This document combines several documents related to Wet swale (wetland channel). Individual documents can be viewed by clicking on the appropriate link below. Fact sheets are not included in this combined document.

Wet swale (wetland channel)

1 Overview
2 Types of wet swale (wetland channel)
3 Design criteria for wet swale (wetland channel)
4 Construction specifications for wet swale (wetland channel)
- 4.1 Access agreements
- 4.2 Construction specifications for swale practices
5 Assessing the performance of wet swale (wetland channel)
6 Operation and maintenance of wet swale (wetland channel)
7 Calculating credits for wet swale (wetland channel)
8 Cost/benefit considerations for wet swale (wetland channel)
9 Additional considerations for wet swale (wetland channel)
10 Links for wet swale (wetland channel)
11 References for wet swale (wetland channel)

Overview

Photo of a wet swale. Courtesy of Limnotech.

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 Infrastructure for stormwater management.

Wet swales occur when the water table is located very close to the surface or water does not readily drain out of the swale. A wet swale acts as a very long and linear shallow biofiltration or linear wetland treatment system. Wet swales do not provide volume reduction and have limited treatment capability. Incorporation of check dams into the design allows treatment of a portion or all of the water quality volume within a series of cells created by the check dams. Wet swales planted with emergent wetland plant species provide improved pollutant removal. Wet swales may be used as pretreatment practices. Wet swales are commonly used for drainage areas less than 5 acres in size.

Function within stormwter treatment train

Wet swales provide limited water quality treatment and no volume control and are not recommended practices unless options for other BMPs are limited.

Wet swales are designed primarily as in-line systems for stormwater quality and typically are used in conjunction with other structural controls in the stormwater treatment train. Wet swales may be used at various locations within a treatment train and can be used for pretreatment, conveyance, and/or primary treatment.

Typical applications

Applications of wet swales can vary extensively. Typical applications include

individual lots for rooftop, driveway, and other on-lot impervious surface;
shared facilities in common areas for individual lots;
areas within loop roads or cul-de-sacs;
landscaped parking lot islands;
within right-of-ways along roads;
common landscaped areas in apartment complexes or other multifamily housing designs;
between buildings in industrial and commercial developments; and
conveyance between detention structures and receiving waters.

Infeasibility criteria

Certain site-specific conditions may make the use of wet swales infeasible. Examples include:

Where ordinances established by the local government with jurisdiction, such as setbacks from structures, conflict with the proposed location
Where high levels of contaminants in soil or groundwater exist
Where the only area available for siting does not allow for a safe overflow pathway to the municipal separate storm sewer system or private storm sewer system
Where reasonable concerns about erosion, slope failure, or down gradient flooding exist and cannot be overcome by swale design modifications
Where there are restrictions on the proximity to building foundations

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

When volume control is constrained at a site and other BMP options (e.g. constructed pond, media filter) are not feasible, a wet swale with check dams provides treatment for a portion or all of the water quality volume stored behind the check dams. For regulatory purposes, wet swales that incorporate check dams into their design 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 wet 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 wet 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 wet swale, but not specifically required by the MPCA CGP.
RECOMMENDED: Indicates design guidance that is helpful for wet swale performance but not critical to the design.

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

Retrofit suitability

The use of wet swales as a retrofit practice primarily depends on existing infrastructure and whether the invert or flowline of the wet swale outlet allow meeting design requirements.

Special receiving waters suitability

The following table provides guidance regarding the use of wet 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.

Summary of design restrictions for special waters.
Link to this table

BMP Group	receiving water
BMP Group	A Lakes	B Trout Waters	C Drinking Water	D Wetlands	E Impaired Waters
Filtration	Some variations NOT RECOMMENDED due to poor phosphorus removal, combined with other treatments	RECOMMENDED	RECOMMENDED	ACCEPTABLE	RECOMMENDED for non-nutrient impairments

Cold climate suitability

In cold climates, some special considerations are HIGHLY RECOMMENDED for surface systems like wet swales to ensure sustained functionality and limit the damage that freezing temperatures and snow and ice removal may cause.

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. Wet swales cannot be used for significant snow storage areas as debris build-up and plant damage 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.

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

Wet swales are not typically a primary practice for providing water quantity control. They are normally either designed off-line using a flow diversion or configured to safely pass large storm flows. In limited cases, wet swales may be able to accommodate the channel protection volume, V_cp, in either an off- or on-line configuration, and in general they can provide some (albeit limited) storage volume. Wet swales can help reduce detention requirements for a site by providing elongated flow paths and longer times of concentration, and provide very limited volumetric losses from infiltration and evapotranspiration. Generally, to meet site water quantity or peak discharge criteria, it is HIGHLY RECOMMENDED that another structural control (e.g., detention) be used.

Caution: It is HIGHLY RECOMMENDED that wet swales have a maximum slope of 1 percent.

Warning: Warning: It is REQUIRED that volume reduction practices, such as infiltration basins, are considered before filtration practices.

Water quality treatment

Wet swales provide some removal of sediment and associated pollutants through filtering and settling. Less significant processes can include evaporation, infiltration, transpiration, biological and microbiological uptake, and soil adsorption. Pollutant removal data for select parameters are provided here.

Water quality performance of wet swales can be diminished when plants die off in the fall and winter months as they are no longer able to uptake water and nutrients.

Limitations

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

Nitrification of water may occur where aerobic conditions exist
Wet swales offer limited water quantity control
The potential for nuisance insects or odors exists when standing water is persistent in the wet swale
Water quality performance can change seasonally
Wet swales are impractical in steep areas, because maintaining a constant water surface elevation or pool becomes too difficult
Wet swales are impractical in extremely flat areas, because the lack of gradient may cause excessive ponding and prevent positive drainage
Vegetation must be periodically trimmed to keep woody vegetation in check
A wet swale can erode during peak rainfall when water volume and velocity are high
Standing water in wet swales may foster mosquitoes, so vector control is recommended
Resuspension of sediment can occur during peak storm events
Standing water causes water temperature to rise, which reduces oxygen in the water and negatively impacts nutrient removal

Types of 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 Infrastructure for 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], [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 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, 3137, 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

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.

Construction specifications for wet swale (wetland channel)

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 Infrastructure for stormwater management.

Wet swale practices can be an important tool for retention and detention of stormwater runoff and treatment of pollutants in stormwater runoff. Because swales incorporate dense vegetation, additional benefits may include cleaner air, carbon sequestration, improved biological habitat, and aesthetic value.

This page provides a discussion of construction specifications for wet swales.

Access agreements

An easement is a legally binding agreement between two parties, and is defined as “a non-possessory right to use and/or enter onto the real property of another without possessing it.“ An easement is required for one party to access, construct, or maintain any feature or infrastructure on the property of another. Easements can be temporary or permanent. For example, temporary easements can be used if limits needed for construction are larger than the permanent easement footprint of constructed features. Having an easement provides a mechanism for enforcement of maintenance agreements to help ensure wet swales are maintained and functioning. See an example access agreement.

Construction specifications for swale practices

Construction of wet swale practice incorporates techniques and steps that may be considered nonstandard. It is recommended that construction specifications include project pretreatment devices, construction sequencing, temporary and permanent erosion control measures measures, excavation and fill, grading, soil decompaction, material specifications, and final stabilization. All of these topics are addressed in further detail below.

Additional specifications for items applicable to swale practices can be found in the Minnesota Department of Transportation’s (MnDOT) Specifications for Construction. The current version of this resource was completed in 2018. Below is a list of MnDOT sections that may be helpful when writing project specifications for wet swales.

1717 - Air, land and water pollution
2101 - Clearing and grubbing
2105 - Excavation and embankment
2511 - Riprap
2571 - Plant installation and establishment
2572 - Protection and restoration of vegetation
2573 - Storm water management
2574 - Soil preparation
2575 - Establishing turf and controlling erosion
3149 - Granular material
3877 - Topsoil material
3878 - Sod
3882 - Mulch material
3884 - Hydraulic erosion control products
3885 - Rolled erosion control products
3897 - Sediment control log

Pre-construction meeting

A pre-construction meeting is recommended and should include a walkthrough of the site with the builder/contractor/subcontractor to identify important features of the work and to review and discuss the plans. This is the best time to identify potential issues related to construction methods and sequencing that will affect site protection, erosion and sediment control, and proper installation of the work.

Site protection

Pretreatment

Pretreatment is a required part of filtration practices. Pretreatment is needed to protect BMPs from the build-up of trash, gross solids, and particulate matter. When the velocity of stormwater decreases, sediment and solids drop out. If pretreatment is not provided, this process will occur in the BMP, resulting in long-term clogging and poor aesthetics.

