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.

• Links to .dwg files for swales
• Swale layout: File:Swale Layout2 (1).pdf
• Typical dry swale profile section with check dams and draintile: File:MIDS Dry Swale Sections-SHEET 1.pdf
• Typical grass channel cross-section without soil amendment: File:MIDS Dry Swale Sections-SHEET 2.pdf

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.

• Karst
• Shallow soils and shallow depth to bedrock
• Shallow groundwater
• Soils with low infiltration capacity
• Separation distances
• Potential stormwater hotspots
• Wellhead protection
• Contaminated soils and groundwater
• Procedures for investigating sites with potential constraints

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 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. 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.

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 also considers the expected watershed flow to be conveyed by the swale in making this preliminary determination of design alternate.
• Swale bottom: It is HIGHLY RECOMMENDED that the swale bottom be no less than 3 feet wide and will be sized with the relative stage-dependent flow driven cross-sectional area in mind.
• Side slopes: It is RECOMMENDED that the maximum side slopes within a swale do not exceed 3H:1V and will be designed with the relative stage-dependent flow driven cross-sectional area in mind.
• Swale depth: Swale depth will be estimated based on the relative stage-dependent flow driven cross-sectional area.
• Infiltration and filtration considerations: The design engineer should review the results of the feasibility check to assist in the selection of swale type. An additional consideration includes watershed soil transport to the site. Watersheds with unstable soils or lack of vegetative cover (e.g., construction, farmland and highly impervious surfaces) can generate and transport excessive sediments to the swale that may affect both infiltration and filtration capacity. In these situations, pretreatment via sedimentation processes is HIGHLY RECOMMENDED. Another consideration is the level of compaction and structure of in-situ soils, when considering dry swales. Construction of developments and roads, for example, significantly alter the parent state of native soils and therefore their hydrologic soil classification should be downgraded for feasibility study purposes.