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Minnesota Stormwater Manual

Design criteria for dry swale (grass swale)
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This page provides a discussion of design elements and design steps for dry swales, which are often called grass swales. The following discussion includes dry swales used as filtration or infiltration practices, with the distinction being the presence of an underdrain for filtration practices.

Green Infrastructure: Dry swales can be an important tool for retention and detention of stormwater runoff. Depending on design and construction, swales may provide additional benefits, including cleaner air, carbon sequestration, improved biological habitat, and aesthetic value.

Terminology

The following terminology is used throughout this design page.

Warning: REQUIRED - Indicates design standards stipulated by the MPCA Construction General Permit (CGP) or other consistently applicable regulations

HIGHLY RECOMMENDED - Indicates design guidance that is extremely beneficial or necessary for proper functioning of the practice, but not specifically required by the MPCA CGP.

RECOMMENDED - Indicates design guidance that is helpful for practice performance but not critical to the design.

Details and CADD images

Use this link to access .pdf diagrams of CADD drawings. To see all filtration CADD images in a combined pdf, click here.

Major design elements - Physical feasibility initial check

Before deciding to use a dry swale practice for stormwater management, it is helpful to consider several items that bear on the feasibility of using such a device at a given location. This section describes considerations in making an initial judgment as to whether or not a dry swale is the appropriate BMP for the site.

Infiltration constraints

The following links provide additional information on specific constraints to infiltration. The Construction Stormwater General Permit prohibits infiltration under certain conditions, which are summarized and discussed in detail at this link.

Contributing drainage area

The RECOMMENDED maximum drainage area is typically 5 acres, although in practicality it is difficult to control the contributing drainage area to a swale.

Site topography and slopes

Unless slope stability calculations demonstrate otherwise (see [1], [2], [3]), it is HIGHLY RECOMMENDED that swales be located a minimum horizontal distance of 200 feet from down-gradient slopes greater than 20 percent, and that slopes in contributing drainage areas be limited to 15 percent

Site location/minimum setback

schematic showing horizontal and vertical setback distances
Schematic showing some horizontal and vertical separation distances from an infiltration BMP. A separation distance may be required, such as with a drinking water well, or recommended, as with an underground tank. (Source: CDM Smith) Not to scale.

If the swale is constructed as an infiltration practice, the following table summarizes horizontal and vertical setback distances for required and recommended minimum distances from an infiltration practice to an above-ground or underground structure. It will be necessary to consult local ordinances for further guidance on siting infiltration practices.

Warning: A minimum setback of 50 feet between a dry swale and a water supply well is REQUIRED by the Minnesota Department of Health Rule 4725.4350

.

Required and recommended minimum vertical and horizontal separation distances. This represents the minimum distance from the infiltration practice to the structure of concern. If the structure is above-ground, the distance is measured from the edge of the BMP to the structure. If the structure is underground, the vertical separation distance represents the distance from the point of infiltration through the bottom of the system to the structure, while the horizontal separation (often called setback) distance is the shortest distance from the edge of the system to the structure.
Link to this table

Structure Distance (feet) Requirement or recommendation Note(s)
Vertical Saturated soil 3 Requirement1
Bedrock 3 Requirement1
Horizontal Public supply well 100 for sensitive wells; 50 for others3 Requirement
Building/structure/property line2 10 Recommended
Surface water none unless local requirements exist If nearby stream is impaired for chloride, see [4]
Septic system 35 Recommended
Contaminated soil/groundwater No specific distance. Infiltration must not mobilize contaminants.
Slope 200 Recommended from toe of slope >= 20%
Karst 1000 up-gradient 100 down-gradient Requirement1 active karst

1 Required under the Construction Stormwater General Permit
2 Minimum with slopes directed away from the building
3If treating an average of 10,000 gallons per day; otherwise separation distance is 300 feet


Depth to groundwater and bedrock

schematic illustrating separation distance from bottom of infiltration BMP to water table or top of bedrock
Schematic illustrating separation distance from bottom of infiltration BMP to water table or top of bedrock. This diagram includes a modified subsoil zone in which the subsoil has been ripped to alleviate compaction.

A separation distance of at least 3 feet is REQUIRED under the MPCA CGP between the bottom elevation of infiltration swales and the elevation of the seasonally high water table. Shallow bedrock areas should be avoided for dry swales with a minimum separation distance of 3 feet.

A field soil properties investigation is HIGHLY RECOMMENDED.

Karst topography

It is HIGHLY RECOMMENDED that infiltration practices not be used in active karst formations without adequate geotechnical testing.