Warning: The Construction Stormwater general permit states: To prevent clogging of the 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 filtration system.

Temporary erosion and sediment control

During construction, it is critical to keep sediment out of the wet swale device as much as practicable. As soon as grading is complete, stabilize slopes to reduce erosion of native soils. Protect temporary soil stockpiles from run-on and run-off from adjacent areas and from erosion by wind. Sweep as often as required if sediment is on paved surfaces to prevent transport offsite by tracking and airborne dust. All sediment and erosion control measures must be properly installed and maintained. When sediment build up reaches 1/2 the height of the device, action is required, such as removing the accumulated sediment or installing additional sediment controls downgradient of the original device. Link here for more information.

Compaction prevention

Preventing and alleviating compaction are crucial during construction of wet swale practices, as compaction can inhibit plant growth and root penetration. The wet swale area should be marked with paint and/or stakes to keep construction traffic from traveling in the area.

Inspection and documentation

Inspections before, during, and after construction are needed to ensure swale practices are built in accordance with the plans and specifications. It is recommended that onsite inspectors are familiar with project plans and specifications to ensure the contractor’s interpretation of the plans are consistent with the designer’s intent. The inspectors should take frequent photos and notes of construction activities and features as work progresses and at all critical points (such as immediately prior to backfilling). They should check dimensions and depths of all installed materials. All materials and products should be verified or tested for conformance with the specifications.

Construction checklists

A construction checklist is found here. Note this checklist may include items not applicable to wet swales.

Construction sequence

Step 1 – Site examination and preparation

It is the responsibility of the contractor to:

Examine the areas for performing earthwork and determine that conditions are satisfactory to proceed, or to correct all unsatisfactory conditions prior to starting work.
Arrange to locate, mark, and protect all existing utilities and underground facilities in the areas of work.
Remove all existing features marked for removal and required earthwork
Ensure entire contributing drainage area is stabilized prior to construction

Step 2 – Excavation

Cut the swale area as shown on the plans. Where possible, excavation should be performed with a backhoe and work should be done from the sides and outside the footprint of the swale area to avoid soil compaction. If it is necessary to work in the swale bottom area, only low ground pressure tracked equipment should be allowed to complete the work. Rubber tire equipment should be strictly prohibited within the swale bottom area, unless working from pavement outside of the basin or trench. The contractor should start the work at the far side of the trench or basin and work their way out.

The contractor is to ensure all laws and regulations are followed regarding stability of excavations. This may require shoring, bracing, sloping, or benching. Materials should not be stockpiled near the edge of the excavation. Drainage and control of water in the excavation must also be considered.

Step 3 – Decompaction

Soil decompaction is required in all wet swale bottom areas. Decompact subsoil with a backhoe ripper attachment or other approved method to a depth of at least 18 inches below subgrade in all locations indicated on the drawings. Also known as soil loosening or soil ripping, this technique has been shown to reduce compaction from construction activities. For more information on alleviating compaction, link here.

Step 4 – Installation of check dams

Check dams may be selected for use within wet swales if permanent ponding and/or rate control is desired. Options include wood, earthen, rock or concrete structures. Materials for the construction of check dams should be stored within a protected staging area until construction, where perimeter erosion control measures are implemented. Check dams should be constructed during the rough grading phase with upstream flows being diverted to a stable conveyance or ponding area until the site is completely stabilized. Check dams must be keyed into the banks and bed of the swale to a depth sufficient to prevent end running and undercutting of the check.

Step 5 – Restoration and plantings

After final grading has been approved, planting or seeding should happen as soon as possible to avoid erosion, sedimentation, and the establishment of weeds. The contractor should notify the designer at least four days in advance of when planting or seeding will occur in advance of delivery of materials to the site to allow for scheduling of site inspections. At least two weeks prior to the planting or seeding dates, any existing weeds should be thoroughly eradicated mechanically or with herbicide within the project area.

Warning: It is REQUIRED that the planting or seeding contractor have proven successful experience installing and maintaining projects of similar scope and scale and provide a superintendent that will be onsite during the entire seeding or planting process.

All seed and plants should be shipped and stored with protection from weather or other conditions that would damage the product. All plants and seeds will be inspected by the designer and items that have become wet, moldy, or otherwise damaged in transit or in storage should be rejected. Plants and seed should arrive within 24 hours of delivery. Plants and seed needs to be protected against drying and damage prior to planting.

It is typical for the plant or seeding contractor to guarantee the work for some length of time. The common minimum for herbaceous plantings or sod is 60 days during the growing season. The growing season in central Minnesota is defined as May 1st through October 31st. A one-year guarantee on containerized plants can help to ensure good establishment and decrease weed infestations. Any watering required to keep the plants healthy should be covered under the cost of the warranty period. It is appropriate to require that the contractor provide some form of surety, such as a letter of credit or other security, to the permitting entity for 150 percent of the estimated costs and quantities of all herbaceous plants or seeding for the duration of the 1-year warranty period. Planting and seeding establishment should meet the requirements within MnDOT Section 2571 (page 478).

Caution: Seeding maintenance requires specialized knowledge and experience in plant and weed identification. Ensure a thorough maintenance plan is established prior to construction and that budget has been allocated for at least three full growing seasons and preferable longer. Native seedings can be more difficult than containerized plantings to establish.

Step 6 – Final stabilization and Closeout

As defined in the NPDES/SDS Construction Stormwater permit, final site stabilization is achieved when all soil disturbing activity is completed and the exposed soils have been stabilized with a vegetative cover with a uniform density of at least 70 percent over the entire site or by equivalent means to prevent soil failure. Simply seeding and mulching is not considered acceptable cover for final stabilization. Final stabilization must consist of an established permanent cover, such as a perennial vegetative cover, concrete, riprap, gravel, rooftops, asphalt, etc

The NPDES permit requires all stormwater treatment systems to meet all permit requirements and be operating as designed prior to submitting the NPDES notice of termination. This can be accomplished as part of the final inspection process.

When a final construction inspection has been completed, log the GPS coordinates for each facility and submit them for entry into the local BMP maintenance tracking database, if available.

Assessing the performance of wet swale (wetland channel)

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 Infrastructure for stormwater management.

Swales retain solids and associated pollutants by settling and filtering. A typical method for assessing the performance of of BMPs with underdrains is therefore measuring and comparing pollutant concentrations at the influent and effluent. If the swale is designed for infiltration, see Assessing the performance of bioretention.

An online manual for assessing BMP treatment performance was developed in 2010 by Andrew Erickson, Peter Weiss, and John Gulliver from the University of Minnesota and St. Anthony Falls Hydraulic Laboratory. The manual advises on a four-level process to assess the performance of a Best Management Practice.

Level 1: Visual Inspection. This includes assessments for infiltration practices and for filtration practices. The website includes links to a downloadable checklist.
Level 2: Capacity Testing. Level 2 testing can be applied to both infiltration and filtration practices.
Level 3: Synthetic Runoff Testing for infiltration and filtration practices. Synthetic runoff test results can be used to develop an accurate characterization of pollutant retention or removal, but can be limited by the need for an available water volume and discharge.
Level 4: Monitoring for infiltration or filtration practices

Level 1 activities do not produce numerical performance data that could be used to obtain a stormwater management credit. BMP owners and operators who are interested in using data obtained from Levels 2 and 3 should consult with the MPCA or other regulatory agency to determine if the results are appropriate for credit calculations. Level 4, Monitoring, is the method most frequently used for assessment of the performance of a BMP.

Use these links to obtain detailed information on the following topics related to BMP performance monitoring:

Operation and maintenance of wet swale (wetland channel)

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 Infrastructure for stormwater management.

The most frequently cited maintenance concern for wet swales is that they provide a breeding ground for mosquitoes. Common operational problems include:

blockage by debris and vegetation;
sediment accumulates in the swale, reducing the storage volume;
slope stabilizing vegetation is lost; and
invasive plants out-compete native vegetation

Design phase maintenance

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

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

Wet swales can be designed, constructed and maintained to minimize the likelihood of being desirable habitat for mosquito populations. Designs that incorporate constant inflows and outflows, habitat for natural predators, and constant permanent pool elevations limit the conditions typical of mosquito breeding habitat (see section on mosquito control). For more information on design information for wet swales, link here.

Construction phase maintenance

Proper construction methods and sequencing play a significant role in reducing problems with operation and maintenance (O&M). Inspections during construction are needed to ensure that the wet swale practice is built in accordance with the approved design standards and specifications. Detailed inspection checklists should be used that include sign-offs by qualified individuals at critical stages of construction, to ensure that the contractor’s interpretation of the plan is acceptable to the professional designer. An example construction phase inspection checklist is provided below.

