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Green Infrastructure: Dry 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.

This page provides a discussion of construction specifications for dry swales (infiltration and filtration-media systems).

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 infiltration practices are maintained and functioning. See an example access agreement.

Construction specifications for swale practices

Construction of swale practices 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, 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 2016. Below is a list of MnDOT sections that may be helpful when writing project specifications for infiltration practices.

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 infiltration and 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 infiltration or 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 infiltration or filtration system.

Temporary erosion and sediment control

During construction, it is critical to keep sediment out of the swale device as much as practicable. Utilizing sediment and erosion control measures will help to keep swale areas from clogging, especially for infiltration and filtration types. 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 1/3 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.

Warning:
  • It is REQUIRED that future infiltration swale locations not be used as temporary sedimentation basins unless 3 feet of cover is left in place during construction.
  • If an infiltration swale area is excavated to final grade (or within 3 feet of) it is REQUIRED that rigorous erosion prevention and sediment controls (e.g., diversion berms) are used to keep sediment and runoff infiltration away from the infiltration area until the contributing watershed is stabilized.

3.2.2.3 Compaction prevention Preventing and alleviating compaction are crucial during construction of infiltration/filtration swale practices, as compaction can reduce infiltration rates by increasing bulk density of the soil. The infiltration/filtration 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

  • Infiltration basin

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

Step 2 – Excavation

For in-situ soil infiltration dry swales

Cut the swale and infiltration 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 infiltration area to avoid soil compaction. If it is necessary to work in the infiltration area, only low ground pressure tracked equipment should be allowed to complete the work. Rubber tire equipment should be strictly prohibited within the infiltration 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.

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.

Warning: It is REQUIRED that infiltration systems not be excavated to final grade until the contributing drainage area has been constructed and fully stabilized.

For filter media dry swales

Cut the swale area and sub-cut the filtration 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 filtration area. The contractor should start the work at the far side of the trench or basin and work their way out.

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 swale bottom areas with the exception of the bottom of the sub-cut for filter media dry swales. 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 increase infiltration and reduce compaction from construction activities. For more information on alleviating compaction, link here.

Step 4 – Subsoil Infiltration Testing for Dry Swales (in-situ soil infiltration swales)

Subsoil infiltration testing is REQUIRED for dry swales depending on in-situ infiltration. After the subsoil is decompacted, test the infiltration area to verify the assumed infiltration rate and that the infiltration area will drain dry within 48 hours.

This can be accomplished by performing double ring infiltrometer tests (ASTM D3385) in the bottom of the basin, or by filling the infiltration basin and timing how long it takes to drain from maximum water depth to dry bottom. The measured infiltration rate should equate to double the designed infiltration rate. If the basin is filled with water to perform this check, be sure sediments are not being washed into the basin during filling. If sediments are washed into the basin, they need to be removed prior to placing infiltration media.

If the basin does not drain dry within 48 hours, 24 hours for special waters, or the infiltration rate is slower than twice what was assumed in the design, additional soil loosening or modification may be necessary. Information on soil testing can be found here.

Step 5 – Installation of materials - filter media (if applicable)

Soil test results should be provided to the designer a minimum of two weeks prior to delivery of planting soil to the site. Submitted test results should include gradation and USDA soil texture classification or certification that the soil mix meets MnDOT specifications or other requirements. Samples of the mixed product should be also provided to the designer two weeks prior to delivery of media to the site. The designer should review the materials as soon as possible to avoid any potential delays in the procurement and review of another media source should the initial submittal not meet specifications.

All accumulated sediment and silt from the bottom of the filtration area should be removed prior to the placement of filtration media. The contractor should make every effort possible to place the filtration media in a way to minimize compaction of the subgrade and the filtration media itself. No construction vehicles are allowed in the filtration area after the media is placed unless approved by designer. Loose placement of filtration media shall be accomplished by dumping from the edges and spreading with the bucket of a backhoe, which is outside of the filtration area, or some other acceptable means determined by the designer. If spreading with a backhoe is not possible for the entire area of the filtration area, only tracked skid steers or other low ground pressure equipment should be permitted in the basin to spread the filtration media. This method should be minimized as much as possible. Travel over placed filtration media should be strictly prohibited.

The contractor should overfill the filtration media areas approximately 20 percent to account for consolidation of the loose soil once wetting occurs. Any small irregularities at the designed finished grade should be worked out with hand tools.

The contractor should contact the designer upon final placement of media for a final inspection prior to planting and mulching. At this inspection, the designer should check thickness and grades after soil wetting occurs and notify the contractor of areas that do not meet the tolerances specified. Tolerances in final grade are commonly vertically +/- 0.1 foot and horizontally +/- 0.5 foot.

If time goes by between the initial placement of infiltration media and planting, the contractor should be required to remove accumulated silt. This work is also a chance to perform any final subgrade grading adjustments required to obtain the finished grades as shown on the drawing.

Step 6 – Restoration and plantings

After final placement of grading and filtration media (if applicable) 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 while maintaining infiltration rates over time through the growth of healthy root systems. 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 preferably longer. Native seedings can be more difficult than containerized plantings to establish.

Step 7 – 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.

For dry swales All filtration (filter media) and infiltration areas must be tested for infiltration rates after they are completed in order to submit the NPDES Notice of Termination. It is highly recommended that all infiltration areas are tested prior to project close out, even if an NPDES permit is not required.

MnDOT projects require at least five tests per acre of infiltration area and a minimum of five tests per infiltration area. Infiltration rates shall meet or exceed double the design rate assumed. The test results from a MnDOT project must be submitted to MnDOT.

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 applicable.

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