Wellhead protection areas

See stormwater and wellhead protection for guidance and recommendations for determining the appropriateness of infiltrating stormwater in a Drinking Water Supply Management Area (DWSMA). For more information on source water protection see Minnesota Department of Health.

Warning: Infiltration is prohibited in areas within a Drinking Water Supply Management Area (DWSMA) as defined in Minn. R. 4720.5100Minn. R. 4720.5100, subp. 13., unless allowed by a local unit of government with a current MS4 permit.

Soils hydrologic soil group mapping (see Design infiltration rates)

See NRCS Web Soil Survey for hydrologic soil descriptions for the swale location. A and B soils are potentially suitable for an infiltration swale. The maximum allowed field-measured infiltration rate shall not exceed 8.3 inches per hour for an infiltration swale.

Major design elements - Practice and site considerations

Several considerations are made in this section for the conceptual design of dry swales. Further design guidance and specifications are made in the following sections.

Conveyance

It is HIGHLY RECOMMENDED that the designer provides non-erosive flow velocities within the swale and at the outlet point to reduce downstream erosion. During the 10-year or 25-year storm (depending on local drainage criteria), discharge velocity should be kept below 4 feet per second for established grassed channels. Erosion control matting or rock should be specified if higher velocities are expected.

Pretreatment

If there is space for pretreatment prior to the swale it should be evaluated. Pretreatment should also be considered when runoff entering the swale has high concentrations of sediment. See the pretreatment section for more information.

If the dry swale is being used to meet the Construction Stormwater General Permit, pretreatment is required.

Warning: To prevent clogging of the infiltration or filtration system, the Permittee(s) must use a pretreatment device such as a vegetated filter strip, small sedimentation basin, or water quality inlet (e.g., grit chamber) to settle particulates before the stormwater discharges into the infiltration or filtration system.

Grading

  • Slope of swale: The longitudinal slope of a dry swale may vary from 0.5 and 2 percent and will affect the selection of swale type. It is HIGHLY RECOMMENDED that the design engineer consider the expected watershed flow to be conveyed by the swale in making the preliminary determination of design type.
  • Swale bottom: It is HIGHLY RECOMMENDED that the swale bottom be no less than 3 feet wide and sized based on the relative stage-dependent flow driven cross-sectional area.
  • Side slopes: It is RECOMMENDED that the maximum side slopes within a swale do not exceed 3H:1V and be designed based on the relative stage-dependent flow driven cross-sectional area.
  • Swale depth: Swale depth will be estimated based on the relative stage-dependent flow driven cross-sectional area.
  • Additional considerations: In watersheds with unstable soils or lack of vegetative cover (e.g., construction, farmland and highly impervious surfaces) or where some compaction occurred during construction, compaction should be alleviated or the hydrologic soil classification for the site should be downgraded for feasibility study purposes.

Filter media

Swales designed for filtration (i.e. swales that have an underdrain) typically have engineered media. The media is comprised of a combination of sand and organic material on top of a pea gravel bed that encases a perforated drain pipe. The media assists in the removal of fine particulate and dissolved pollutants, improving on the overall performance of swale systems. See design specifications for media.

Soils with high infiltration rates (A and B soils) typically do not utilize engineered media. Swales constructed on these soils are suitable for infiltration and underdrains are not needed.

Underdrains

Underdrains are used when drawdown requirements cannot be me (e.g. C and D soils) or when there are other constraints to infiltration (see constraints to infiltration). Underdrains are comprised of a perforated, PVC pipe laid within filter media to convey runoff to either a stable day-lit area, a second form of treatment, or the storm sewer. A solid-walled PVC section of piping should be connected to the perforated drain pipe with a “tee” junction piece and extended to the swale’s surface to serve as an inspection and cleanout access point. These observation/maintenance ports are spaced throughout the system. See specifications for underdrains.

Treatment

Stormwater treatment in dry swales varies by design. For swales designed as infiltration practices, pollutants are attenuated through settling of sediment and adsorption of pollutants on soil media. Pollutants not attenuated by these processes will infiltrate deeply into the vadose zone, where they may be adsorbed, undergo chemical change, or leach to groundwater.

For swales designed as filtration practices, pollutants are attenuated through settling of sediment and adsorption of pollutants. Engineered media, which typically has a relatively organic matter content, is effective in attenuating metals, most organics, and bacteria. Soluble pollutants, such as nitrate and chloride, may be taken up by vegetation but will largely be captured by the underdrain and returned to the stormwater drainage system. Unless lined, some infiltration will occur below the underdrain in filtration systems

The use of impermeable check dams or weirs can enhance treatment by increasing the volume of water retained and increasing the contact time between soil or media and runoff water.