Wet swale construction inspection checklist.
Link to this table
To access an Excel version of form (for field use), click here.

Construction Sequence	Satisfactory / Unsatisfactory	Comments
Project:
Location:
Site Status:
Date:
Time:
Inspector:
1. Pre-Construction
Pre-construction meeting
Runoff diverted (Note type of bypass)
Facility area cleared
Project benchmark near site
Facility location staked out
Temporary erosion and sediment protection properly installed
2. Excavation
Size, location, and inverts per plans
Side slopes stable
Lateral slopes completely level
Longitudinal slopes within design range
Groundwater/bedrock verified
Stockpile location not adjacent to excavation area and stabilized with vegetation and/ or silt fence
Verify stockpile is not eroding
3. Structural Components
Outlets installed pre plans
Pretreatment devices installed per plans
Soil bed composition and texture conforms to specifications
Inlets installed per plans
4. Vegetation
For native wet swales, plants and materials ordered 6 months prior to construction
For native wet swales, construction planned to allow for adequate planting and establishment of plant community
Complies with planting specs
Topsoil complies with specs in composition and placement
Soil properly stabilized for permanent erosion control
5. Final Inspection
Project:
Dimensions per plans
Check dams operational
Pre-treatment operational
Inlet/outlet operational
Effective stand of vegetation stabilized per specifications
Construction generated sediments removed
Contributing watershed stabilized before flow is diverted to the practice
Comments:
Actions to be taken:

Post-construction operation and maintenance

Proper maintenance is critical to the successful operation of a wet swale. Without regular maintenance, wet swales can fill in with sediment and lose important vegetation. This can lead to a reduction or elimination of pollutant removal capacity. Warning: A maintenance plan clarifying maintenance responsibility is REQUIRED. Effective long-term operation of filtration practices necessitates a dedicated and routine maintenance schedule with clear guidelines and schedules. Proper maintenance will not only increase the expected lifespan of the facility but will improve aesthetics and property value.

Inspection and maintenance planning

A maintenance plan clarifying maintenance responsibilities is REQUIRED. Effective long-term operation of wet swales necessitates a dedicated and routine maintenance schedule with clear guidelines and schedules. Proper maintenance will not only increase the expected lifespan of the facility but will improve aesthetics and property value. Some important post-construction considerations are provided below along with RECOMMENDED maintenance standards.

A site-specific O&M plan that includes the following considerations should be prepared by the designer prior to putting the stormwater practice into operation:
- Inspection checklist
- Routine maintenance checklist (see below)
- Operating instructions for any outlet components
- Vegetation maintenance schedule (see item 2 in checklist below and section below)

Wet swale operation and maintenance checklist.
Link to this table
To access an Excel version of form (for field use), click here.

Maintenance Item	Satisfactory / Unsatisfactory	Comments
Project:
Location:
Site Status:
Date:
Time:
Inspector:
1. Debris Cleanout (Monthly)
Project:
Contributing areas clean of litter and vegetative debris
Filtration or infiltration facility clean
Inlets and outlets clear
2. Vegetation (Monthly)
Vegetation maintenance complies with O&M plan
Vegetation meets performance standards (including control of specified invasive species)
No evidence of erosion
Maintenance of adequate water depths for desired wetland plant species
Have sediment accumulations reduced wet swale volume significantly or are plants “choked” with sediment
3. Inundated Portion of Swale (Monthly)
Floating or floatable debris removal required
Visible pollution
Eutrophication level of the wet swale
No evidence of erosion
4. Sediment Deposition (Annual)
Area clean of sediment
Contributing drainage area stabilized and free of erosion
Winter accumulation of sand removed each spring
5. Outlet/Overflow Spillway (Annual, After Major Storms)
Good condition, no need for repair
No evidence of erosion
No evidence of any blockages
No evidence of structural deterioration
6. Other (Monthly)
Encroachment on easement area (if applicable)
Complaints from residents (if applicable)
Any public hazards (specify)
Comments:
Actions to be taken:

A legally binding and enforceable maintenance agreement should be executed between the practice owner and the local review authority to ensure the following:
- Sediment should be cleaned out of any sedimentation chamber when it accumulates to a depth equal to ½ the total depth to the outlet, or when greater than 1.5 feet, whichever is less. The sediment chamber outlet devices should be cleaned/repaired when drawdown times exceed 36 hours. Trash and debris should be removed as necessary; and
- Silt/sediment should be removed from the swale bottom when the accumulation exceeds one inch.
Adequate access must be provided for inspection, maintenance and landscaping upkeep, including appropriate equipment and vehicles.
Wet swales generally should not be used as dedicated snow storage areas, but can be with the following considerations.
- Snow storage should not occur in areas designated as potential stormwater hotspots for road salt. NOTE: Chloride will not be attenuated in filtration BMPs such as wet swales.
- When used for snow storage, or if used to treat parking lot runoff, the BMP area should be planted with salt tolerant and non-woody plant species.
- Practices should always be inspected for sand build-up on the surface following the spring melt event.
General maintenance activities and schedule are provided below.

Summary of typical maintenance regime

The list below highlights the assumed maintenance regime for a wet swale.

First year after planting
- Adequate water is crucial to plant survival and temporary irrigation may be needed unless rainfall is adequate until plants mature
- Inspect after significant rain events (e.g. ½ inch)
As needed
- Prune and weed to maintain appearance
- Remove trash and debris
- Mow filter strip/grass channel (if present)
- Replace vegetation whenever percent cover of acceptable vegetation falls below 90 percent or project specific performance requirements are not met. If vegetation suffers for no apparent reason, consult with horticulturist and/or test soil as needed
Semi-annually
- Inspect inflow and pretreatment systems for clogging (off-line systems) and remove any sediment
- Inspect filter strip/grass channel for erosion or gullying. Sod as necessary
- Herbaceous vegetation, trees and shrubs should be inspected to evaluate their health and replanted as appropriate to meet project goals
- Remove any dead or severely diseased vegetation
Annually in fall
- Inspect and remove any sediment and debris build-up in pretreatment areas
- Inspect inflow points and wet swale bottom for buildup of road sand associated with spring melt period, remove as necessary, and replant areas that have been impacted by sand/salt build up
Annually in spring
- Cut back and remove previous year’s plant material and remove accumulated leaves if needed (or conduct controlled burn where appropriate)

Estimated hours to perform maintenance activities

All estimated hours listed below would be to perform maintenance on a wet swale system approximately 1,000 square feet in size that has adequate pretreatment and where seed and/or live plants have been installed appropriately.

Plant Establishment Period (First two years)
- Monthly weeding – 12 visits (6 per year) at 1 hour per visit
- Vegetation replacement – 1 overseeding or replanting effort, 2 hours (assuming 10 percent warrants replacement)
- Spring cleanup (cut back of previous years vegetation) – 2 cleanups (1 per year) at 2 hours each
- Erosion, sediment, and pretreatment cleanout – 2 cleanouts (1 per year) at 1 hour each (assuming vacuum truck clean-out of any sump catch basins)
Regular Maintenance (After first two years)
- Bi-monthly (every other month) weeding – 3 visits per year at 1 hour per visit
- Vegetation replacement – 1 overseeding or replanting effort per year on average, 1 hour (assuming 5 percent warrants replacement)
- Spring cleanup (cut back of previous years vegetation) – 1 per year at 2 hours
- Erosion, sediment, and pretreatment cleanout – 2 hours per year on average (assuming vacuum truck clean-out of any sump catch basins once per year, and at least one bi-yearly (every other year) sediment removal from the bottom of the swale)

Erosion protection and sediment monitoring, removal, and disposal

Regular inspection of not only the BMP but also the immediate surrounding catchment area is necessary to ensure a long lifespan of the water quality improvement feature. Erosion should be identified as soon as possible to avoid the contribution of significant sediment to the BMP.

Pretreatment devices need to be maintained for long-term functionality of the entire BMP. Accumulated sediment in filter strips, rock diaphragms, water quality sump catch basins, or any pretreatment features will need to be inspected yearly. Timing of cleaning of these features is dependent on their design and sediment storage capabilities. In watersheds with erosion or high sediment loadings, the frequency of clean out will likely be increased. A vacuum truck is typically used for sediment removal. It is possible that any sediment removed from pretreatment devices or from the bottom of a dry swale may contain high levels of pollutants. All sediments, similar to those retrieved from a stormwater pond during dredging, may be subjected to the MPCA’s guidance for reuse and disposal.

