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)
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.
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.
Applications of wet swales can vary extensively. Typical applications include
Certain site-specific conditions may make the use of wet swales infeasible. Examples include:
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:
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.
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.
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 | ||||
---|---|---|---|---|---|
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 |
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.
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, Vcp, 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.
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.
The following general limitations should be recognized when considering installation of wet swales.
Types of wet swale (wetland channel)
Use this link to access .pdf diagrams of CADD drawings.
Click on file link to see all filtration CADD images in a combined pdf: File:All filtration cadd images combined 2.pdf
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
For more information on design information for individual infiltration and filtration practices, link here.
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.
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.
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.
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.
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.
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.
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.
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.
If there is space for pretreatment prior to the swale it should be evaluated. See the pretreatment section for more information.
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.
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 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.
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.
Considering management of snow, the following are recommended
For more information and example photos, see the section on snow and ice management.
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.
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.
See MNDOT Standard Specification 3601.
See MNDOT Standard Specifications 2461, 2573, 3137, 3301, 3491, 3601.
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 |
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.
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:
B. Determine how the swale will fit into the overall stormwater treatment system, including:
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.
See Major design elements
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.
1. Calculate the following runoff control volumes.
2. Once the runoff control volume is determined for design, compute the following design parameters to determine the swale size required.
<math>q = (0.00236/n) · Y · 1.67 · S · 0.5 </math>
Where:
Where:
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 Vwq.
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 Vwq at the downstream end should not exceed 18 inches.
Check for erosive velocities and modify design as appropriate based on local conveyance regulations. Provide 6 inches of freeboard.
The water level should draw down to the flow line of the controlling check dam elevation within 48 hours.
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.
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).}}
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.
See Operation and Maintenance section for guidance on preparing an O&M plan.
See Cost Considerations section for guidance on preparing a cost estimate that includes both construction and maintenance costs.
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.
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 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.
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.
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.
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.
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.
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.
A construction checklist is found here. Note this checklist may include items not applicable to wet swales.
It is the responsibility of the contractor to:
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.
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.
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.
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.
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).
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.
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 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:
The most frequently cited maintenance concern for wet swales is that they provide a breeding ground for mosquitoes. Common operational problems include:
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:
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.
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.
Project: | ||
Location: | ||
Site Status: | ||
Date: | ||
Time: | ||
Inspector: | ||
Construction Sequence | Satisfactory / Unsatisfactory | Comments |
---|---|---|
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: |
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.
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.
Wet swale operation and maintenance checklist.
Link to this table
To access an Excel version of form (for field use), click here.
Project: | ||
Location: | ||
Site Status: | ||
Date: | ||
Time: | ||
Inspector: | ||
Maintenance Item | Satisfactory / Unsatisfactory | Comments |
---|---|---|
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: |
The list below highlights the assumed maintenance regime for a wet swale.
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.
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.
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.
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.
The service life of swales depends upon the pollutant of concern.
Infiltration is not a primary function of wet swales.
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 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).
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.
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.
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 |
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 |
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 |
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 |
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.
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.
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)
______________________________________________________________________________________ ______________________________________________________________________________________
Inlets & Pretreatment Structures (Inspect in Spring and Fall)
______________________________________________________________________________________ ______________________________________________________________________________________
Swale (Inspect after large storms for first two years, Inspect yearly in spring or after large storms after first two years)
______________________________________________________________________________________ ______________________________________________________________________________________
Outlet/Emergency Overflow (Inspect in Spring and Fall)
______________________________________________________________________________________ ______________________________________________________________________________________
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
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
This page provides a discussion of how wet swales can achieve stormwater credits.
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.
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.
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.
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.
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.
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 (VWQ) 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. VWQ can vary depending on the stormwater management objective(s). For construction stormwater, VWQ is 1 inch times new impervious surface area. For MIDS, the VWQ 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.
The water quality volume (Vwq) achieved behind each check dam (instantaneous volume), in cubic feet, is given by
<math> V_{wq} = 1728 h^2 * (h * H + B_w)]/(2S) </math>
where
Add the Vwq 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
<math> M_{TSS} = M_{TSS_f} </math>
where
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
The Stormwater Manual provides a recommended value for RTSS 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
Water not captured by a check dam but conveyed in the swale are assigned a removal value of 0.20 (20 percent).
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:
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:
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 | |||||
---|---|---|---|---|---|---|---|---|---|---|
Constructed basin BMPs | Filter BMPs | Infiltrator BMPs | Swale or strip BMPs | Reuse | Manu- factured devices |
|||||
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. |
Users should refer to the MIDS Calculator section of the WIKI for additional information and guidance on credit calculation using this approach.
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:
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.
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.
The following guidance manuals have been developed to assist BMP owners and operators on how to plan and implement 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:
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:
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).
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:
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:
Use these links to obtain detailed information on the following topics related to BMP performance monitoring:
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 |
Cu, Cr | Low | |
Nutrients | Total Nitrogen, TKN | Low |
Bacteria | Fecal Coliform, E. coli | Medium |
1Results are for total metals only
Cost/benefit considerations for wet swale (wetland channel)
Additional considerations for wet swale (wetland channel)
Links for wet swale (wetland channel)
This page was last edited on 19 March 2018, at 19:45.
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