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.
The following terminology is used throughout this design page.
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.
Use this link to access .pdf diagrams of CADD drawings. To see all stormwater step pool CADD images in a combined pdf, click here.
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 (applicable to stormwater step pools without an underdrain).
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.
The RECOMMENDED maximum drainage area is typically 5 acres when the practice is being used to achieve volume reduction and water quality improvements for compliance with the MPCA CGP.
Stormwater step pools are often used for drainage areas greater than 5 acres (up to 50 acres) as a practice that can be used to safely convey stormwater down a steep slope, gulley, or ravine while minimizing scour and channel erosion potential. However, as the drainage area increases over 5 acres, the ability of the practice to provide volume reduction and water quality benefits is diminished.
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. Swale gradient is a primary driver of selection of stormwater step pools over wet or dry Swales. Proposed swale gradient and general soil erodibility should be considered at this stage, in conjunction with expected discharges, to decide if a stormwater step pool is preferable and feasible. See Practice and Site Considerations.
Warning: A minimum setback of 50 feet between an infiltration practice (stormwater step pool without underdrain) and a water supply well is REQUIRED by the Minnesota Department of Health Rule 4725.4350.
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 as infiltration practices with a minimum separation distance of 3 feet. A field soil properties investigation is HIGHLY RECOMMENDED.
It is HIGHLY RECOMMENDED that underdrains and an impermeable liner be used for stormwater step pools with filter media in karst terrain because infiltration is typically not allowed in karst areas. Geotechnical investigations are REQUIRED in karst 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.
See NRCS Web Soil Survey for hydrologic soil descriptions for the stormwater step pool location. A and B soils are potentially suitable for a stormwater step pool without an underdrain (infiltration practice). C and D soils are potentially suitable for a stormwater step pool with an underdrain (filtration practice). The maximum allowed infiltration rate shall not exceed 8.3 in/hr.
Several considerations are made in this section for the conceptual design. Further design guidance and specifications are in the following sections.
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 prior to the stormwater step pool such as vegetated filter strips or side slopes, small sedimentation basins, water quality inlets, or other pretreatment BMPs should be evaluated. See the pretreatment section for more information. Pretreatment is REQUIRED for all infiltration and filtration devices.
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 consider the expected watershed flow to be conveyed by the stormwater step pool in making this preliminary determination of design alternate. Final slope will depend on expected flow combined with maximum depth (to determine shear stress), bank/soil stability and practical D50 of rock for the site.
Stormwater step pool bottom It is HIGHLY RECOMMENDED that the stormwater step pool bottom be no less than 4 feet wide and sized with the relative stage-dependent flow driven cross-sectional area in mind. Maximizing the pool segments can reduce scour potential in steps and the next pool as well as increase water quality treatment potential.
Side slopes It is RECOMMENDED that the maximum side slopes within a stormwater step pool do not exceed 3H:1V and 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, rate control, and water quality needs. Use the proposed flow and velocity to determine whether the step pool will require scour protection. If scour protection is necessary, see potential erosion control methods.
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 REQUIRED. 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.
Filtration media is comprised of a combination of sand and organic material on top of a pea gravel bed that typically 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 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. Underdrains may be required to increase drawdown within pool areas, thereby increasing storage capacity of the system.
A stormwater step pool’s water quality function can utilize sedimentation, infiltration, filtration (adsorption via carbon bonding to metals, or chemical interaction between iron and phosphorus, for instance) and, to a lesser extent, denitrification via biochemical mechanisms (nitrate conversion to nitrogen gas by bacteria). 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 may be removed before being conveyed via drain pipe to downstream stormwater treatment practices or a receiving body of water. The addition of an underdrain to the filter media facilitates conveyance but may transport dissolved phosphorus from media with higher phosphorus index or when soil particles reach a breakthrough point, where soluble phosphate, and other dissolved nutrients, will no longer be adsorbed and will pass through the soil (Erickson, Weiss, Gulliver, 2013). This problem can be remedied by the use of iron-, calcium-, aluminum-, or magnesium-enhanced sand in place of compost or peat-sand media. 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. For each treatment scenario being considered, the designer should refer to the respective sections of the manual during the feasibility check and design stages:
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 and sedges 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, ponding duration, and salt tolerance (where such deicing materials are used in the drainage area). As discussed above, while not desirable, some stormwater step pools will be used for snow storage; these areas should not be planted with woody vegetation.