If a grassed filter strip is used as pretreatment, they should be mowed as frequently as a typical lawn. Native vegetated filter strips can be maintained less frequently, such as once per year (e.g., mow and remove cut material or prescribed burn). Depending on the contributing watershed, grassed BMPs may also need to be swept before mowing. All grassed BMPs should be swept annually with a stiff bristle broom or equal to remove thatch and winter sand. The University of Minnesota’s Sustainable Urban Landscape Series website provides guidance for turf maintenance, including mowing heights.

Seeding, planting, and landscaping maintenance

Plant selection during the design process is essential to limit the amount of maintenance required. It is also critical to identify who will be maintaining the BMP in perpetuity and to design the plantings or seedings accordingly. The decision to install containerized plants or to seed will dictate the appearance of the BMP for years to come. Inundated areas are typically planted with live plant material such as plugs (as opposed to seed); however, it may be feasible to vegetate these areas using seed if the practice is constructed off-line and the seed is able to grow sufficiently prior to inundation. If the BMP is designed to be seeded with an appropriate native plant based seed mix, it is essential the owner have trained staff or the ability to hire specialized management professionals. Seedings can provide plant diversity and dense coverage that helps maintain drawdown rates, but landscape management professionals that have not been trained to identify and appropriately manage weeds within the seeding may inadvertently allow the BMP to become infested and the designed plant diversity be lost. The following are minimum requirements for seed establishment and plant coverage.

At least 50 percent of specified vegetation cover at end of the first growing season, not including REQUIRED cover crop
At least 90 percent of specified vegetation cover at end of the third growing season, not including REQUIRED cover crop
Supplement seeding/plantings to meet project specifications if cover requirements are not met
Tailor percent coverage requirements to project goals and vegetation. For example, percent cover required for turf after one growing season would likely be 100 percent, whereas it would be lower for other vegetation types.

For information on plant selection, link here.

For proper nutrient control, swales must not be fertilized unless a soil test from a certified lab indicates nutrient deficiency. If this is the case, apply the minimum rate of appropriate nutrients to provide a suitable environment for vegetation establishment while also minimizing the mobilization (and loss) of nutrients to downstream receiving waters. Irrigation may be needed during establishment, depending on soils, precipitation, and if stormwater flows are kept off-line during establishment.

Weeding is especially important during the plant establishment period, when vegetation cover is not 100 percent yet. Some weeding will always be needed. It is also important to budget for some plant replacement (at least 5 to 10 percent of the original plantings or seedings) during the first few years in case some of the plants or seed that were originally installed don’t become vigorous. It is highly recommended that the install contractor be responsible for a plant warranty period. Typically, plant warranty periods can be 60 days or up to one year from preliminary acceptance through final inspections. If budget allows, installing larger plants (#1 container vs. 4” pot) during construction can decrease replacement rates if properly cared for during the establishment period.

Weeding in years after initial establishment should be targeted and thorough. Total eradication of aggressive weeds at each maintenance visit will ultimately reduce the overall effort required to keep the BMP weed free. Mulch is generally not recommended for use in swales since flowing water typically washes it downstream; however, mulch may be appropriate in planting beds or around individual trees on upper sideslopes and adjacent areas.

Rubbish and trash removal will likely be needed more frequently than in the adjacent landscape. Trash removal is important for prevention of mosquitoes and for the overall appearance of the BMP.

Sustainable service life

The service life of swales depends upon the pollutant of concern.

Infiltration rate service life before clogging

Infiltration is not a primary function of wet swales.

Nitrogen reduction

Nitrate is generally less than one-third of the total nitrogen in urban stormwater runoff. Denitrification is a bacterial reaction occurring under anaerobic conditions that may occur in swales that pond water for extended periods of time. Denitrification converts nitrate in stormwater to nitrogen gas, requiring a source of organic matter. Denitrification occurs under anoxic conditions where carbon is supplied via rooted plants via sediments comprised of decomposing organic material. Denitrification is also controlled by temperature with colder temperatures limiting microbial processing of nitrogen is limited. (Erickson, Weiss and Gulliver, 2013).

Wet swales have an internal water storage (i.e., inundation) zone. If this zone is deep enough and flow rates are low enough, soluble nitrogen will be removed through denitrification, a microbially-mediated process that occurs only under anoxic conditions. Denitrification requires organic matter as a carbon source, which is supplied by decaying root matter and mulch. Particulate bound nitrogen in stormwater runoff will typically be removed through sedimentation. Lastly, plants uptake nitrogen since it is essential for plant growth. All of these processes are self-sustaining with routine maintenance, and the nitrogen reduction service life of a wet swale should be very long. In very shallow or high flow wet swales (i.e., oxygenated systems), denitrification is not an important process, and leaching of nitrate may occur. In systems having soils with a high organic matter content, organic nitrogen can be converted to nitrate, resulting in loss of nitrogen through leaching (Liging and Davis, 2014).

Phosphorus reduction

Phosphorus removal in wet swales is achieved primarily through sorption of phosphorus to trapped sediments. Therefore, it is beneficial to intermittently remove sediment (with its attached phosphorus) from the bottom of wet swales. Sediment should be disposed in an acceptable manner (e.g., landfill).

Heavy metals retention

Metals are typically retained in wet swale systems (including wet swales) through sedimentation and adsorption processes. Therefore, it is beneficial to intermittently remove sediment (with its attached metals) from the bottom of wet swales. Sediment should be disposed in an acceptable manner (e.g., landfill). Since there are a finite amount of sorption sites for metals in a particular soil/media, there will be a finite service life for the removal of dissolved metals. Morgan et al. (2011) investigated cadmium, copper, and zinc removal and retention with batch and column experiments. Using synthetic stormwater at typical stormwater concentrations, they found that 6 inches of filter media composed of 30 percent compost and 70 percent sand will last 95 years until breakthrough (i.e., when the effluent concentration is 10 percent of the influent concentration). They also found that increasing compost from 0 percent to 10 percent more than doubles the expected lifespan for 10 percent breakthrough in 6 inches of filter media for retainage of cadmium and zinc. Using accelerated dosing laboratory experiments, Hatt et al. (2011) found that breakthrough of Zn was observed after 2000 pore volumes, but did not observe breakthrough for Cd, Cu, and Pb after 15 years of synthetic stormwater passed through the media. However, concentrations of Cd, Cu, and Pb on soil/media particles exceeded human and/or ecological health levels, which could have an impact on disposal if the soil/media needed replacement. Since the majority of metals retainage occurs in the upper 2 to 4 inches of the soil/media (Li and Davis, 2008), long-term metals capture may only require rejuvenation of the upper portion of the media. If concentrations of metals in runoff are anticipated to be elevated, wet swale design should include soil amendments as indicated above.

Polycyclic aromatic hydrocarbons (PAHs) reduction

Accumulation of polycyclic aromatic hydrocarbons (PAHs) in sediments has been found to be so high in some stormwater retention ponds that disposal costs for the dredging spoils were prohibitively high. Research has shown that rain gardens, on the other hand, are “a viable solution for sustainable petroleum hydrocarbon removal from stormwater, and that vegetation can enhance overall performance and stimulate biodegradation.” (Lefevre et al., 2012). Given that wet swales provide some of the same functions as stormwater retention ponds (i.e., inundated portions) and rain gardens (i.e., higher sideslopes), it would be expected they provide some PAH management. However, swales performance in PAH management has not been the focus of any identified studies.

Typical maintenance problems and activities

The following table summarizes common maintenance concerns, suggested actions, and recommended maintenance schedule.

