The following terminology is used throughout this "Design Section":
HIGHLY RECOMMENDED - Indicates design guidance that is extremely beneficial or necessary for proper functioning of the bioretention practice, but not specifically required by the MPCA CGP.
RECOMMENDED - Indicates design guidance that is helpful for bioretention practice performance but not critical to the design.
Drainage Area: It is HIGHLY RECOMMENDED that the following infiltration practices be designed with the indicated maximum drainage areas.
Site Topography and Slopes: Unless slope stability calculations demonstrate otherwise, it is HIGHLY RECOMMENDED that infiltration practices 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.
Soils: It is HIGHLY RECOMMENDED that native soils in proposed infiltration areas have a minimum infiltration rate of 0.2 inches per hour (typically Hydrologic Soil Group A, B and C soils). Initially, soil infiltration rates can be estimated from NRCS soil data, and confirmed with an on-site infiltration evaluation or geotechnical investigation (see Step 6 of the Design procedures section for investigation procedures). It is HIGHLY RECOMMENDED that native soils have silt/clay contents less than 40 percent and clay content less than 20 percent, and that infiltration practices not be situated in fill soils.
Depth to groundwater table and bedrock:
Local authorities may require greater separation depths.
Site Location / Minimum Setbacks: It is HIGHLY RECOMMENDED that infiltration practices not be hydraulically connected to structure foundations or pavement, to avoid seepage and frost heave concerns, respectively. If groundwater contamination is a concern, it is RECOMMENDED that groundwater mapping be conducted to determine possible connections to adjacent groundwater wells.
Recommended minimum setback requirements. This represents the minimum distance from the infiltration practice to the structure of concern. If the structure is aboveground, the distance is measured from the edge of the permeable pavement to the structure. If the structure is underground, the setback distance represents the distance from the point of infiltration through the bottom of the permeable pavement system to the structure.
Link to this table
Setback from | Minimum Distance [feet] |
---|---|
Property Line | 10 |
Building Foundation* | 10 |
Private Well | 50 |
Septic System Tank/Leach Field | 35 |
* Minimum with slopes directed away from the building. |
Karst: It is HIGHLY RECOMMENDED that infiltration practices not be used in active karst formations without adequate geotechnical testing.
It is HIGHLY RECOMMENDED that a flow splitter or diversion structure be provided to divert the Vwq to the infiltration practice and allow larger flows to bypass the practice, unless the infiltration practice is sized to retain Vcp, Vp10 or Vp100. Where a flow splitter is not used, it is HIGHLY RECOMMENDED that contributing drainage areas be limited to the appropriate size given the BMP and an overflow be provided within the practice to pass part of the Vwq to a stabilized watercourse or storm drain. It is also HIGHLY RECOMMENDED that overflow associated with the Vp10 or Vp100 storm (depending on local drainage criteria) be controlled such that velocities are non-erosive at the outlet point (to prevent downstream slope erosion), and that when discharge flows exceed 3 cubic feet per second, the designer evaluate the potential for erosion to stabilized areas and infiltration facilities.
It is HIGHLY RECOMMENDED that the following pre-treatment sizing guidelines be followed:
It is HIGHLY RECOMMENDED that pre-treatment practices be designed such that exit velocities from the pre-treatment systems are non-erosive (less than 3 feet per second) and flows are evenly distributed across the width of the practice (e.g., by using a level spreader).
Space Occupied: Space varies depending on the depth of the practice. Typically, infiltration trenches are 3 to 12 feet deep with a width less than 25 feet. A dry well is essentially a smaller version of an infiltration trench, consistent with the fact that the drainage area to an infiltration trench is typically five times greater (or larger) than that of a dry well. Underground infiltration systems are larger practices that range in depth from approximately 2 to 12 feet. The surface area of all infiltration practices is a function of MPCA’s 48-hour drawdown requirement and the infiltration capacity of the underlying soils.
Practice Slope: It is RECOMMENDED that the bottom of all infiltration practices be flat, in order to enable even distribution and infiltration of stormwater. It is RECOMMENDED that the longitudinal slope range only from the ideal 0 percent up to 1 percent, and that lateral slopes be held at 0 percent.
Side Slopes: It is HIGHLY RECOMMENDED that the maximum side slopes for an infiltration practice be 1:3 (V:H).
Depth: The depth of an infiltration practice is a function of the maximum drawdown time and the design infiltration rate.
Aesthetics: infiltration basins can be effectively integrated into the site planning process, and aesthetically designed as attractive green spaces planted with native vegetation. Infiltration trenches are less conducive to site aesthetics, but the surface of trenches can be designed with turf cover crops if desired.