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, appropriately chosen vegetation must be selected to withstand the expected depth and duration of flows, the calculated shear stress and duration of light exposure. Underdrains, if incorporated, may affect the drawdown time and media saturation times thereby impacting this selection.
Considering management of snow, the following are recommended
For more information and example photos, see the section on snow and ice management.
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.
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 used in stormwater step pool designs should follow guidance on material specifications within the Bioretention section of the MN Stormwater Manual.
The following are RECOMMENDED for filtration practices with underdrains.
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 | |
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:
B. Determine how the stormwater step pool will fit into the overall stormwater treatment system, including:
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.
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:
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.
Refer to “Design Guidelines for Step Pool Conveyance, Anne Arundel County Government, Department of Public Works, Bureau of Engineering” for step-pool hydraulic design.
\( Area = 2WD / 3 \)
and
\( Hydraulic radius = (2W<sup>2</sup>D)/(3W<sup>2</sup> + 8D<sup>2</sup>) \)
where
\( n = D/6 (21.6 log(D/D<sub>50</sub>) + 14) \)
where
\( Q = (1.49/n) A Rh<sup>2/3</sup> S<sup>1/2</sup> \)
where:
\( F<sub>r</sub> = V / (gD)<sup>1/2</sup> \)
\( Maximum allowable velocity = C ((2g ((ρ<sub>s</sub> - ρ<sub>w</sub>)ρ<sub>w</sub>))1/2 D<sub>50</sub><sup>0.5</sup> \)
where C = 0.86 for prevailing supercritical flow and 1.2 for prevailing subcritical flow g = 32.2 ft/sec2 ρs = stone density (lb/ft3) ρw = water density (lb/ft3) D50 = cobble stone diameter (ft)
\( h<sub>f</sub> or h<sub>f<sub>cascade</sub></sub> = D + (V<sup>2</sup>/2g) - 0.25 \)
Check for erosive velocities and modify design as appropriate based on local conveyance regulations. Provide 6 inches of freeboard.
Design control to pass Vwq in 48 hours.
Inlets to stormwater step pools must be provided with energy dissipaters such as riprap or geotextile reinforcement. Pre-treatment of runoff is typically provided by a sediment forebay located at the inlet. Enhanced swale systems that receive direct concentrated runoff may have a 6-inch drop to a pea gravel diaphragm flow spreader at the upstream end of the control. A pea gravel diaphragm and gentle side slopes should be provided along the top of channels to provide pre-treatment for lateral sheet flows.
Follow the design procedures identified in the Unified Sizing Criteria section of the Manual to determine the volume control and peak discharge requirements for water quality, recharge (not required), channel protection, overbank flood and extreme storm.
Model the proposed development scenario using a surface water model appropriate for the hydrologic and hydraulic design considerations specific to the site. This includes defining the parameters of the stormwater step pool practice defined above: ponding elevation and area (defines the ponding volume), filtration rate and method of application (effective filtration area), and outlet structure and/or flow diversion information. The results of this analysis can be used to determine whether or not the proposed design meets the applicable requirements. If not, the design will have to be re-evaluated.
A. Volume: Volume reduction may be achieved where in-situ soils, depth to bedrock and depth to groundwater are suitable (see “Design Criteria for infiltration”), though gradients dictating the use of stormwater step pools may limit actual infiltration potential as well as present bluff instability risks. Therefore, it is advisable that stormwater step pools be designed to filter the water quality volume of runoff as opposed to relying on infiltration. Stormwater step pool systems shall be sufficient to filter a water quality volume of 1 inch of runoff from the new impervious surfaces created by the project. If this criterion is not met, increase the storage volume of the filtration practice or treat excess water quality volume (Vwq) in an upstream or downstream BMP (see Step 5). B. Drawdown: Filtration practices shall discharge through the soil or filter media in 48 hours or less. Additional flows that cannot be infiltrated or filtered in 48 hours should be routed to bypass the system through a stabilized discharge point.
