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Minnesota Stormwater Manual

Design criteria for high-gradient stormwater step-pool swale
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This page provides a discussion of design elements and design steps for high-gradient stormwater step-pool swale (step pools). The following discussion includes step pools used as filtration or infiltration practices, with the distinction being the presence of an underdrain for filtration practices.

Green Infrastructure: Step pools 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.

Terminology

The following terminology is used throughout this design page.

Warning: REQUIRED - Indicates design standards stipulated by the MPCA Construction General Permit (CGP) or other consistently applicable regulations

HIGHLY RECOMMENDED - Indicates design guidance that is extremely beneficial or necessary for proper functioning of the practice, but not specifically required by the MPCA CGP.

RECOMMENDED - Indicates design guidance that is helpful for practice performance but not critical to the design.

Details and CADD images

Use this link to access .pdf diagrams of CADD drawings. To see all filtration CADD images in a combined pdf, click here.

Major design elements - Physical feasibility initial check

Before deciding to use a stormwater step pool practice 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 stormwater step pool practice is the appropriate BMP for the site. The following links provide additional information on specific constraints to infiltration.

Infiltration constraints

If a step pool is being considered for infiltration, the following links provide additional information on specific constraints to infiltration. The Construction Stormwater General Permit prohibits infiltration under certain conditions, which are summarized and discussed in detail at this link.

Contributing drainage area

The RECOMMENDED maximum drainage area is typically 5 acres.

Site topography

Unless slope stability calculations demonstrate otherwise, it is HIGHLY RECOMMENDED that stormwater step pools 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

Site location/minimum setback

Warning: A minimum setback of 50 feet between an infiltration practice and a water supply well is REQUIRED by the Minnesota Department of Health Rule 4725.4350.

Depth to groundwater and bedrock

A separation distance of at least 3 feet is REQUIRED under the MPCA CGP between the bottom elevation of a stormwater step pool and the elevation of the seasonally high water table. Shallow bedrock areas should be avoided for stormwater step pools with a minimum separation distance of 3 feet. A field soil properties investigation is HIGHLY RECOMMENDED.

Karst topography

If stormwater step pools 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.

Wellhead protection areas

See stormwater and wellhead protection for guidance and recommendations for determining the appropriateness of infiltrating stormwater in a Drinking Water Supply Management Area (DWSMA). For more information on source water protection see Minnesota Department of Health.

Soils hydrologic soil group mapping (link to “Design infiltration rates, in inches per hour, for A, B, C, and D soil groups” Table)

See NRCS Web Soil Survey for hydrologic soil descriptions for the stormwater step pool location. A and B soils are potentially suitable for an infiltration practice. The maximum allowed infiltration rate shall not exceed 1.63 in/hr.

Major design elements - practice and site considerations

Several considerations are made in this section for the conceptual design. Further design guidance and specifications is madeare in the following sections.

Conveyance

It is HIGHLY RECOMMENDED that the designer provides non-erosive flow velocities within the stormwater step pool 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 grassed channels. Erosion control matting or rock should be specified if higher velocities are expected.

Pretreatment

If there is space for pretreatment prior to the stormwater step pool 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.

Grading

Slope of stormwater step pool The longitudinal slope of a stormwater step pool may vary from 2% up to 10% and greater slopes if necessary. It is HIGHLY RECOMMENDED that the design engineer also considers the expected watershed flow to be conveyed by the stormwater step pool in making this preliminary determination of design alternate. Stormwater step pool bottom It is HIGHLY RECOMMENDED that the stormwater step pool bottom be no less than 4 feet wide and will be sized with the relative stage-dependent flow driven cross-sectional area in mind. Side slopes It is RECOMMENDED that the maximum side slopes within a stormwater step pool do not exceed 3H:1V and will be designed with the relative stage-dependent flow driven cross-sectional area in mind. Stormwater step pool depth Stormwater step pool depth will be estimated based on the relative stage-dependent flow driven cross-sectional area. Infiltration and filtration considerations The design engineer should review the results of the feasibility check to assist in the selection of stormwater step pool type. An additional consideration includes watershed soil transport to the site. Watersheds with unstable soils or lack of vegetative cover (e.g., construction, farmland and highly impervious surfaces) can generate and transport excessive sediments to the stormwater step pool that may affect both infiltration and filtration capacity. In these situations, pretreatment via sedimentation processes is HIGHLY RECOMMENDED. Another consideration is the level of compaction and structure of in-situ soils, when considering stormwater step pools. Construction of developments and roads, for example, significantly alter the parent state of native soils and therefore their hydrologic soil classification should be downgraded for feasibility study purposes.