Typical maintenance problems and activities for wet swales
Link to this table

Inspection Focus	Common Maintenance Problems	Maintenance Activity	Recommended Maintenance Schedule
Drainage Area	Erosion of catchment area contributing significant amount of sediment	In case of severely reduced drawdown time, scrape bottom of basin and remove sediment. Disc or otherwise aerate/scarify basin bottom. De-thatch if basin bottom is turf grass. Restore original design cross section or revise section to increase infiltration rate and restore with vegetation as necessary.	Bi‐monthly April through October
Site Erosion	Scouring at inlets	Correct earthwork to promote non‐erosive flows that are evenly distributed	As necessary
Site Erosion	Unexpected flow paths into practice	Correct earthwork to eliminate unexpected drainage or created additional stable inlets as necessary	As necessary
Vegetation	Severe weed establishment	Limit the ability for noxious weed establishment by properly mowing, mulching or timely herbicide or hand weeding. Refer to the MDA Noxious Weed List	Bi‐monthly April through October
Vegetation	Vegetative cover	Add seed/plants to maintain ≥95% vegetative cover.	Bi‐monthly April through October
Pretreatment	Pretreatment screens or sumps reach capacity	Remove sediment and oil/grease from pretreatment devices/structures	Minimum yearly or as per manufacturer's recommendations
Pretreatment	Vegetative filter strip failure	Reduce height of vegetative filter strip that may be limiting in‐flow. Re‐establish vegetation to prevent erosion. Leave practice off‐line until full reestablishment.	Mow grass filter strips monthly. Restore as necessary

Maintenance agreements

A Maintenance Agreement is a legally binding agreement between two parties, and is defined as ”a nonpossessory right to use and/or enter onto the real property of another without possessing it.“ Maintenance Agreements are often required for the issuance of a permit for construction of a stormwater management feature and are written and approved by legal counsel. Maintenance Agreements are often similar to Construction Easements. A Maintenance Agreement is required for one party to define and enforce maintenance by another party. The Agreement also defines site access and maintenance of any features or infrastructure if the property owner fails to perform the required maintenance. Maintenance Agreements are commonly established for a defined period such as five years for a residential site or 10 to 20 years for a commercial/governmental site after construction of the filtration practice. Maintenance agreements often define the types of inspection and maintenance that would be required for that filtration practice and what the timing and duration of the inspections and maintenance may be. Essential inspection and maintenance activities include but are not limited to sediment removal, erosion monitoring and correction, and vegetative maintenance and weeding. If maintenance is required to be performed due to failure of the site owner to properly maintain the filtration practices, payment or reimbursement terms of the maintenance work are defined in the Agreement. Below is an example list of maintenance standards from an actual Maintenance Agreement.

Live plantings and seeding areas shall be watered as necessary to achieve performance standards.
Weeding and vegetation management (e.g., mowing, spot spraying) shall be conducted as necessary to achieve performance standards.
Dead plant material, garbage, and other debris shall be removed from the swale at least annually.
Silt/sediment should be removed from the swale bottom when the accumulation exceeds one inch.
Side slopes must be inspected for erosion and the formation of rills or gullies at least annually, and erosion problems must be corrected immediately.
If properly planned, designed, constructed, and maintained (including protected from sediment and compaction and incorporating sufficient pretreatment), a wet swale is likely to retain its effectiveness for well over 20 years. After that time, inspection will reveal whether interventions are warranted.

In some project areas, a drainage easement may be required. Having an easement provides a mechanism for enforcement of maintenance agreements to help ensure swales are maintained and functioning. Drainage easements also require that the land use not be altered in the future. Drainage easements exist in perpetuity and are required property deed amendment to be passed down to all future property owners. As defined by the Maintenance Agreement, the landowner should agree to provide notification immediately upon any change of the legal status or ownership of the property. Copies of all duly executed property transfer documents should be submitted as soon as a property transfer is made final.

Maintenance inspection reports

The maintenance inspection report for dry swale with check dams can be used with some modifications (link here). The contents of the inspection form are provided below. For another source of information on visual indicators, see Chesapeake Stormwater visual indicators form.

Maintenance Inspection Report for Dry Swale with Check Dams and Stormwater Step Pool. Can be used for wet swales with exceptions, as noted in footnotes.

Date: ____________________________________________________________________

Inspector Name/Address/Phone Number: _______________________________________

Site Address: ______________________________________________________________

Owner Name/Address/Phone Number: _________________________________________

Drainage Area Stabilization (Inspect after large storms for first two years, Inspect yearly in spring or after large storms after first two years)

Erosion control/planting/seeding necessary: __________________________________________________
Mowing, pruning and debris removal necessary: _______________________________________________
Observations:

______________________________________________________________________________________ ______________________________________________________________________________________

Inlets & Pretreatment Structures (Inspect in Spring and Fall)

Repair needed: _________________________________________________________________________
Debris & sediment removal required: _______________________________________________________
Erosion evident: _________________________________________________________________________
Water by-passing inlet: ___________________________________________________________________
Vegetation control necessary: _____________________________________________________________
Observations:

______________________________________________________________________________________ ______________________________________________________________________________________

Swale (Inspect after large storms for first two years, Inspect yearly in spring or after large storms after first two years)

Condition of infiltration area¹: ______________________________________________________________
Condition of check dams: _________________________________________________________________
Surface erosion evident: __________________________________________________________________
Debris/sediment removal required: _________________________________________________________
Adequate drawdown/standing water²: _______________________________________________________
Weeding and pruning necessary: ___________________________________________________________
Mulch replacement necessary³: _____________________________________________________________
Observations:

______________________________________________________________________________________ ______________________________________________________________________________________

Outlet/Emergency Overflow (Inspect in Spring and Fall)

Overflow type: _________________________________________________________________________
Debris/sediment removal required: _________________________________________________________
Repair needed: _________________________________________________________________________
Observations:

______________________________________________________________________________________ ______________________________________________________________________________________

¹For wet swale, check condition of inundated area
²For wet swale with check dam, drawdown applies to the water elevation at the botton of weir
³Not applicable for wet swale

Calculating credits for wet swale (wetland channel)

Warning: Models are often selected to calculate credits. The model selected depends on your objectives. For compliance with the Construction Stormwater permit, the model must be based on the assumption that an instantaneous volume is captured by the BMP.

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 Infrastructure for stormwater management.

Recommended pollutant removal efficiencies, in percent, for wet swale BMPs. Sources. NOTE: removal efficiencies are 100 percent for water that is infiltrated. TSS=total suspended solids; TP=total phosphorus; PP=particulate phosphorus; DP=dissolved phosphorus; TN=total nitrogen
TSS	TP	PP	DP	TN	Metals²	Bacteria³	Hydrocarbons
40/20¹	0	0	0	15⁴	35	35	ND⁵
¹ 40 percent credit if a check dam is employed; 20 percent credit if no check dam is employed; ² Value represents the median removal for total Cd, Cr, Cu, Pb, and Zn using data from the International Stormwater BMP database (2016 summaries); removal for dissolved metal is 0; ³ Data from the International Stormwater BMP database, 2016, for fecal coliform bacteria; ⁴ From the International Stormwater BMP database, 2016, for total nitrogen; ⁵ No data found.

Credit refers to the quantity of stormwater or pollutant reduction achieved either by an individual Best Management Practice BMP or cumulatively with multiple BMPs. Stormwater credits are a tool for local stormwater authorities who are interested in

providing incentives to site developers to encourage the preservation of natural areas and the reduction of the volume of stormwater runoff being conveyed to a best management practice (BMP);
complying with permit requirements, including antidegradation (see [4]; [5]);
meeting the MIDS performance goal; or
meeting or complying with water quality objectives, including Total Maximum Daily Load (TMDL) Wasteload Allocations (WLAs).

This page provides a discussion of how wet swales can achieve stormwater credits.

Overview

A wet swale acts as a very long and linear shallow biofiltration or linear wetland treatment system. Wet swales do not provide volume reduction and have limited treatment capability. Incorporation of check dams into the design allows treatment of a portion or all of the water quality volume within a series of cells created by the check dams. Wet swales planted with emergent wetland plant species provide improved pollutant removal. Wet swales may be used as pretreatment practices. Wet swales are commonly used for drainage areas less than 5 acres in size.

Pollutant Removal Mechanisms

Wet swales without check dams primarily remove pollutants through filtration during conveyance of stormwater runoff. Wet swales do not achieve significant volume reduction. Check dams may be incorporated into wet swale design to enhance settling and filtration of solids.

Location in the Treatment Train

Wet swales provide limited water quality treatment and no volume control and are not recommended practices unless options for other BMPs are limited.

Wet swales are designed primarily as in-line systems for stormwater quality and typically are used in conjunction with other structural controls in the stormwater treatment train. Wet swales may be used at various locations within a treatment train and can be used for pretreatment, conveyance, and/or primary treatment.

Methodology for calculating credits

This section describes the basic concepts and equations used to calculate credits for Total Suspended Solids (TSS).

Wet swale practices generate credits for TSS. Wet swale practices are moderately effective at reducing concentrations of metals. They are somewhat effective at removing bacteria. This article does not provide information on calculating credits for pollutants other than TSS, but references are provided that may be useful for calculating credits for other pollutants.

Assumptions and Approach

In developing the credit calculations, it is assumed the swale is properly designed, constructed, and maintained in accordance with the Minnesota Stormwater Manual. If any of these assumptions is not valid, the BMP may not qualify for credits or credits should be reduced based on reduced ability of the BMP to achieve volume or pollutant reductions. For guidance on design, construction, and maintenance, see the appropriate article within the Manual.

