Design criteria for wet swale (wetland channel)
Design criteria for wet swale (wetland channel)

Wet swale section.

Green Infrastructure: 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. See the section Green Stormwater Infrastructure (GSI) and sustainable stormwater management.

Use this link to access .pdf diagrams of CADD drawings.

Links to .dwg files for swales

Click on file link to see all filtration CADD images in a combined pdf: File:All filtration cadd images combined 2.pdf

Swale layout: File:Swale Layout2 (1).pdf
Typical dry swale profile section with check dams and draintile: File:MIDS Dry Swale Sections-SHEET 2.pdf
File:MIDS Dry Swale Sections-SHEET 2 (2).pdf
Typical grass channel cross-section without soil amendment: File:MIDS Dry Swale Sections-SHEET 1.pdf
Typical wet swale check dam cross section, profile section with check dams, and profile section: File:MIDS WET Swale Sections SHEET 2 (1).pdf
Cascade profile, riffle pool sequence profile, riffle weir cross-section: File:Stormwater-Step-Pool.pdf
typical earthen, rock, structural check dams; profile and cross-section: File:VARIOUS CHECK DAMS Layout1 (1).pdf
Typical wet and dry swale cross-sections: File:Wet and dry swales Layout2 (1).pdf

Design phase maintenance considerations

Caution: Maintenance considerations are an important component of design

Implicit in the design guidance is the fact that many design elements of infiltration and filtration systems can minimize the maintenance burden and maintain pollutant removal efficiency. Key examples include

limiting drainage area;
providing easy site access (REQUIRED);
providing pretreatment (REQUIRED); and
utilizing native plantings (see Plants for Stormwater Design).

For more information on design information for individual infiltration and filtration practices, link here.

Major design elements

Physical feasibility initial check

Before deciding to use a wet swale 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.

Wet swales can be used for conveyance of runoff from larger drainage areas, but the water quality function of the wet swale will be diminished.

Site topography

Unless slope stability calculations demonstrate otherwise (for guidance on calculating slope stability, see [1], [2]), 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 required 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 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 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. Use the proposed flow and velocity to determine whether the swale will require scour protection. If scour protection is necessary, see potential erosion control methods.
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

See MNDOT Standard Specification 3601.

Weir

See MNDOT Standard Specifications 2461, 2573, , 3301, 3491, 3601.

Plants

See MNDOT Standard Specifications 2571, 2574, 2575, 3861, 3876, 3878.

Refer to the swale plant 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). For the bottom of the swale, preference should be given to robust non-clump forming grasses or sedges 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.

Wet swale materials specifications.
Link to this table

Parameter	Specification	Size	Note
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
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
Check dam (rock, rip rap)	Per local criteria	Size per requirements based on 10-year design flow	Cannot get water quality volume credit when using a permeable check dam
Check dam (earth)	Per local criteria	Size per requirements based on 10-year design flow	Use clayey soils with low permeability
Check dam (precast concrete)	Per pre-cast manufacturer	Size per requirements based on 10-year design flow	Testing of pre-cast concrete required: 28 day strength and slump test; all concrete design (cast-in-place or pre-cast) not using previously approved State or local standards requires design drawings sealed and approved by a licensed professional structural engineer. Embed at least 3’ into side slopes

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

1. Calculate the following runoff control volumes.

Calculate the Water Quality Volume (V_wq): 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 V_wq is to be treated by other BMPs, subtract that portion from the V_wq to determine the part of the V_wq to be treated by the dry swale.

V_wq = 1 inch x Area_{impervious surface}

Calculate Channel Protection Volume (V_cp)

V_cp = 24 hour extended detention of post-development 1-yr 24-hr storm event

Calculate Overbank Flood Protection Volume (V_p10)

V_p10 = peak discharge from the 10-yr storm to 10-yr predevelopment rates

Calculate Extreme Flood Volume (V_p100).

Vp100 = peak discharge from the 100-yr storm to 100-yr predevelopment rates

2. Once the runoff control volume is determined for design, compute the following design parameters to determine the swale size required.

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 Q_wq

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

Where:

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

Step 6. Compute number of check dams

Profile of swale with structural check dams (not to scale). Source: Virginia DOT BMP Design Manual, Chapter 6. Click on image to enlarge.

Space check dams in a channel so the crest of the downstream dam is at the elevation of the toe of the upstream dam. Click on image to enlarge.

Profile of Swale with earthern check dams (not to scale). Source: Oregon Department of Environmental Quality Erosion and Sediment Control Manual.

The number of check dams should be computed based on swale slope, length, and treatment objectives. For example, a swale designed to contain the entire Vwq may require more check dams than a swale that only contains a portion of the V_wq.

Channel slopes between 0.5 and 2 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 0.5 to 2 percent range. Energy dissipation will be required below the drops. Spacing between the drops should not be closer than 50 feet. Depth of the V_wq 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

The water level should draw down to the flow line of the controlling check dam elevation within 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 swale plant list 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.

Related pages

Design criteria for wet swale (wetland channel) Design criteria for wet swale (wetland channel)

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