Filter media

Filtration media is comprised of a combination of sand and organic material on top of a pea gravel bed that encases a perforated drain pipe. The media assists in the removal of fine particulate and dissolved pollutants, improving on the overall performance of stormwater step pools.

Underdrains

Underdrains are comprised of a perforated, level PVC pipe laid within filter media to convey runoff to either a stable day-lit area, a second form of treatment, or connected to the storm sewer. Solid-walled PVC section of piping area connected to the perforated drain pipe with a “tee” junction piece and extended to the stormwater step pool’s surface to serve as an inspection and cleanout access point. These observation/maintenance wells are spaced throughout the system.

Treatment

Stormwater treatment in stormwater step pools varies by design, relying on several functions. Organic and mineral sediments suspended in stormwater flows are deposited onto the stormwater step pool bottom, depending on their size and mass as well as water retention time in a process termed sedimentation. Though stormwater step pools 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. The second function in pollutant removal is sorption of particulate matter via the stormwater step pool soils and vegetation as it passes through the system. For stormwater step pools with or without filter media, a portion of the stormwater flows percolate through soil where fine particulate and dissolved pollutants are treated. In fully infiltrating soils, 100% of that portion of pollutant is removed from the surface conveyance to downstream waterbodies, though some shallow ground water connections to nearby water bodies or aquifers should be considered. For stormwater step pools with filter media and underdrains, a significant portion of dissolved contaminants are removed before being conveyed via drain pipe to downstream stormwater treatment practices or a receiving body of water. The last form of treatment stormwater step pools provide is plant uptake of pollutants. In most cases, stormwater step pools are not considered a volume reduction practice unless there is suitable in-situ soil for infiltration to occur, though some volume reduction can occur through evapotranspiration.

Vegetation

Vegetation plays a crucial role in stormwater step pool treatment capacity, flow attenuation and stabilization of the device (i.e., erosion control). It is HIGHLY RECEOMMENDED that preference is given to robust native, non-clump forming grasses as the predominant plant type within the stormwater step pool 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.

Landscaping

Stormwater step pools 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 stormwater step pool, aesthetically chosen vegetation may only be possible on the surface tops of the stormwater step pools.

Snow considerations

Considering management of snow, the following are recommended

  • Plan a plow path during design phase and tell snowplow operators where to push the snow. Plan trees around (not in) plow path, with a 16 foot minimum between trees.
  • Plan for snow storage (both temporary during construction and permanent). Don’t plow into raingardens routinely. Raingardens should be a last resort for snow storage (i.e. only for very large snow events as “emergency overflow”.
  • Snow storage could be, for example, a pretreatment moat around a raingarden, i.e. a forebay for snow melt.

For more information and example photos, see the section on snow and ice management.

Safety

Stormwater step pools do not pose any major safety hazards. Potential hazards could occur from the steep side slope and rock checks of the stormwater step pools if they are close to pedestrian traffic or roadways with no shoulders.

Materials specification

Erosion control

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 stormwater step pool 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 stormwater step pool 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.

Filter media

Filter media used in stormwater step pool designs should follow guidance on material specifications within the Bioretention section of the MN Stormwater Manual.

Underdrains

The following are RECOMMENDED for filtration practices with underdrains.