Warning: Pretreatment is required for all filtration and infiltration practices

Unlike other BMPs such as bioretention and permeable pavement, credits for swales are calculated in two ways. First, if check dams are incorporated into the design, the water quality volume (V_WQ) is assumed to be delivered instantaneously to the BMP and stored as water ponded behind the check dam, above the soil or filter media, and below the overflow point of the check dam. V_WQ can vary depending on the stormwater management objective(s). For construction stormwater, V_WQ is 1 inch times new impervious surface area. For MIDS, the V_WQ is 1.1 inches times impervious surface area.

Second, if check dams are not incorporated into the swale, water will be filtered as it is conveyed along the swale. Some settling also occurs as the water is conveyed. The extent of filtration is a function of the channel roughness, including vegetation effects, and the slope of the swale, which affects the velocity of the water and thus settling.

Total suspended solids

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

The water quality volume (V_wq) achieved behind each check dam (instantaneous volume), in cubic feet, is given by

where

h = check dam height (inches)

H = horizontal component of the swale side slope (1 vertical : H horizontal)(inches)

S = slope (unitless); and

Bw = channel bottom width (inches)

Add the V_wq for each check dam together to obtain the cumulative water quality volume for the swale.

TSS reduction credits correspond with the volume captured by swale check dams and is given by

where

M_TSS = TSS removal (pounds); and

M_{TSS_f} = TSS removal from filtered water (pounds).

The event-based mass of pollutant removed through filtration, in pounds, is given by

<math> M_{TSS_f} = 0.0000624 V_{total} EMC_{TSS} R_{TSS} </math>

where

V_total is the total volume of water captured by the BMP (cubic feet);

EMC_TSS is the event mean concentration (mg/L); and

R_TSS is the TSS pollutant removal percentage for filtered runoff.

The Stormwater Manual provides a recommended value for R_TSS of 0.40 (40 percent) removal for filtered water. Alternate justified percentages for TSS removal can be used if proven to be applicable to the BMP design.

The above calculations may be applied on an event or annual basis and are given by

<math> M_{TSS_f} = 2.72\ F\ V_{F_{annual}}\ EMC_{TSS}\ R_{TSS} </math>

where

F is the fraction of annual volume filtered through the BMP; and

V_annual is the annual volume treated by the BMP, in acre-feet.

Water not captured by a check dam but conveyed in the swale are assigned a removal value of 0.20 (20 percent).

Methods for calculating credits

This section provides specific information on generating and calculating credits from swale BMPs for Total Suspended Solids (TSS). Pollution reductions (“credits”) may be calculated using one of the following methods:

Quantifying pollution reductions based on accepted hydrologic models
MIDS Calculator
Quantifying pollution reductions based on values reported in literature
Quantifying pollution reductions based on field monitoring

Credits based on models

Warning: The model selected depends on your objectives. For compliance with the Construction Stormwater permit, the model must be based on the assumption that an instantaneous volume is captured by the BMP.

Users may opt to use a water quality model or calculator to compute TSS pollutant removal for the purpose of determining credits for wet swales. The available models described in the following sections are commonly used by water resource professionals, but are not explicitly endorsed or required by the Minnesota Pollution Control Agency.

Use of models or calculators for the purpose of computing pollutant removal credits should be supported by detailed documentation, including:

Model name and version
Date of analysis
Person or organization conducting analysis
Detailed summary of input data
Calibration and verification information
Detailed summary of output data

The following table lists water quantity and water quality models that are commonly used by water resource professionals to predict the hydrologic, hydraulic, and/or pollutant removal capabilities of a single or multiple stormwater BMPs. The table can be used to guide a user in selecting the most appropriate model for computing volume, TSS, and/or TP removal for constructed basin BMPs. In using this table to identify models appropriate for constructed ponds and wetlands, use the sort arrow on the table and sort by Constructed Basin BMPs. Models identified with an X may be appropriate for using with constructed basins.

Comparison of stormwater models and calculators. Additional information and descriptions for some of the models listed in this table can be found at this link. Note that the Construction Stormwater General Permit requires the water quality volume to be calculated as an instantaneous volume, meaning several of these models cannot be used to determine compliance with the permit.
Link to this table
Access this table as a Microsoft Word document: File:Stormwater Model and Calculator Comparisons table.docx.

Model name	BMP Category						Assess TP removal?	Assess TSS removal?	Assess volume reduction?	Comments
Model name	Constructed basin BMPs	Filter BMPs	Infiltrator BMPs	Swale or strip BMPs	Reuse	Manu- factured devices	Assess TP removal?	Assess TSS removal?	Assess volume reduction?	Comments
Center for Neighborhood Technology Green Values National Stormwater Management Calculator	X	X	X		X		No	No	Yes	Does not compute volume reduction for some BMPs, including cisterns and tree trenches.
CivilStorm							Yes	Yes	Yes	CivilStorm has an engineering library with many different types of BMPs to choose from. This list changes as new information becomes available.
EPA National Stormwater Calculator	X		X		X		No	No	Yes	Primary purpose is to assess reductions in stormwater volume.
EPA SWMM	X		X		X		Yes	Yes	Yes	User defines parameter that can be used to simulate generalized constituents.
HydroCAD	X		X	X			No	No	Yes	Will assess hydraulics, volumes, and pollutant loading, but not pollutant reduction.
infoSWMM	X		X		X		Yes	Yes	Yes	User defines parameter that can be used to simulate generalized constituents.
infoWorks ICM	X	X	X	X			Yes	Yes	Yes
i-Tree-Hydro			X				No	No	Yes	Includes simple calculator for rain gardens.
i-Tree-Streets							No	No	Yes	Computes volume reduction for trees, only.
LSPC	X		X	X			Yes	Yes	Yes	Though developed for HSPF, the USEPA BMP Web Toolkit can be used with LSPC to model structural BMPs such as detention basins, or infiltration BMPs that represent source control facilities, which capture runoff from small impervious areas (e.g., parking lots or rooftops).
MapShed	X	X	X	X			Yes	Yes	Yes	Region-specific input data not available for Minnesota but user can create this data for any region.
MCWD/MWMO Stormwater Reuse Calculator					X		Yes	No	Yes	Computes storage volume for stormwater reuse systems
Metropolitan Council Stormwater Reuse Guide Excel Spreadsheet					X		No	No	Yes	Computes storage volume for stormwater reuse systems. Uses 30-year precipitation data specific to Twin Cites region of Minnesota.
MIDS Calculator	X	X	X	X	X	X	Yes	Yes	Yes	Includes user-defined feature that can be used for manufactured devices and other BMPs.
MIKE URBAN (SWMM or MOUSE)	X		X		X		Yes	Yes	Yes	User defines parameter that can be used to simulate generalized constituents.
P8	X		X	X		X	Yes	Yes	Yes
PCSWMM	X		X		X		Yes	Yes	Yes	User defines parameter that can be used to simulate generalized constituents.
PLOAD	X	X	X	X		X	Yes	Yes	No	User-defined practices with user-specified removal percentages.
PondNet	X						Yes	No	Yes	Flow and phosphorus routing in pond networks.
PondPack	X		[				No	No	Yes	PondPack can calculate first-flush volume, but does not model pollutants. It can be used to calculate pond infiltration.
RECARGA			X				No	No	Yes
SELECT	X	X	X	X		X	Yes	Yes	Yes	User defines parameter that can be used to simulate generalized constituents.
SHSAM						X	No	Yes	No	Several flow-through structures including standard sumps, and proprietary systems such as CDS, Stormceptors, and Vortechs systems
SUSTAIN	X	X	X	X	X		Yes	Yes	Yes	Categorizes BMPs into Point BMPs, Linear BMPs, and Area BMPs
SWAT	X	X	X				Yes	Yes	Yes	Model offers many agricultural BMPs and practices, but limited urban BMPs at this time.
Virginia Runoff Reduction Method	X	X	X	X	X	X	Yes	No	Yes	Users input Event Mean Concentration (EMC) pollutant removal percentages for manufactured devices.
WARMF	X	X					Yes	Yes	Yes	Includes agriculture BMP assessment tools. Compatible with USEPA Basins
WinHSPF	X		X	X			Yes	Yes	Yes	USEPA BMP Web Toolkit available to assist with implementing structural BMPs such as detention basins, or infiltration BMPs that represent source control facilities, which capture runoff from small impervious areas (e.g., parking lots or rooftops).
WinSLAMM	X	X	X	X			Yes	Yes	Yes
XPSWMM	X		X		X		Yes	Yes	Yes	User defines parameter that can be used to simulate generalized constituents.

MIDS Calculator

Users should refer to the MIDS Calculator section of the WIKI for additional information and guidance on credit calculation using this approach.

