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Minnesota Stormwater Manual

Construction specifications for wet swale (wetland channel)
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Green Infrastructure: Swales can be an important tool for retention and detention of stormwater runoff. Depending on design and construction, swales may provide additional benefits, including cleaner air, carbon sequestration, improved biological habitat, and aesthetic value. See the section Green Stormwater Infrastructure (GSI) and sustainable stormwater management.

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.

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.


Related pages

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