  • The minimum pipe diameter is 4 inches.
  • Install 2 or more underdrains for each practice system in case one clogs. At a minimum provide one underdrain for every 1,000 square feet of surface area.
  • Include at least 2 observation /cleanouts for each underdrain, one at the upstream end and one at the downstream end. Cleanouts should be at least 4 inches diameter vertical non-perforated schedule 40 PVC pipe, and extend to the surface. Cap cleanouts with a watertight removable cap.
  • Construct underdrains with Schedule 40 or SDR 35 smooth wall PVC pipe.
  • Install underdrains with a minimum slope of 0.5 percent, particularly in HSG D soils (Note: to utilize Manning’s equation the slope must be greater than 0).
  • Include a utility trace wire for all buried piping.
  • For underdrains that daylight on grade, include a marking stake and animal guard.
  • For each underdrain have an accessible knife gate valve on its outlet to allow the option of operating system as either bioinfiltration, biofiltration system or both. The valve should enable the ability to make adjustments to the discharge flow so the sum of the infiltration rate plus the under-drain discharge rate equal a 48 hour draw-down time.
  • Perforations should be 3/8 inches. Use solid sections of non-perforated PVC piping and watertight joints wherever the underdrain system passes below berms, down steep slopes, makes a connection to a drainage structure, or daylights on grade.
  • Spacing of collection laterals should be less than 25 feet.
  • Underdrain pipes should have a minimum of 3 inches of washed #57 stone above and on each side of the pipe (stone is not required below the pipe). Above the stone, two inches of choking stone is needed to protect the underdrain from blockage.
  • Avoid filter fabric.
  • Pipe socks may be needed for underdrains imbedded in sand. If pipe socks are used, then use circular knit fabric.
  • The procedure to size underdrains is typically determined by the project engineer. An example for sizing underdrains is found in Section 5.7 of the North Carolina Department of Environment and Natural Resources Stormwater BMP Manual.

Rock (MNDOT – specs)

Weir (MNDOT – specs)

Plants (MNDOT specs)

Refer to the vegetation section of the manual for selection of Minnesota native plants to be used in stormwater step pools. Care must be taken to specify plants for their position in the system (stormwater step pool bottom, side slopes and buffer). Preference towards robust non-clump forming grasses or sedges should be given to the stormwater step pool bottom 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.

Open vegetated swale materials specifications.
Link to this table

Parameter Specification Size Note Design application
Dry swale soil USCS; ML, SM, SC n/a soil with a higher percent organic content is preferred dry swale, step pool
Dry swale sand ASTM C-33 fine aggregate concrete sand 0.02” to 0.04” dry swale, step pool
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 dry swale, wet swale, step pool
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 dry swale, wet swale, step pool
Filter Strip sand/gravel pervious berm sand: per dry swale sand gravel; AASHTO M-43 sand: 0.02” to 0.04” gravel: 1/2” to 1” Mix with approximately 25% loam soil to support grass cover crop; see Bioretention planting soil notes for more detail dry swale, step pool
Pea gravel diaphragm and curtain drain ASTM D 448 varies (No. 6) or (1/8” to 3/8”) use clean bank-run gravel dry swale, step pool
Underdrain gravel per pre-cast manufacturer 1.5” to 3.5” dry swale, step pool
Underdrain ASTM D-1785 or AASHTO M-278 6” rigid Schedule 40 PVC 3/8” perf. @ 6” o.c.; 4 holes per row dry swale, step pool
Rip rap per local criteria size per requirements based on 10- year design flow dry swale, wet swale, step pool


Design procedure – design steps

Step 1. Make a preliminary judgment

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 stormwater step pool 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 stream restoration plantings with respect to developing appropriate planting plans and habitat improvement features.

Consider basic issues for initial suitability screening

Make a preliminary judgment as to whether site conditions are appropriate for the use of a stormwater step pool, and identify its function in the overall treatment system. A. Consider basic issues for initial suitability screening, including:

  • Site drainage area
  • Site topography and slopes
  • Regional or local depth to ground water and bedrock
  • Bottom of the channel and pools
  • Site location/minimum setbacks
  • Presence of active karst

B. Determine how the stormwater step pool will fit into the overall stormwater treatment system, including:

  • Decide whether the stormwater step pool is the only BMP to be employed, or if there are other BMPs addressing some of the treatment requirements.
  • Decide where on the site the stormwater step pool will most likely be located.