Credits Based on Reported Literature Values

A simplified approach to computing a credit would be to apply a reduction value found in literature to the pollutant mass load or event mean concentration (EMC) of the wet swale. A more detailed explanation of the differences between mass load reductions and EMC reductions can be found here.

Designers may use the pollutant reduction values reported here or may research values from other databases and published literature.

Designers who opt for this approach should:

Select the median value from pollutant reduction databases that report a range of reductions, such as from the International BMP Database.
Select a pollutant removal reduction from literature that studied a wet swale device with site characteristics and climate similar to the device being considered for credits.
When using data from an individual study, review the article to determine that the design principles of the studied wet swale are close to the design recommendations for Minnesota, as described here, and/or by a local permitting agency.
Preference should be given to literature that has been published in a peer-reviewed publication.

The following references summarize pollutant reduction values from multiple studies or sources that could be used to determine credits. Users should note that there is a wide range of monitored pollutant removal effectiveness in the literature. Before selecting a literature value, users should compare the characteristics of the monitored site in the literature against the characteristics of the proposed wet swale, considering such conditions as watershed characteristics, swale sizing, and climate factors.

International Stormwater Best Management Practices (BMP) Database Pollutant Category Summary Statistical Addendum: TSS, Bacteria, Nutrients, and Metals
- Compilation of BMP performance studies published through 2011
- Provides values for TSS, Bacteria, Nutrients, and Metals
- Applicable to grass strips, bioretention, bioswales, detention basins, green roofs, manufactured devices, media filters, porous pavements, wetland basins, and wetland channels

Effectiveness Evaluation of Best Management Practices for Stormwater Management in Portland, Oregon
- Appendix M contains Excel spreadsheet of structural and non-structural BMP performance evaluations
- Provides values for sediment, nutrients, pathogens, metals, quantity, air purification, carbon sequestration, flood storage, avian habitat, aquatics habitat and aesthetics
- Applicable to Filters, Wet Ponds, Porous Pavements, Soakage Trenches, Flow through Stormwater Planters, Infiltration Stormwater Planters, Vegetated Infiltration Basins, Swales, and Treatment Wetlands
The Illinois Green Infrastructure Study
- Figure ES-1 summarizes BMP effectiveness
- Provides values for TN, TSS, peak flows / runoff volumes
- Applicable to Permeable Pavements, Constructed Wetlands, Infiltration, Detention, Filtration, and Green Roofs
New Hampshire Stormwater Manual
- Volume 2, Appendix B summarizes BMP effectiveness
- Provides values for TSS, TN, and TP removal
- Applicable to basins and wetlands, stormwater wetlands, infiltration practices, filtering practices, treatment swales, vegetated buffers, and pre-treatment practices
BMP Performance Analysis. Prepared for US EPA Region 1, Boston MA.
- Appendix B provides pollutant removal performance curves
- Provides values for TP, TSS, and Zn
- Pollutant removal broken down according to land use
- Applicable to Infiltration Trench, Infiltration Basin, Bioretention, Grass Swale, Wet Pond, and Porous Pavement
Weiss, P.T., J.S. Gulliver and A.J. Erickson. 2005. The Cost and Effectiveness of Stormwater Management Practices: Final Report
- Table 8 and Appendix B provides pollutant removal efficiencies for TSS and P
- Applicable to Wet Basins, Stormwater Wetlands, Bioretention Filter, Sand Filter, Infiltration Trench, and Filter Strips/Grass Swales

Credits Based on Field Monitoring

Field monitoring may be used to calculate stormwater credits in lieu of desktop calculations or models/calculators as described. Careful planning is HIGHLY RECOMMENDED before commencing a program to monitor the performance of a BMP. The general steps involved in planning and implementing BMP monitoring include the following.

Establish the objectives and goals of the monitoring.
1. Which pollutants will be measured?
2. Will the monitoring study the performance of a single BMP or multiple BMPs?
3. Are there any variables that will affect the BMP performance? Variables could include design approaches, maintenance activities, rainfall events, rainfall intensity, etc.
4. Will the results be compared to other BMP performance studies?
5. What should be the duration of the monitoring period? Is there a need to look at the annual performance vs the performance during a single rain event? Is there a need to assess the seasonal variation of BMP performance?
Plan the field activities. Field considerations include:
1. Equipment selection and placement
2. Sampling protocols including selection, storage, delivery to the laboratory
3. Laboratory services
4. Health and Safety plans for field personnel
5. Record keeping protocols and forms
6. Quality control and quality assurance protocols
Execute the field monitoring
Analyze the results

The following guidance manuals have been developed to assist BMP owners and operators on how to plan and implement BMP performance monitoring.

Urban Stormwater BMP Performance Monitoring

Geosyntec Consultants and Wright Water Engineers prepared this guide in 2009 with support from the USEPA, Water Environment Research Foundation, Federal Highway Administration, and the Environment and Water Resource Institute of the American Society of Civil Engineers. This guide was developed to improve and standardize the protocols for all BMP monitoring and to provide additional guidance for Low Impact Development (LID) BMP monitoring. Highlighted chapters in this manual include:

Chapter 2: Designing the Program
Chapters 3 & 4: Methods and Equipment
Chapters 5 & 6: Implementation, Data Management, Evaluation and Reporting
Chapter 7: BMP Performance Analysis
Chapters 8, 9, & 10: LID Monitoring

Evaluation of Best Management Practices for Highway Runoff Control (NCHRP Report 565)

AASHTO (American Association of State Highway and Transportation Officials) and the FHWA (Federal Highway Administration) sponsored this 2006 research report, which was authored by Oregon State University, Geosyntec Consultants, the University of Florida, and the Low Impact Development Center. The primary purpose of this report is to advise on the selection and design of BMPs that are best suited for highway runoff. The document includes the following chapters on performance monitoring that may be a useful reference for BMP performance monitoring, especially for the performance assessment of a highway BMP:

Chapter 4: Stormwater Characterization
- 4.2: General Characteristics and Pollutant Sources
- 4.3: Sources of Stormwater Quality data
Chapter 8: Performance Evaluation
- 8.1: Methodology Options
- 8.5: Evaluation of Quality Performance for Individual BMPs
- 8.6: Overall Hydrologic and Water Quality Performance Evaluation
Chapter 10: Hydrologic Evaluation
- 10.5: Performance Verification and Design Optimization

Investigation into the Feasibility of a National Testing and Evaluation Program for Stormwater Products and Practices.

In 2014 the Water Environment Federation released this White Paper that investigates the feasibility of a national program for the testing of stormwater products and practices. The information contained in this White Paper would be of use to those considering the monitoring of a manufactured BMP. The report does not include any specific guidance on the monitoring of a BMP, but it does include a summary of the existing technical evaluation programs that could be consulted for testing results for specific products (see Table 1 on page 8).

Caltrans Stormwater Monitoring Guidance Manual (Document No. CTSW-OT-13-999.43.01)

The most current version of this manual was released by the State of California, Department of Transportation in November 2013. As with the other monitoring manuals described, this manual does include guidance on planning a stormwater monitoring program. However, this manual is among the most thorough for field activities. Relevant chapters include:

Chapter 4: Monitoring Methods and Equipment
Chapter 5: Analytical Methods and Laboratory Selection
Chapter 6: Monitoring Site Selection
Chapter 8: Equipment Installation and Maintenance
Chapter 10: Pre-Storm Preparation
Chapter 11: Sample Collection and Handling
Chapter 12: Quality Assurance / Quality Control
Chapter 13: Laboratory Reports and Data Review
Chapter 15: Gross Solids Monitoring

Optimizing Stormwater Treatment Practices: A Handbook of Assessment and Maintenance

This online manual was developed in 2010 by Andrew Erickson, Peter Weiss, and John Gulliver from the University of Minnesota and St. Anthony Falls Hydraulic Laboratory with funding provided by the Minnesota Pollution Control Agency. The manual advises on a four-level process to assess the performance of a Best Management Practice, involving:

Level 1: Visual Inspection
Level 2: Capacity Testing
Level 3: Synthetic Runoff Testing
Level 4: Monitoring
Level 1 activities do not produce numerical performance data that could be used to obtain a stormwater management credit. BMP owners and operators who are interested in using data obtained from Levels 2 and 3 should consult with the MPCA or other regulatory agency to determine if the results are appropriate for credit calculations. Level 4, Monitoring, is the method most frequently used for assessment of the performance of a BMP.

Use these links to obtain detailed information on the following topics related to BMP performance monitoring:

Other pollutants

According to the International BMP Database, studies have shown wet swales are somewhat effective at reducing concentrations of bacteria, metals, and nitrogen. This database provides an overview of BMP performance in relation to various pollutant categories and constituents that were monitored in BMP studies within the database. The report notes that effectiveness and range of unit treatment processes can vary greatly depending on BMP design and location. The following table shows a list of the constituents and associated pollutant category for the monitored “media filters” data. The constituents shown all had data representing decreases in effluent pollutant loads for the median of the data points and the 95% confidence interval about the median.

