The following terminology is used throughout this Design Section:

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 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.

## Major design elements

### Physical feasibility initial check

Drainage Area: It is HIGHLY RECOMMENDED that the following infiltration practices be designed with the indicated maximum drainage areas.

• Dry well – 1 acre.
• Infiltration Trench – 5 acres.
• Underground Infiltration System – 10 acres.
• Infiltration Basin – between 5 and 50 acres.

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.

Warning: It is REQUIRED that impervious area construction be completed and pervious areas established with dense and healthy vegetation prior to introduction of stormwater into an infiltration practice.

Depth to groundwater table and bedrock:

Warning: A separation distance of 3 feet is REQUIRED between the bottom of the infiltration practice and the elevation of the seasonally high water table (saturated soil) or top of bedrock (i.e. there must be a minimum of 3 feet of undisturbed soil beneath the infiltration practice and the seasonally high water table or top of bedrock). See also Step 8 under the Design procedures section.

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.

Warning: The minimum setbacks in the table below are REQUIRED by the Minnesota Department of Health for the design and location of infiltration practices. It will be necessary to consult local ordinances for further guidance on siting infiltration practices.

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.

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.

Wellhead Protection Areas: It is HIGHLY RECOMMENDED to review the Stormwater and wellhead protection regarding stormwater infiltration in Wellhead Protection Areas.

### Conveyance

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.

### Pre-treatment

Warning: It is REQUIRED that some form of pre-treatment, such as a plunge pool, sump pit, filter strip, sedimentation basin, grass channel, or a combination of these practices be installed upstream of the infiltration practice.

It is HIGHLY RECOMMENDED that the following pre-treatment sizing guidelines be followed:

• Before entering an infiltration practice, stormwater should first enter a pre-treatment practice sized to treat a minimum volume of 25 percent of the Vwq.
• If the infiltration rate of the native soils exceeds 2 inches per hour a pre-treatment practice capable of treating a minimum volume of 50 percent of the Vwq should be installed.
• If the infiltration rate of the native soils exceeds 5 inches per hour a pre-treatment practice capable of treating a minimum volume of 100 percent of the Vwq should be installed.

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).

Caution: - Use low-impact earth moving equipment - DO NOT Overexcavate

### Treatment

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).

Schematic showing the effective infiltration area for side slopes less than 1V:3H.

Depth: The depth of an infiltration practice is a function of the maximum drawdown time and the design infiltration rate.

Warning: The REQUIRED drawdown time for infiltration practices is 48 hours or less, and so the depth of the practice should be determined accordingly.
Warning: Groundwater Protection: It is REQUIRED that runoff from potential stormwater hotspots (PSHs) not be infiltrated unless adequate pre-treatment has been provided. Infiltration of runoff from confirmed hotspot areas, industrial areas with exposed significant materials, or vehicle fueling and maintenance areas is PROHIBITED.

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.

### Landscaping

Warning: It is REQUIRED that impervious area construction be completed and pervious areas established with dense and healthy vegetation prior to introduction of stormwater into an infiltration practice.

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.

### Safety

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.

Warning: If a dry well or infiltration trench is greater than five feet deep, it is REQUIRED that OSHA health and safety guidelines be followed for safe construction practices.

Additional information on safety for construction sites is available from OSHA.

## Design procedure

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.

### Design steps

#### Step 1. Make a preliminary judgment

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:

• site drainage area (See the Summary of infiltration practices for given drainage areas table below);
• site topography and slopes;
• soil infiltration capacity;
• regional or local depth to groundwater and bedrock;
• site location/ minimum setbacks; and
• presence of active karst.

B. Determine how the infiltration practice will fit into the overall stormwater treatment system.

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

Summary of infiltration practices for given drainage areas.

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

#### Step 2. Confirm design criteria and applicability

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.

#### Step 3. Perform field verification of site suitability

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):

• thickness, in inches or decimal feet;
• Munsell soil color notation;
• soil mottle or redoximorphic feature color, abundance, size and contrast;
• USDA soil textural class with rock fragment modifiers;
• soil structure, grade size and shape;
• soil consistency, root abundance and size;
• soil boundary; and
• occurrence of saturated soil, impermeable layers/lenses, groundwater, bedrock or disturbed soil.

It is HIGHLY RECOMMENDED that the field verification be conducted by a qualified geotechnical professional.

#### Step 4. Compute runoff control volumes

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).

Warning: If the infiltration practice is being designed to meet the requirements of the MPCA Permit, the REQUIRED treatment volume is the water quality volume of 1 inch of runoff from the new impervious surfaces created from the project.

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).

#### Step 5. Select design variant based on physical suitability evaluation

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).

#### Step 6. Size infiltration practice (Note: Steps 6, 7, 8 and 9 are iterative)

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.
Warning: Pre-treatment prior to infiltration is REQUIRED to remove total suspended solids and other pollutants associated with stormwater (see Step 9)
Infiltration Rates: If the infiltration rate is not measured, tabled values for design infiltration rates provide infiltration rates for the design of infiltration practices. These infiltration rates represent the long-term infiltration capacity of a practice and are not meant to exhibit the capacity of the soils in the natural state. Select the design infiltration rate based on the least permeable soil horizon within the first 5 feet below the bottom elevation of the proposed infiltration practice.