Step 2. Confirm design criteria and applicability

A. Determine whether a media filter must comply with the MPCA CGP. To determine if permit compliance is required, see Permit Coverage and Limitations. B. Check with local officials, watershed organizations, and other agencies to determine if there are any additional restrictions and/or surface water or watershed requirements that may apply.

Step 3. Perform field verification of site suitability

Refer to “Design Guidelines for Step Pool Conveyance, Anne Arundel County Government, Department of Public Works, Bureau of Engineering”. Stormwater step pools consist of an open channel conveyance with alternating riffles and pools. These systems are best suited for ditches, outfalls, ephemeral and intermittent channels with longitudinal profile slopes that are less than 10%.

However, the design can be easily adapted for sites where the slope exceeds 10 percent. For these sites, the size and quantity of the cobbles and rows of boulders inherent in the design computations are increased to mitigate for the stability issues associated with steep slopes. It is noted that the utilization of two or more rows of boulders typically will result in a water cascade. In extreme slope situations (>50%), the designer may elect to use imbricated riprap (stones are usually two to three foot long boulders that overlap and stack on top of one another to provide structural integrity), gabion baskets, retaining walls, drop storm drain structures or other structural/geotechnical slope treatment methods to safely traverse the grade.

Connection of high-gradient, stormwater step pools to downstream water resources areas must identify and inventory problem areas such as erosion, buffer deficiencies, headcuts and infrastructure impacts that may affect stormwater step pool design and feasibility. For projects that drain to stream channels with active incisions, it is imperative that proper tie-in design be established between the stormwater step pool system and the connecting downstream channel. This could be accomplished by installing an in-stream weir at the proper elevation to promote upstream floodplain connection and prevent headcut erosion from unraveling the proposed stormwater step pool. An example design solution for tying the stormwater step pool with downstream incised perennial channel is presented in the Swale CAD section. It is noted that each case should be evaluated carefully and that design engineers propose appropriate solutions based on the individual circumstance surrounding each case. Additionally, the design engineer is responsible for notifying and obtaining all required approvals from the Local, State and Federal authorities.

Evaluation of in-situ soils for infiltration capacity: Refer to the Infiltration page of the MN Stormwater Manual. Confirm infiltration rate of in-situ soils. Link to step 3 in “Design Criteria for infiltration” Link to “Recommended number of soil borings, etc.” Perform groundwater mounding analysis:

  • Stormwater infiltration and groundwater mounding
  • When should a mounding analysis be conducted?
  • How to predict the extent of a mound
  • Example mound calculations

Step 4. Select design variant based on physical suitability evaluation

Once the physical suitability evaluation is complete, it is HIGHLY RECOMMENDED that the better site design principles be applied in sizing and locating the stormwater step pool on the development site. Given the drainage area and slope, select the appropriate 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 BMPs versus relying on a single practice. Compute watershed runoff values.

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

Refer to “Design Guidelines for Step Pool Conveyance, Anne Arundel County Government, Department of Public Works, Bureau of Engineering” for step-pool hydraulic design.

Map a preliminary vertical alignment for the project

  • Measure the elevation difference “ΔE” between the top and the bottom of the proposed stormwater step pool. In the event that the proposed stormwater step pool connects to an incised downstream channel, the elevation of the floodplain terrace shall be used as the downstream elevation. An in-stream weir design with a top of weir elevation set at the floodplain terrace is necessary at the tie-in location.
  • Compute the average outfall slope, S, by dividing ΔE by Ldesign.
  • Stormwater step pool segments utilized for water quality shall not exceed 5% in longitudinal slope. If the overall slope exceeds 5%, estimate the length of boulder cascade to use for traversing the grade. Boulder cascades maybe placed at 2H:1V or 50% slope. A maximum 5 ft of vertical drop shall be permitted at any single cascade location. Multiple cascades maybe required along the length of the project to traverse steeper grades. The location of the cascade shall be selected to minimize site disturbances and environmental impacts.
  • Assume that the length of the pools is equal to the length of the riffles at 10ft. The minimum length of pools and riffles shall not be less than 10 ft.
  • Assume a fixed one foot drop along the length of the riffle.
  • Assume a minimum 18 inch fixed pool depth.
  • Assume no elevation drop along the length of the pool to allow for dead storage ponding and promote filtration/infiltration.
  • Alternate pool and riffle channels using an even length distribution along the horizontal alignment. Three consecutive pools separated by cobble riffle grade control structures shall be used following a cascade.
  • Using the assumptions above, ΔE-ΔEcascade in feet will equal the number of riffle channels and associated pools.
  • Boulders shall be used in-stream to transition the in-stream weir with the downstream bed elevation. A maximum 5% longitudinal profile slope shall be used to establish the grade transition.