It is RECOMMENDED that vegetation associated with infiltration practices be established to blend into the surrounding area, that native species be used wherever possible. It is HIGHLY RECOMMENDED that deep rooted plants such as prairie grass be used, because they increase the infiltration capacity of the underlying soils. Dry wells and infiltration trenches can be covered with permeable topsoil and planted with grass to match the surrounding landscape.
Due to soil compaction concerns, it is HIGHLY RECOMMENDED that infiltration areas not be used for recreational purposes unless a soil amendment is used to off-set compaction.
It is HIGHLY RECOMMENDED that vegetation associated with infiltration practices be regularly maintained and bare areas seeded. Mowing practices can be used to maintain native vegetation.
It is RECOMMENDED that soil testing be conducted in infiltration practices, to determine if fertilizer application is warranted. Incorporating mulch or compost into the soil or planting with salt tolerant grasses can counter soil fertility problems caused by high chloride concentrations
It is HIGHLY RECOMMENDED that designs include an observation well consisting of an anchored six-inch diameter perforated PVC pipe fitted with a cap to facilitate periodic inspection and maintenance.
Dry wells, infiltration trenches and subsurface infiltration systems do not pose any major safety hazards. Infiltration basins should have similar side slope considerations as ponds and wetlands.
Additional information on safety for construction sites is available from OSHA.
The following steps outline a recommended design procedure for infiltration practices in compliance with the MPCA Permit for new construction. Design recommendations beyond those specifically required by the permit are also included and marked accordingly.
When riser pipe outlets are used in infiltration basins, it is HIGHLY RECOMMENDED that they be constructed with manholes that either have locks or are sufficiently heavy to prevent easy removal.
Fencing of dry wells and infiltration trenches is neither necessary nor desirable. Infiltration basins may warrant fencing in some situations.
Make a preliminary judgment as to whether site conditions are appropriate for the use of an infiltration practice, and identify the function of the practice in the overall treatment system.
A. Consider basic issues for initial suitability screening, including:
B. Determine how the infiltration practice will fit into the overall stormwater treatment system.
Summary of infiltration practices for given drainage areas.
Link to this table
Infiltration Practice | Drainage Area |
---|---|
Infiltration Basin | 5 to 50 acres |
Infiltration Trench | less than 5 acres |
Dry Wells | less than one acre |
Underground Infiltration Systems | less than 10 acres |
A. Determine whether the infiltration practice must comply with the MPCA Permit.
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.
If the initial evaluation indicates that an infiltration practice would be a good BMP for the site, it is RECOMMENDED that a minimum of 3 soil borings or pits be dug (in the same location as the proposed infiltration practice) to verify soil types and infiltration capacity characteristics and to determine the depth to groundwater and bedrock.
It is RECOMMENDED that the minimum depth of the soil borings or pits be 5 feet below the bottom elevation of the proposed infiltration practice.
It is HIGHLY RECOMMENDED that soil profile descriptions be recorded and include the following information for each soil horizon or layer (Source: Site Evaluation for Stormwater Infiltration, Wisconsin Department of Natural Resources Conservation Practice Standards, 2004):
It is HIGHLY RECOMMENDED that the field verification be conducted by a qualified geotechnical professional.
Calculate the Water Quality Volume (Vwq), Channel Protection Volume (Vcp), Overbank Flood Protection Volume (Vp10), and the Extreme Flood Volume (Vp100) (see Unified sizing criteria).
If part of the overall Vwq is to be treated by other BMPs, subtract that portion from the Vwq to determine the part of the Vwq to be treated by the infiltration practice.
The design techniques in this section are meant to maximize the volume of stormwater being infiltrated. If the site layout and underlying soil conditions permit, a portion of the Channel Protection Volume (Vcp), Overbank Flood Protection Volume (Vp10), and the Extreme Flood Volume (Vp100) may also be managed in the infiltration practice (see Step 7).
Once the Physical Suitability Evaluation is complete, it is HIGHLY RECOMMENDED that the designer apply the better site design principles in sizing and locating the infiltration practice(s) on the development site. Given the water quality volume and the drainage area, select the appropriate infiltration practice for the first iteration of the design process.
Note: Information collected during the site suitability evaluation (see Steps 1 and 3) should be used to explore the potential for multiple infiltration practices versus relying on a single infiltration facility. The use of smaller infiltration practices dispersed around a development is usually more sustainable than a single regional facility that is more likely to have maintenance and groundwater mounding problems (Source: Site Evaluation for Stormwater Infiltration, Wisconsin Department of Natural Resources Conservation Practice Standards, 2004).
After following the steps outlined above, the designer will presumably know the location of naturally occurring permeable soils, the depth to the water table, bedrock or other impermeable layer, and the contributing drainage area. While the first step in sizing an infiltration practice is selecting the type of infiltration practice for the site, the basic design procedures are very similar.