Wet swale pollutant load reduction
Link to this table

Pollutant Category	Constituent	Treatment Capabilities (Low = < 30%; Medium = 30-65%; High = 65 -100%)
Metals¹	Cd, Pb, Zn	Medium
Metals¹	Cu, Cr	Low
Nutrients	Total Nitrogen, TKN	Low
Bacteria	Fecal Coliform, E. coli	Medium

¹Results are for total metals only

References and suggested reading

Ahearn, Dylan, and Richard Tveten. "Legacy LID: Stormwater Treatment in Unimproved Embankments along Highway Shoulders in Western Washington." In Proceedings of the 2008 International Low Impact Development (LID) Conference, pp. 16-19. 2008.
Barrett, Michael E., Michael Vincent Keblin, Patrick M. Walsh, Joseph F. Malina Jr, and Randall J. Charbeneau. Evaluation of the performance of permanent runoff controls: summary and conclusions. No. TX-99/2954-3F,. 1998.
Barrett, Michael E., Patrick M. Walsh, Joseph F. Malina Jr, and Randall J. Charbeneau. "Performance of vegetative controls for treating highway runoff." Journal of environmental engineering 124, no. 11 (1998): 1121-1128.
Barrett, Michael, Anna Lantin, and Steve Austrheim-Smith. "Storm water pollutant removal in roadside vegetated buffer strips." Transportation Research Record: Journal of the Transportation Research Board 1890, no. 1 (2004): 129-140.
Bureau of Environmental Services. 2006. Effectiveness Evaluation of Best Management Practices for Stormwater Management in Portland, Oregon. Bureau of Environmental Services, Portland, Oregon.
California Stormwater Quality Association. "California Stormwater BMP Handbook-New Development and Redevelopment." California Stormwater Quality Association, Menlo Park, CA (2003).
Caltrans. 2004. BMP Retrofit Pilot Program Final Report, Report No., CTSW-RT-01-050. Division of Environmental Analysis, California Dept. of Transportation, Sacramento, CA
CDM Smith. 2012. Omaha Regional Stormwater Design Manual Chapter 8 Stormwater Best Management Practices. Kansas City, MO.
Dorman, M. E., H. Hartigan, F. Johnson, and B. Maestri. Retention, detention, and overland flow for pollutant removal from highway stormwater runoff: interim guidelines for management measures. Final report, September 1985-June 1987. No. PB-89-133292/XAB.
Consultants, Geosyntec, and Wright Water Engineers. "Urban stormwater BMP performance monitoring." (2002).
Leisenring, M., J. Clary, and P. Hobson. "International Stormwater Best Management Practices (BMP) Database Pollutant Category Summary Statistical Addendum: TSS, Bacteria, Nutrients, and Metals July 2012." (2012): 1-31.
Gulliver, J. S., A. J. Erickson, and PTe Weiss. "Stormwater treatment: Assessment and maintenance." University of Minnesota, St. Anthony Falls Laboratory. Minneapolis, MN. http://stormwaterbook. safl. umn. edu (2010).
Guo, James CY, Gerald E. Blackler, T. Andrew Earles, and Ken MacKenzie. "Incentive index developed to evaluate storm-water low-impact designs." Journal of Environmental Engineering 136, no. 12 (2010): 1341-1346.
Harper, Harvey H. "Effects of stormwater management systems on groundwater quality." FDEP Project# WM190. Florida Department of Environmental Regulation, Tallahassee, FL (1988).
Jaffe, et. al. 2010. The Illinois Green Infrastructure Study. Prepared by the University of Illinois at Chicago, Chicago Metropolitan Agency for Planning, Center for Neighborhood Technology, Illinois-Indiana Sea Grant.
Jurries, Dennis. "Biofilters (Bioswales, Vegetative Buffers, & Constructed Wetlands) for Storm Water Discharge Pollution Removal." Quality, S. o. OD o. E.(Ed.).
Kearfott, Pamela J., Michael E. Barrett, and Joseph F. Malina. Stormwater quality documentation of roadside shoulders borrow ditches. Center for Research in Water Resources, University of Texas at Austin, 2005.
Kim, Yun Ki, and Seung Rae Lee. "Field infiltration characteristics of natural rainfall in compacted roadside slopes." Journal of geotechnical and geoenvironmental engineering 136, no. 1 (2009): 248-252.
Leisenring, M., J. Clary, and P. Hobson. "International Stormwater Best Management Practices (BMP) Database Pollutant Category Summary Statistical Addendum: TSS, Bacteria, Nutrients, and Metals July 2012." (2012): 1-31.
New Hampshire Department of Environmental Services. 2008. New Hampshire Stormwater Manual. Volume 2 Appendix B. Concord, NH.
Transportation Officials, Oregon State University. Dept. of Civil, Environmental Engineering, University of Florida. Dept. of Environmental Engineering Sciences, GeoSyntec Consultants, and Low Impact Development Center, Inc. Evaluation of Best Management Practices for Highway Runoff Control. No. 565. Transportation Research Board, 2006.
State of California, Department of Transportation. 2013. Caltrans Stormwater Monitoring Guidance Manual. Sacramento, CA.
TetraTech. 2008. BMP Performance Analysis. Prepared for US EPA Region 1, Boston, MA.
Torres, Camilo. "Characterization and Pollutant Loading Estimation for Highway Runoff in Omaha, Nebraska." (2010).
Water Environment Federation. 2014. Investigation into the Feasibility of a National Testing and Evaluation Program for Stormwater Products and Practices. A White Paper by the National Stormwater Testing and Evaluation of Products and Practices (STEPP) Workgroup Steering Committee.
WEF, ASCE/EWRI. 2012. Design of Urban Stormwater Controls, WEF Manual of Practice No. 23, ASCE/EWRI Manuals and Reports on Engineering Practice No. 87. Prepared by the Design of Urban Stormwater Controls Task Forces of the Water Environment Federation and the American Society of Civil Engineers/Environmental & Water Resources Institute.
Weiss, Peter T., John S. Gulliver, and Andrew J. Erickson. "The Cost and Effectiveness of Stormwater Management Practices Final Report." (2005).

Cost/benefit considerations for wet swale (wetland channel)

Additional considerations for wet swale (wetland channel)

Links for wet swale (wetland channel)

References for wet swale (wetland channel)

Wet swale (wetland channel) combined

Contents

Overview

Function within stormwter treatment train

Typical applications

Infeasibility criteria

MPCA permit applicability

Retrofit suitability

Special receiving waters suitability

Cold climate suitability

Water quantity treatment

Water quality treatment

Limitations

Types of wet swale (wetland channel)

Design criteria for wet swale (wetland channel)

Design phase maintenance considerations

Major design elements

Physical feasibility initial check

Contributing drainage area

Site topography

Depth to groundwater and bedrock

Karst topography

Soils hydrologic soil group mapping

Practice and site considerations

Conveyance

Pretreatment

Anticipated flow

Grading

Treatment

Vegetation

Landscaping

Snow considerations

Safety

Materials specification

Erosion control (MNDOT – product by velocity)

Rock

Weir

Plants

Design procedure – design steps

Step 1. Make a preliminary judgment

Step 2. Confirm design criteria and applicability

Step 3. Perform field verification of site suitability

Step 4. Select design variant based on physical suitability evaluation

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

Step 6. Compute number of check dams

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

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

Step 9. Design inlets and sediment forebay(s)

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

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

Step 13. Prepare vegetation and landscaping plan

Step 14. Prepare operation and maintenance plan

Step 15. Prepare cost estimate

Construction specifications for wet swale (wetland channel)

Access agreements

Construction specifications for swale practices

Pre-construction meeting

Site protection

Pretreatment

Temporary erosion and sediment control

Compaction prevention

Inspection and documentation

Construction checklists

Construction sequence

Step 1 – Site examination and preparation

Step 2 – Excavation

Step 3 – Decompaction

Step 4 – Installation of check dams

Step 5 – Restoration and plantings

Step 6 – Final stabilization and Closeout

Assessing the performance of wet swale (wetland channel)

Operation and maintenance of wet swale (wetland channel)

Design phase maintenance

Construction phase maintenance

Post-construction operation and maintenance

Inspection and maintenance planning

Summary of typical maintenance regime

Estimated hours to perform maintenance activities

Erosion protection and sediment monitoring, removal, and disposal

Seeding, planting, and landscaping maintenance