Vegetation

Vegetation plays a crucial role in dry swale treatment capacity, flow attenuation and stabilization of the swale itself (i.e., erosion control). It is HIGHLY RECEOMMENDED that preference is given to robust native, non-clump forming grasses as the predominant plant type within the swale flow area. Care must also be taken to consider species selection in light of sun exposure duration/timing as well as soil moisture, ponding depth and ponding duration.

Landscaping

Swales can be effectively integrated into the site planning process and aesthetically designed as attractive green spaces planted with native vegetation. Because vegetation is fundamental to the performance and function of the swale, aesthetically chosen vegetation may only be possible on the surface of the swales.

Snow considerations

Considering management of snow, the following are recommended

  • Plan a plow path during design phase and tell snowplow operators where to push the snow. Plan trees around (not in) plow path, with a 16 foot minimum between trees.
  • Plan for snow storage (both temporary during construction and permanent). Don’t plow into dry swales routinely. Dry swales should be a last resort for snow storage (i.e. only for very large snow events as “emergency overflow”.
  • Snow storage could be, for example, a designed pretreatment area.

For more information and example photos, see the section on snow and ice management.

Safety

Swales do not pose any major safety hazards. Potential hazards could occur from the steep side slope and checks of the swales if they are close to pedestrian traffic or roadways with no shoulders.

Materials specification

Erosion control

The use of temporary erosion control materials is REQUIRED in the design and construction of all swale types to allow for the establishment of firmly-rooted, dense vegetative cover. The dry swale bottom and side slopes up to the 10 year event should use robust erosion control matting that can resist the expected shear stresses associated with channelized flows. The matting should have a minimum life expectancy of three years. Upper banks of the swale slope should be protected by either similar matting or a straw/coconut blend erosion control blanket. See MNDOT specifications for guidance on selection of erosion control products.

Filter media

Filter media used in dry swale designs should follow guidance on material specifications within the Bioretention section of the MN Stormwater Manual.

Underdrains

The following are RECOMMENDED for filtration practices with underdrains.

  • The minimum underdrain pipe diameter is 4 inches.
  • Install 2 or more underdrains for each practice system in case one clogs. At a minimum provide one underdrain for every 1,000 square feet of surface area.
  • Include at least 2 observation /cleanouts for each underdrain, one at the upstream end and one at the downstream end. Cleanouts should be at least 4 inch diameter vertical non-perforated schedule 40 PVC pipe, and extend 6-12 inches above the surface. Cap cleanouts with a watertight removable cap.
  • Construct underdrains with Schedule 40 or SDR 35 smooth wall PVC pipe.
  • Install underdrains with a minimum slope of 0.5 percent, particularly in HSG D soils (Note: to utilize Manning’s equation the slope must be greater than 0).
  • Include a utility trace wire for all buried piping.
  • For underdrains that daylight on grade, include a marking stake and animal guard.
  • For each underdrain have an accessible knife gate valve on its outlet to allow the option of operating the system as either bioinfiltration, biofiltration system or both. The valve should enable the ability to make adjustments to the discharge flow so the sum of the infiltration rate plus the underdrain discharge rate equal a 48 hour draw-down time.
  • Pipe perforations should be 3/8 inches and prevent migration of filter media and aggregate into the pipe. Use solid sections of non-perforated PVC piping and watertight joints wherever the underdrain system passes below berms, down steep slopes, makes a connection to a drainage structure, or daylights on grade.
  • Spacing of collection laterals should be less than 25 feet.
  • Underdrain pipes should have a minimum of 3 inches of washed #57 stone above and on each side of the pipe (stone is not required below the pipe). Above the stone, two inches of choking stone is needed to protect the underdrain from blockage.
  • Avoid filter fabric.
  • Pipe socks may be needed for underdrains embedded in sand. If pipe socks are used, use circular knit fabric.
  • The procedure to size underdrains is typically determined by the project engineer. An example for sizing underdrains is found in Section 5.7 of the North Carolina Department of Environment and Natural Resources Stormwater BMP Manual.

Plants

Refer to the vegetation section of the manual for selection of Minnesota native plants to be used in swales. Care must be taken to specify plants for their position in the system (swale bottom, side slopes and buffer). Preference towards robust non-clump forming grasses or sedges should be given to the swale bottom that can withstand flow forces as well as provide adequate filtration functions. It is also important to understand draw-down time not only within the channel itself, but in either in-situ soils or the filter media as plants have variable tolerance to the depth and duration of inundation as well as soil moisture period. Lastly, care should be taken to understand sun exposure requirements of various plants to ensure a robust, dense establishment of vegetative cover.

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