Infiltration Location: Given the steps performed in the Physical Suitability Evaluation, identify the most suitable location for the infiltration practice.
The infiltration capacity and existing hydrologic regime of natural basins are inheritably different than constructed practices and may not meet the General Permit requirements for constructed practices. In the event that a natural depression is being proposed to be used as an infiltration system, the design engineer must demonstrate the following information:
The design engineer should also demonstrate that operation of the natural depression under post-development conditions mimics the hydrology of the system under pre-development conditions.
If the infiltration rates are measured the tests shall be conducted at the proposed bottom elevation of the infiltration practice. If the infiltration rate is measured with a double-ring infiltrometer the requirements of ASTM D3385 shall be used for the field test.
The measured infiltration rate shall be divided by a correction factor selected from the table below. The correction factor adjusts the measured infiltration rates for the occurrence of less permeable soil horizons below the surface and the potential variability in the subsurface soil horizons throughout the infiltration site. This correction factor also accounts for the long-term infiltration capacity of the stormwater management facility.
Total correction factors Ddvided into measured infiltration rates
To select the correction factor from the table above, determine the ratio of the design infiltration rates for each location an infiltration measurement was performed. To determine this ratio, the design infiltration rate for the surface textural classification is divided by the design infiltration rate for the least permeable soil horizon. For example, a device with a loamy sand (0.8 inches per hour.) at the surface and least permeable layer of loam (0.3 inches per hour.) will have a design infiltration rate ratio of about 2.7 and thus a correction factor of 3.5. The depth of the least permeable soil horizon should be within 5 feet of the proposed bottom of the device or to the depth of a limiting layer. In this exercise, if an infiltration rate of 2.5 inches per hour is measured, the adjustment rate would be 0.71 inch per hour.
\(D = i x t\)
where
\(A_i = V_w/D\)
where
For above ground practices that have angular sides slopes (infiltration basins with sides slopes shallower that 1V:3H)
\(Ai = V_w/D\)
where
Since there is potentially a significant amount of infiltration that could occur though the sides of the practice, the design engineer should take this surface area into consideration thereby potentially reducing the overall footprint of the stormwater infiltration practice. For underground practices (e.g. infiltration trenches, dry wells, subsurface infiltration practices)
\(A_i = V_w / n D\)
where
Since underground facilities have potentially more surface area in contact with permeable soils, these practices should take these areas into consideration. Only that portion of the sides that is in contact with naturally permeable material should be used in calculating the effective infiltration area of the practice.
For subsurface infiltration practices, use the procedure described above or technique recommended by manufacturer and approved by the local or state authority.
\(V = A D\)
where
For those practices that do not involve a media filter (e.g. infiltration basin and underground infiltration systems) this volume represents the total storage volume (design volume) of the practice. For those practices which do involve a media filter (e.g. infiltration trenches and dry wells) this volume represents the void space and the total storage volume will be greater. The following formula can be used to determine the total storage volume (design volume) for infiltration trenches and dry wells
\(V_t = A n D_i\)
where
It is HIGHLY RECOMMENDED that the outlet for the infiltration practice shall safely convey stormwater using all of the following mechanisms (Infiltration Basin, Wisconsin Department of Natural Resources Conservation Practice Standard, 10/04).
Ground water mounding, the process by which a mound of water forms on the water table as a result of recharge at the surface, can be a limiting factor in the design and performance of infiltraand seasonally saturated soils (or from bedrock) is required (5 feet RECOMMENDED) to maintain the hydraulic capacity of the practice and provide adequate water quality treatment. A groundwater mounding analysis is RECOMMENDED to verify this separation for infiltration practices.
The most widely known and accepted analytical methods to solve for groundwater mounding are based on the work by Hantush (1967) and Glover (1960). The maximum groundwater mounding potential should be determined through the use of available analytical and numerical methods. Detailed groundwater mounding analysis should be conducted by a trained hydrogeologist or equivalent as part of the site design procedure.
See the section on pre-treatment for specific pre-treatment design guidance
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, channel protection, overbank flood and extreme storm.
Perform hand calculations or model the proposed development scenario using a surface water model appropriate for the hydrologic and hydraulic design considerations specific to the site (see also the section on stormwater modeling). This includes defining the parameters of the infiltration practice defined above: elevation and area (defines the storage volume), infiltration rate and method of application (effective infiltration 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 (back to Step 5).
The following items are specifically REQUIRED by the MPCA Permit:
Other design requirements may apply to a particular site. The applicant should confirm local design criteria and applicability (see Step 3).
A landscaping plan for an infiltration]] basin]] or trench should be prepared to indicate how the enhanced swale system will be stabilized and established with vegetation. Landscape design should specify proper grass species and wetland plants based on specific site, soils and hydric conditions present along the channel. Further information on plant selection and use occurs 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.