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Minnesota Stormwater Manual

Design criteria for wet swale (wetland channel)
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Major design elements

Physical feasibility initial check

Before deciding to use a 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 wet swale practice is the appropriate BMP for the site.

Contributing drainage area

The RECOMMENDED maximum drainage area is typically 5 acres.

Site topography

Unless slope stability calculations demonstrate otherwise (for guidance on calculating slope stability, 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.

Depth to groundwater and bedrock

In general, there is no minimum separation distance required with wet swales and wetland channels. However, some source water protection requirements may dictate a separation distance if there is a sensitive underlying aquifer, which means that a liner might be required for portions of the swale or channel with standing water. A Level 2 liner is recommended. A field soil properties investigation is HIGHLY RECOMMENDED.

Karst topography

If wet swales are used in karst areas, it is RECOMMENDED that maximum pool depths be 3 to 5 feet. Impermeable liners maybe needed. Geotechnical investigations are necessary in karst areas.

Soils hydrologic soil group mapping

See NRCS Web Soil Survey for hydrologic soil descriptions for the swale location. C and D soils are potentially suitable for wet swales. Infiltration abstractions are considered negligible in these conditions.

Practice and site considerations

Several considerations are made in this section for the conceptual design of swales types. Further design guidance and specifications is 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 vegetated 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. See the pretreatment section for more information.

Anticipated flow

Although local drainage criteria may require a certain frequency event be used in the design, it is HIGHLY RECOMMENDED that larger events be considered depending on the adjacent property and associated risks.

Grading

  • Slope of swale: The longitudinal slope of a wet swale may vary from 0.5 to 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 this preliminary determination of design alternate.
  • Swale bottom: It is HIGHLY RECOMMENDED that the swale bottom be no less than 3 feet wide and 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.
  • 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 filtration capacity. In these situations, pretreatment via sedimentation processes is HIGHLY RECOMMENDED. 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.

Treatment

Stormwater treatment in swales varies by design, relying on several functions. Organic and mineral sediments suspended in stormwater flows are deposited onto the swale bottom, depending on their size and mass as well as water retention time in a process termed sedimentation. Though swales generally do not detain or retain water for extended periods, this function can be enhanced through the use of check dams or weirs that hold back flows for a design period.

A second function in pollutant removal is sorption of particulate matter via the swales soils and vegetation as it passes through the system. Wet swales also provide opportunity for plant uptake of pollutants.

Wet swales are not considered a volume reduction practice, though some volume reduction can occur through evapotranspiration.

Vegetation

Vegetation plays a crucial role in swale treatment capacity, flow attenuation and as well as in providing stabilization of the swale itself (i.e., erosion control). It is HIGHLY RECOMMENDED 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. Special considerations for wet swales include plants tolerant of wet conditions and salt spray or runoff with elevated concentrations of sodium and chloride.

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 tops of the swales.

Green Infrastructure: Because they utilize vegetation, swales provide additional benefits, including cleaner air, carbon sequestration, improved biological habitat, and aesthetic value.

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 for snow storage (both temporary during construction and permanent). Don’t plow into wet swales if possible.
  • Snow storage could be, for example, a pretreatment forebay for snow melt.

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 rock checks of the swales if they are close to pedestrian traffic or roadways with no shoulders.

Materials specification

Erosion control (MNDOT – product by velocity)

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 swale bottom and side slopes up to the 10-yr event should use robust erosion control matting that can resists 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.

Rock (MNDOT – specs)

Weir (MNDOT – specs)

Plants (MNDOT specs)

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.

