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Minnesota Stormwater Manual

Construction specifications for high-gradient stormwater step-pool swale
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Green Infrastructure: Stormwater step pool practices can be an important tool for retention and detention of stormwater runoff and treatment of pollutants in stormwater runoff. Because stormwater step pools incorporate dense vegetation and infiltration/filtration, additional benefits may include cleaner air, carbon sequestration, improved biological habitat, and aesthetic value.

This page provides a discussion of construction specifications for stormwater step pools.

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 stormwater step pool practices

Construction of stormwater step pools 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 2018. 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 stormwater step pool as much as practicable. Utilizing sediment and erosion control measures will help to keep the stormwater step pool 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 1/2 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.

Potential techniques that could be used to divert runoff and isolate the dry swale may include one or more of the following:

  • Cofferdam and bypass pump
  • Bypass channel
  • Place plastic sheeting over swale when rain in forecasted – anchor with staples, sand bags, etc.
  • Isolate BMP from runoff using sand bags, fiber logs, coir logs, silt fence, or other barrier
  • Temporary pond and bypass pumping
Warning:
  • It is REQUIRED that future stormwater step pool locations not be used as temporary sedimentation basins unless 3 feet of cover is left in place during construction.
  • If a stormwater step pool 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 away from the stormwater step pool area.

Compaction prevention

Preventing and alleviating compaction are crucial during construction of stormwater step pool practices, as compaction can reduce infiltration rates by increasing bulk density of the soil. The stormwater step pool 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 stormwater step pools 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.

Stormwater step pool 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
Soil tested for permeability
Soil tested for phosphorus content (include test method)
Verify site was not overdug
Project benchmark near site
Facility location staked out
Temporary erosion and sediment protection properly installed
2. Excavation
Lateral slopes completely level
Soils not compacted during excavation
Longitudinal slopes within design range
Stockpile location not adjacent to excavation area and stabilized with vegetation and/ or silt fence
Verify stockpile is not causing compaction and that it is not eroding
Was underlying soil ripped or loosened
Size, location, and inverts per plans
Side slopes stable
Groundwater / bedrock verified
3. Check dams
Dimensions per plans
Spacing and grade installed per plans
Materials per specifications
4. Structural Components
Outlets installed pre plans
Underdrain installed to grade
Pretreatment devices installed per plans
Soil bed composition and texture conforms to specifications
Inlets installed per plans
Underdrain installed per plans
5. Vegetation
For native stormwater step pools, plants and materials ordered 6 months prior to construction
For native stormwater step pools, 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
6. Final Inspection
Dimensions per plans
Pretreatment operational
Inlet/outlet operational
Check dams operational
Soil/filter bed permeability verified
Effective stand of vegetation stabilized
Construction generated sediments removed
Contributing watershed stabilized before flow is diverted to the practice
Comments:
Actions to be taken:


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 stormwater step pools with no infiltration or filter media, cut the stormwater step pool 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 stormwater step pool area to avoid soil compaction. If it is necessary to work in the stormwater step pool bottom area, only low ground pressure tracked equipment should be allowed to complete the work. Rubber tire equipment should be strictly prohibited within the stormwater step pool 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.

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 stormwater step pool 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 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 HIGHLY RECOMMENDED for swales designed as infiltration practices. 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 is recommended for trout 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 filtration media 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 stormwater step pool 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 or mix in 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. Follow herbicide recommendations regarding duration to wait between application and seeding/planting.

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 preferable longer. Native seedings can be more difficult than containerized plantings to establish.

For information on plants recommended or suitable for swales, link here.

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

All filtration (filter media) and infiltration areas must be tested for infiltration rates after they are completed in order to submit the NPDES 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.

Minnesota Department of Transportation example construction protocols

Preliminary analysis and selection

Recommended number of soil borings, pits or permeameter tests for bioretention design. Designers select one of these methods.
Link to this table

Surface area of stormwater control measure (BMP)(ft2) Borings Pits Permeameter tests
< 1000 1 1 5
1000 to 5000 2 2 10
5000 to 10000 3 3 15
>10000 41 41 202

1an additional soil boring or pit should be completed for each additional 2,500 ft2 above 12,500 ft2
2an additional five permeameter tests should be completed for each additional 5,000 ft2 above 15,000 ft2


Field verification testing prior to pond construction

  • Soil hydraulic group represent what is stated in SWPPP (Stormwater Pollution Prevention Plan)
  • Seasonally high water table not discovered within 3 feet of the excavated pond base within a test pit
  • Commonly will test bottom of proposed pond for soil compaction (subsequent subsoil ripping) prior to media placement
  • Commonly will test bottom of proposed pond for insitu infiltration rate by test pit or water filled barrel placed on pond base surface

Filter media and material testing

  • Existing soil (option 1 below) or Washed sand (option 2 below), and compost certification
  • Washed course aggregate choker certification
  • Other treatment material certification of iron filings, activated charcoal, pH buffers, minerals, etc.
  • Geotextile separation fabric certification
  • Drain-tile certification (if filtration is specified)
  • Seed source certification
  • Barrel test verification of infiltration rate using 2.5 feet of imported 3877 Type G media

Field verification testing/inspection/verification during construction

  • Water drains away in 48 hours
  • Infiltration drainage rate does not exceed 8.3 inches per hour
  • No tracking/equipment in pond bottom
  • No sediment deposits from ongoing construction activity, media perimeter controls kept functional
  • Forebay is trapping settleable solids, floating materials, and oil/grease
  • Area staked off

Notice of Termination (NOT) verification

  • Option 1. Amending existing HSG soils with compost or other treatment material. Test the infiltration rate of each infiltration basin using a double ring infiltrometer prior to completion of the basin. Conduct the test at the finished grade of the basin bottom, prior to blending the compost with the in-situ soils or sand. Ensure infiltration rates meet or exceed greater of two times the designed infiltration rate or 2 inches per hour. Conduct a minimum of five tests per representative acre of basin area and a minimum of five tests per basin. Conduct double ring infiltrometer tests in accordance with ASTM standards. Thoroughly wet test areas prior to conducting infiltrometer tests.
  • Option 2. Importing 3877 Type G Filter Topsoil Borrow (may be amended with other treatment material). Ensure infiltration rates meet or exceed greater of two times the designed infiltration rate or 2 inches per hour, or rate specified in the plan. Conduct a minimum of five tests per representative acre of basin area and a minimum of five tests per basin. Conduct double ring infiltrometer tests in accordance with ASTM standards. Thoroughly wet test areas prior to conducting infiltrometer tests. Amend soils with additional washed sand if rates less than specified in the contract, or compost if rates exceed 8.3 inches per hour.

The permanent stormwater management system must meet all requirements in sections 15, 16, and 17 of the CSW permit and must operate as designed. Temporary or permanent sedimentation basins that are to be used as permanent water quality management basins have been cleaned of any accumulated sediment. All sediment has been removed from conveyance systems and ditches are stabilized with permanent cover.



Related pages

Stormwater step pools are currently not included as a BMP in the MIDS calculator. The swale main channel BMP can be used, but the maximum allowable slope is 4 percent. To dtermine volume retention for slopes greater than 4 percent, you will need to develop a relationship between the slope and volume retained. To do this, determine volume retention at 0.5 percent slope increments for your site at slopes ranging from 0.5 to 4 percent. Determine the appropriate regression for volume retention and slope and calculate the volume retained at the slope for your site. The relationship is not linear. Links to MIDS calculator information are provided below.

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