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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.

• Links to .dwg files for swales
• Swale layout: File:Swale Layout2 (1).pdf
• Typical dry swale profile section with check dams and draintile: File:MIDS Dry Swale Sections-SHEET 1.pdf
• Typical grass channel cross-section without soil amendment: File:MIDS Dry Swale Sections-SHEET 2.pdf

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.

• Karst
• Shallow soils and shallow depth to bedrock
• Shallow groundwater
• Soils with low infiltration capacity
• Separation distances
• Potential stormwater hotspots
• Wellhead protection
• Contaminated soils and groundwater
• Procedures for investigating sites with potential constraints

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.

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.

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.