Open vegetated swale materials specifications.
Link to this table

Parameter Specification Size Note Design application
Dry swale soil USCS; ML, SM, SC n/a soil with a higher percent organic content is preferred dry swale, step pool
Dry swale sand ASTM C-33 fine aggregate concrete sand 0.02” to 0.04” dry swale, step pool
Check dam (pressure treated) AWPA Standard C6 6” by 6” or 8” by 8” do not coat with creosote; embed at least 3’ into side slopes dry swale, wet swale, step pool
Check Dam (natural wood) Black Locust, Red Mulberry, Cedars, Catalpa, White Oak, Chestnut Oak, Black Walnut 6” to 12” diameter; notch as necessary do not use the following, as these species have a predisposition towards rot: Ash, Beech, Birch, Elm, Hackberry, Hemlock, Hickories, Maples, Red and Black Oak, Pines, Poplar, Spruce, Sweetgum, Willow dry swale, wet swale, step pool
Filter Strip sand/gravel pervious berm sand: per dry swale sand gravel; AASHTO M-43 sand: 0.02” to 0.04” gravel: 1/2” to 1” Mix with approximately 25% loam soil to support grass cover crop; see Bioretention planting soil notes for more detail dry swale, step pool
Pea gravel diaphragm and curtain drain ASTM D 448 varies (No. 6) or (1/8” to 3/8”) use clean bank-run gravel dry swale, step pool
Underdrain gravel per pre-cast manufacturer 1.5” to 3.5” dry swale, step pool
Underdrain ASTM D-1785 or AASHTO M-278 6” rigid Schedule 40 PVC 3/8” perf. @ 6” o.c.; 4 holes per row dry swale, step pool
Rip rap per local criteria size per requirements based on 10- year design flow dry swale, wet swale, step pool


Design procedure – design steps

It is important to acknowledge that each site has unique and defining features that require site-specific design and analysis. The guidance provided below is intended to provide the fundamentals for designing swale systems to meet regulatory requirements but is not intended to substitute engineering judgment regarding the validity and feasibility associated with site-specific implementation. Designers need to be familiar with the hydrologic and hydraulic engineering principles that are the foundation of the design and they should also enlist the expertise of qualified individuals in stormwater management and conveyance system plantings with respect to developing appropriate planting plans and habitat improvement features.

Step 1. Make a preliminary judgment

Considering basic issues for initial suitability screening, make a preliminary judgment as to whether site conditions are appropriate for the use of a swale, and identify its function in the overall treatment system.

A. Consider basic issues for initial suitability screening, including:

  • site drainage area,
  • site topography and slopes,
  • regional or local depth to ground water and bedrock,
  • the seasonally high water table may inundate the swale; but not above the design bottom of the channel,
  • site location/minimum setbacks, and
  • presence of active karst.

B. Determine how the swale will fit into the overall stormwater treatment system, including:

  • deciding whether the swale is the only BMP to be employed, or if are there other BMPs addressing some of the treatment requirements; and
  • deciding where on the site the swale will most likely be located.

Step 2. Confirm design criteria and applicability

A. Determine whether a media filter must comply with the MPCA Construction Stormwater General Permit (CGP). To determine if permit compliance is required, see Permit Coverage and Limitations.

B. Check with local officials, watershed organizations, and other agencies to determine if there are any additional restrictions and/or surface water or watershed requirements that may apply.

Step 3. Perform field verification of site suitability

See Major design elements

Step 4. Select design variant based on physical suitability evaluation

Once the physical suitability evaluation is complete, it is HIGHLY RECOMMENDED that the better site design principles be applied in sizing and locating the wet swale practice(s) on the development site. Given the drainage area, select the appropriate swale practice for the first iteration of the design process.

Note: Information collected during the physical suitability evaluation (see Step 1) should be used to explore the potential for multiple swale practices versus relying on a single facility. Compute watershed runoff values.

Step 5. Compute runoff control volumes and other key design parameters

If the swale is being designed to meet the requirements of the MPCA Permit, the REQUIRED treatment volume is the water quality volume of 1 inch of runoff from the new impervious surfaces created from the project. If part of the overall Vwq is to be treated by other BMPs, subtract that portion from the Vwq to determine the part of the Vwq to be treated by the dry swale.

For swales, compute the following design parameters:

A. Calculate the maximum discharge loading per foot of swale width

\(q = (0.00236/n) · Y · 1.67 · S · 0.5 \)

Where:

q = discharge per foot of length of the swale, from Manning’s equation (cfs/ft);
Y = allowable depth of flow (inches);
S = slope of swale (percent) (0.5 to 2 percent); and
n = Manning’s “n” roughness coefficient (use 0.15 for short prairie grass, 0.25 for dense grasses such as bluegrass, buffalo grass, blue grama grass and other native grass mixtures).

