the practice.

Limitations

The following general limitations should be recognized when considering installation of dry swales.

  • Limited monitoring data are available and field longevity is not well documented.
  • Failure can occur due to improper siting, design, construction and maintenance.
  • Systems are susceptible to clogging by sediment and organic debris.
  • When used as an infiltration practice, there is a risk of groundwater contamination depending on subsurface conditions, land use and aquifer susceptibility.
  • They are not ideal for stormwater runoff from land uses or activities with the potential for high sediment or pollutant loads.
  • They are not recommended for areas with steep slopes. Use step pools for these situations.
  • Swales are difficult to protect from sediment-laden runoff during construction


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