Experience has demonstrated that, although the drawdown period is 48 hours, there is often some residual water pooled in the infiltration practice after 48 hours. This residual water may be associated with reduced head, water gathered in depressions within the practice, water trapped by vegetation, and so on. The drawdown period is therefore defined as the time from the high water level in the practice to 1 to 2 inches above the bottom of the facility. This criterion was established to provide the following: wet-dry cycling between rainfall events; unsuitable mosquito breeding habitat; suitable habitat for vegetation; aerobic conditions; and storage for back-to-back precipitation events. This time period has also been called the period of inundation.\(Hydraulic Power = ρ Q S \)
where
\( ρ Q<sub>pre</sub> (ΔE/L<sub>pre</sub>) = ρ Q<sub>post</sub> x (ΔE/L<sub>post</sub>) \)
\( L<sub>add</sub> = L<sub>pre</sub> (Q<sub>post</sub>/Q<sub>pre</sub>) - Lpre \)
\( Friction head loss = fL<sub>add</sub>V<sub>out</sub><sup>2</sup> / 2D<sub>out</sub><sup>g</sup> \)
The general form of the Bernoulli Equation is given by: (Potential + Kinetic + Static) Energies SPSC entrance = (Potential + Kinetic + Static) Energies stormwater step pool outlet + Head loss within stormwater step pool system
The solved form of the Bernoulli Equation is given by
\( D<sub>in</sub> + ((9Q<sup>2</sup>)/(4gD<sub>in</sub><sup>2</sup>W<sub>in</sub><sup>2</sup>)) = ΔE = D<sub>out</sub> + ((9Q<sup>2</sup>)/(4gD<sub>out</sub><sup>2</sup>W<sub>out</sub><sup>2</sup>)) + ((9fL<sub>add</sub>Q<sup>2</sup>)/(4gD<sub>out</sub><sup>3</sup>W<sub>out</sub><sup>2</sup>)) \)
where
Grading plan: Develop a grading plan based on the preliminary profile and cross-section typical design.
Dimensions: Adjust the preliminary profile dimensions to accommodate site specific concerns/impacts. Minimum design parameters for hydraulic, water quality, and quantity management criteria should be rechecked based on adjustments to the riffle/pool channels to ensure that safe and adequate conveyance is still maintained.
Rock checks: Adjust the preliminary rock check dimensions to accommodate site specific concerns/impacts. Minimum design parameters for hydraulic, water quality criteria should be rechecked based on adjustments to the channels profile and bank and bed stability to ensure that safe and adequate conveyance is still maintained.
Filter bed: The sand/woodchip mix filter bed shall have a minimum depth of 18 inches under the riffle channel and a minimum width of 4 ft and shall be placed as the substrate drainage material along the entire project length. The actual dimensions of the sand/woodchip mix filter bed will be determined based on the required water quality volume. Typically, construction of the step-pool system shall begin at the downstream end and proceed upstream to the project outfall. The outlet pool is designed to be placed at the lowest point in the project reach. This is often in the receiving wetland or stream/ floodplain, but can also be located in upland settings where the stormwater step pool system discharges to another stormwater BMP or adequate storm conveyance system.
Footer boulders: Footer boulders shall be placed at the interface of the pools and riffles as shown below. Additional boulders shall be placed on top of the footer boulders at the weir elevation upstream of the footer boulders to form the riffle channel parabolic shape.Continue the process of alternating pools and riffles up through the system to the entry pool. If the entry pool ties to an existing pipe outfall, additional armoring of the pool maybe needed to address the pipe exit velocities associated with supercritical hydraulic conditions. The designer may elect to use a larger size pool at the project entry to dissipate the outfall velocity and/or to address pretreatment concerns.
Upstream drainage: If the stormwater step pool is proposed downstream of a pipe system, it is desirable that the top invert of the weir associated with the entry pool is set at or above the invert of the discharge pipe or culvert. It is the responsibility of the design engineer to check the adequacy of the upstream drainage system
Site stabilization: Course woodchips and compost should be used throughout the limit of disturbance for site stabilization. All areas should be seeded and planted as well as blanketed/matted. Jute blanket should be used within the stormwater step pool bottom and side slopes. Erosion control blanket with biodegradable netting can be used above the side slopes within the floodplain.
Excess materials: It is advisable that excess materials, i.e., cobbles and boulders, be placed at the edge of the cross-section for use during the maintenance phase to correct any physical instability.
A landscaping plan for a stormwater step pool should be prepared to indicate how the enhanced system will be stabilized and established with vegetation. Landscape design should specify proper species and based on specific site, soils, sun exposure and hydric conditions present along the channel. Further information on plant selection and use can be found in the Minnesota plant lists section.
See Operation and Maintenance section for guidance on preparing an O&M plan.
See Cost Considerations section for guidance on preparing a cost estimate that includes both construction and maintenance costs.
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.