Design the typical cross-section for the riffle/cascade and pool channel segments

  • The riffle/cascade and pool channels shall be parabolic in shape.
  • Design the riffle/cascade and pool channels to carry the Qdesign for the unmanaged 100-year storm flow in a parabolic shape. The area and hydraulic radius of a parabola are computed as

\( Area = 2WD/3 \)

\( Hydraulic radius = (2W<sup>2</sup>D)/(3W<sup>2</sup> + 8D<sup>2</sup>) \)

where

W = width of the riffle or pool section; and
D = depth of the riffle or pool.
  • The minimum freeboard for lined waterways or outlets shall be 0.25 ft above design high water in areas where erosion-resistant vegetation cannot be grown and maintained. No freeboard is required if vegetation can be grown and maintained. (USDA, 2006.)
  • Select a trial constructed riffle channel width (W). The width is the dimension perpendicular to the flow.
  • Select a trial constructed riffle channel depth (D). The Width/Depth ratio shall not be less than 2.
  • The dead storage depth within the pool shall not be considered when checking for adequacy of conveyance.
  • Design using a trial cobble with a d50 of 6 inches. The density of the stone shall be specified. The depth of the cobble material is equal to 2 x d50 (MDSHA, Highway Drainage Manual, 1981). Boulders shall be used to line cascade segments.
  • Calculate the Manning’s n roughness coefficient based on the constructed depth, D, associated with the 100-year ultimate flow conditions and the cobble size (USDA, 2006)\[ n = D/6 (21.6 log(D/D<sub>50</sub>) + 14) \]

where

n = Manning’s n, use 0.05 for cascades;
D = depth of water in the riffle channel associated with unmanaged;
100-year Q design, ft; and
D50 = cobble size, ft.
  • Use the Manning formula to calculate the flow and velocity associated with the trial parameters D, W, and d50. The design flow shall meet or exceed the 100-year ultimate flow conditions.

\( Q = (1.49/n) A Rh<sup>2/3</sup> S<sup>1/2</sup> \)

where:

Q = 100 year ultimate flow (cfs);
1.49 = conversion factor;
n = Manning’s n, determined by USDA, 2006 equation;
A = cross-section area of a riffle channel, which for a parabola = 2/3(W)(D), where W is top constructed width (ft) and D is the constructed depth (ft);
Rh = hydraulic radius (ft), calculated using Chow 1959 relationship for parabolas;
S = average slope over entire length of project (ft/ft); and
V = velocity in the riffle channel (ft/sec), V = Q/A.
  • Using small incremental depths (0.1 ft), develop a hydraulic rating curve/table for the channel to ensure that subcritical flow conditions prevail to the greatest extent possible. This is achieved by calculating the Froude Number, Fr. A Froude number exceeding 1 indicates that the flow is supercritical. A Froude number of less than 1 indicates that the flow is subcritical in nature. The Isbash coefficient for high turbulence should be used when sizing the cobble stones to accommodate supercritical conditions. Increasing the cobble size or the width depth ratio of the riffle channel can increase roughness and reduce velocity. This can further assist in meeting subcritical flow conditions. Refer to the design example at the end of this document for an example of a hydraulic rating curve table.

<math> Fr = V / (gD)1/2 <math>

  • The design velocity shall be checked to ensure that it is below the maximum allowable velocity estimated from the Isbash formula below (NRCS, 2007). A graphical solution of the Isbash formula is also shown. This will be an iterative design process. Spreadsheets can be used to streamline the calculations.
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