The infiltration capacity and existing hydrologic regime of natural basins are 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:

• infiltration capacity of the system under existing conditions (inches/hour);
• existing drawdown time for the high water level (HWL); and
• a natural overflow elevation.

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.

Warning: The measured infiltration rate shall be divided by a safety factor of 2

The safety factor of 2 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.

Depth: The depth of an infiltration practice is a function of the maximum drawdown time and the design infiltration rate.
Warning: To avoid an infiltration trench being classified as a Class V injection well, it is strongly recommended that the length of the trench be at least 2 times greater than the depth of the trench.

Given the assumed infiltration rate for the practice, determine the maximum depth using the following equation

$$D = I_R DDT_{calc}$$

where

D = maximum depth of practice (inches);
IR = infiltration rate (inches/hour); and
DDTcalc = maximum drawdown time (48 hours).
Effective Infiltration Area: Given the water quality volume (Vwq) and the maximum depth of the practice (D) calculate the effective infiltration area where the effective infiltration area is defined as the area of the facility that is used to infiltrate runoff and does not include the area used for site access, berms and/or pre-treatment. For above ground practices that are rectangular in nature (infiltration basins with 1V:3H side slopes or steeper)

$$A_i = V_w/D$$

where

Ai = effective infiltration area at the bottom of practice (square feet);
Vw = design volume (e.g. Vwq) (cubic feet); and
D = maximum depth of practice (feet) Note: bottom of the infiltration practice must be at least 3 feet from the seasonally high groundwater table.

For above ground practices that have angular sides slopes (infiltration basins with sides slopes shallower that 1V:3H)

$$Ai = V_w/D$$

where

Ai = effective infiltration area at half the volume of the practice (see figure below) (square feet);
Vw = design volume (e.g. Vwq) (cubic feet); and
D = maximum depth of practice (feet) Note: bottom of the infiltration practice should be at least 3 feet from the seasonally high groundwater table.

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

Ai = effective infiltration area is the sum of the bottom area and the sides of the practice (square feet);
Vw = design volume (e.g. Vwq) (feet3);
n = porosity of filter media (range of porosity values for sands and gravels: 0.25 to 0.5); and
D = maximum depth of practice (feet) Note: maximum of 12 feet, and separated by at least 3 feet from seasonally high ground-water table.

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.

Volume: The preliminary volume of the infiltration practice is determined by multiplying the average basin area by the depth of the practice. The total storage volume for infiltration basin and underground infiltration systems is

$$V = A D$$

where

V = design volume for infiltration basin and underground infiltration system (cubic feet);
A = average basin area (square feet); and
D = depth of practice (feet).

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

Vt = design volume for infiltration trenches and dry wells (cubic feet);
A = average basin area (square feet);
n = porosity of filter media (range of porosity values for sands and gravels: 0.25 to 0.5); and
D = depth of practice (feet).

#### Step 7. Size outlet structure and/or flow diversion structure, if needed (Note: Steps 6, 7, 8 and 9 are iterative)

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).

Drawdown valve: infiltration systems may be designed with a drawdown valve for the removal of standing water for maintenance and winter diversion.
Emergency spillway:
Warning: A means to release discharge in excess of the infiltration volume safely into the downstream stormwater conveyance system is REQUIRED.
Freeboard: It is HIGHLY RECOMMENDED that two feet of freeboard be provided from the 100-year flood elevation of the infiltration practice to the lowest basement floor elevation of residential, commercial, industrial and institutional buildings located adjacent to the BMP, unless local requirements recommend otherwise.
Drop Structure: Infiltration trenches or subsurface infiltration systems may be designed with a drop structure sized to handle the overflow. This additional volume of stormwater may be directed into the existing stormwater system or it may be diverted to a downstream BMP.

#### Step 8. Perform ground-water mounding analysis (Note: Steps 6, 7, 8 and 9 are iterative)

Groundwater 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.

#### Step 9. Determine pre-treatment volume and design pre-treatment measures (Note: Steps 6, 7, 8 and 9 are iterative).

See the section on pre-treatment for specific pre-treatment design guidance

#### Step 10. Check volume, peak discharge rates and period of inundation against State, local and watershed organization requirements (Note: Steps 6, 7, 8 and 9 are iterative).

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

Warning:
• Volume - Infiltration or filtration systems shall be sufficient to infiltrate or 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 bioretention practice or treat excess water quality volume (Vwq) in an upstream or downstream BMP (see Step 5). Retrofit and supplemental systems do not need to meet this requirement, provided new impervious surfaces are not created.
• Peak Discharge Rates - Since most bioretention systems are not designed for quantity control they generally do not have peak discharge limits. However outflow must be limited such that erosion does not occur down gradient.
• Period of Inundation - Bioretention 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. The period of inundation is 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.

Other design requirements may apply to a particular site. The applicant should confirm local design criteria and applicability (see Step 3).

#### Step 11. Prepare Vegetation and Landscaping Plan.

A landscaping plan for an infiltration 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.

#### Step 12. Prepare Operation and Maintenance (O&M) Plan.

See Operation and Maintenance section for guidance on preparing an O&M plan.

#### Step 13. Prepare Cost Estimate.

See Cost Considerations section for guidance on preparing a cost estimate that includes both construction and maintenance costs.