B. Use a recommended hydrologic model to compute Qwq

C. Minimum swale length (in feet) = Qwq / q

Where:

Qwq = the water quality peak discharge (cubic feet per second)

Step 6. Compute number of check dams

  • Design to contain entire Vwq.
  • Channel slopes up to 1 percent are recommended unless topography necessitates a steeper slope, in which case 6- to 12-inch drop structures can be placed to limit the energy slope to within the recommended 0 to 1 percent range. Energy dissipation will be required below the drops. Spacing between the drops should not be closer than 50 feet. Depth of the Vwq at the downstream end should not exceed 18 inches.

Step 7. Check 2-year and 10-year velocity erosion potential and freeboard

Check for erosive velocities and modify design as appropriate based on local conveyance regulations. Provide 6 inches of freeboard.

Step 8. Design low flow control at downstream headwalls and checkdams

Design control to pass Vwq in 48 hours.

Step 9. Design inlets and sediment forebay(s)

Inlets to swales must be provided with energy dissipaters such as riprap or geotextile reinforcement. Pre-treatment of runoff is typically provided by a sediment forebay located at the inlet.

Step 11. Check volume, peak discharge rates and drawdown time against state, local, and watershed organization requirements (NOTE: steps are iterative)

Follow the design procedures identified in the Unified Sizing Criteria section of the Manual to determine the volume control and peak discharge requirements for water quality, recharge (not required), channel protection, overbank flood and extreme storm.

Model the proposed development scenario using a surface water model appropriate for the hydrologic and hydraulic design considerations specific to the site. This includes defining the parameters of the swale practice defined above: ponding elevation and area (defines the ponding volume), filtration rate and method of application (effective filtration area), and outlet structure and/or flow diversion information. The results of this analysis can be used to determine whether or not the proposed design meets the applicable requirements. If not, the design will have to be re-evaluated.

{{:Swale systems shall be sufficient to filter a water quality volume of 1 inch of runoff from the new impervious surfaces created by the project. If this criterion is not met, increase the storage volume of the filtration practice or treat excess water quality volume (Vwq) in an upstream or downstream BMP (see Step 5).}}

Step 12. Finalize the cross-section and profile design for the project

  • Grading plan: Develop a grading plan based on the preliminary profile and cross-section typical design.
  • Dimensions: Adjust the preliminary profile dimensions to accommodate site specific concerns/impacts. Minimum design parameters for hydraulic, water quality, and quantity management criteria should be rechecked based on adjustments to the channels to ensure that safe and adequate conveyance is still maintained.
  • Ditch checks: Adjust the preliminary ditch check dimensions to accommodate site specific concerns/impacts. Minimum design parameters for hydraulic, water quality criteria should be rechecked based on adjustments to the channels profile and bank and bed stability to ensure that safe and adequate conveyance is still maintained.
  • Site stabilization: Course woodchips and compost should be used throughout the limit of disturbance for site stabilization. All areas should be seeded and planted as well as blanketed/matted. Jute blanket should be used within the swale bottom and side slopes. Erosion control blanket with biodegradable netting should be used above the side slopes within the floodplain.
  • Excess materials: It is advisable that excess materials, i.e., cobbles and boulders, be placed at the edge of the cross-section for use during the maintenance phase to correct any physical instability as long as the excess materials do not impede flow or create a hazard.

Step 13. Prepare vegetation and landscaping plan

A landscaping plan for a wet swale should be prepared to indicate how the enhanced swale system will be stabilized and established with vegetation. Landscape design should specify proper species and based on specific site, soils, sun exposure and hydric conditions present along the channel. Further information on plant selection and use can be found in the Minnesota plant lists section.

Step 14. Prepare operation and maintenance plan

See Operation and Maintenance section for guidance on preparing an O&M plan.

Step 15. Prepare cost estimate

See Cost Considerations section for guidance on preparing a cost estimate that includes both construction and maintenance costs.

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