Design criteria for bioretention

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

Before deciding to use a bioretention 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. The following list of considerations will help in making an initial judgment as to whether or not a bioretention practice is the appropriate BMP for the site.

• Drainage Area:

The RECOMMENDED maximum drainage area is typically 5 acres, but can be greater if the discharge to the basin has received adequate pretreatment and the basin is properly designed, constructed, and maintained. For larger sites, multiple bioretention areas can be used to treat site runoff provided appropriate grading is present to convey flows.

• Site Topography and Slopes: It is RECOMMENDED that sloped areas immediately adjacent to the bioretention practice be less than 33 percent but greater than 1 percent to promote positive flow towards the practice.

• Soils: No restrictions; engineered media HIGHLY RECOMMENDED; underdrain is HIGHLY RECOMMENDED where parent soils are HSG C or D.

• Depth to Ground Water and Bedrock:

Warning: A separation distance of 3 feet is REQUIRED between the bottom of the bioretention 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). Note that if underlying soils are ripped to alleviate compaction, the requirement is a 2 foot minimum between the bottom of the ripped zone and a 3 foot minimum from the bottom of the infiltration practice. If there is only a 3 foot separation distance between the bottom of the infiltration practice and the elevation of the seasonally high water table or bedrock, limit ripping depth to 12 inches).

• Karst: Underdrains and an impermeable liner may be desirable in some karst areas; specific site geotechnical assessment is RECOMMENDED

• Site Location / Minimum Setbacks: It is HIGHLY RECOMMENDED that bioinfiltration 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. The table below provides the minimum setbacks REQUIRED by the Minnesota Department of Health for the design and location of bioretention practices.

Minimum setback requirements (for bioretention practices that treat a volume of 1000 gallons or more)
Link to this table

Setback from	Minimum Distance (feet)
Property Line	10
Building Foundation - Minimum with slopes directed away from the building	10
Private Well	50
Public Water Supply Well	50
Septic System Tank/Leach Field	50

Karst: It is HIGHLY RECOMMENDED that bioinfiltration practices not be used in active karst formations without adequate geotechnical testing.

Wellhead Protection Areas: It is HIGHLY RECOMMENDED to review the Minnesota Department of Health guidance on stormwater infiltration in Wellhead Protection Areas.

Conveyance

It is Highly Recommended that the designer provides non-erosive flow velocities at theoutlet 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. Common overflow systems within the structure consist of a yard drain inlet, where the top of the yard drain inlet is placed at the elevation of the shallow ponding area. A stone drop of about 12 inches or small stilling basin could be provided at the inlet of bioretention areas where flow enters the practice through curb cuts or other concentrated flow inlets. In cases with significant drop in grade this erosion protection should be extended to the bottom of the facility.

Underdrains

The following are RECOMMENDED for bioretention areas with underdrains:

• a 4 inch minimum pipe diameter;
• install 2 or more underdrains for each bioretention system in case one clogs;
• include at least 2 observation /cleanouts for each underdrain, one at the upstream end and one at the downstream end;
• provide clean-out on either side of each underdrain (minimum of one per every 1,000 square feet of surface area);
• 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 under-drains that daylight on grade, include a marking stake and animal guard;
• the under-drain should 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 for the Water Quality Volume;
• perforations should be 3/8 inches;
• spacing of collection laterals should be less than 25 feet;
• use solid sections of non-perforated PVC piping and watertight joints wherever the under-drain system passes below berms, down steep slopes, makes a connection to a drainage structure or daylight on grade;
• 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, either filter fabric or two inches of choking stone is needed to protect the underdrain from blockage;
• avoid filter fabric if there is any question about the future stability of the drainage area;
• above the filtering device, install a minimum of 2 inches of washed sand;
• use choking stone (#8 or #89) in lieu of filter fabric if there is potential for higher sediment loads that would lead to clogging; and
• pipe socks are also not recommended.

The procedure to size underdrains is typically determined by the project engineer. An example for sizing underdrains is found inSection 5.7 of the North Carolina Department of Environment and Natural Resources Stormwater BMP Manual.

Pretreatment

Pre-treatment refers to features of a bioretention area that capture and remove coarse sediment particles.

Warning: To prevent clogging of the infiltration or filtration system with trash, gross solids, and particulate matter, use of a pretreatment device such as a vegetated filter strip, vegetated swale, small sedimentation basin (forebay), or water quality inlet (e.g., grit chamber) to settle particulates before the stormwater discharges into the infiltration or filtration system is REQUIRED.

For applications where runoff enters the bioretention system through sheet flow, such as from parking lots, or residential back yards, a grass filter strip with a pea gravel diaphragm is the preferred pre-treatment method. The width of the filter strip depends on the drainage area, imperviousness and the filter strip slope. The minimum RECOMMENDED vegetated filter strip width is 3 feet. The width should increase with increasing slope of the filter strip. Slopes should not exceed 8 percent. Pretreatment filter strips greater than 15 feet in width will provide diminishing marginal utility on the installation cost.

For retrofit projects and sites with tight green space constraints, it may not be possible to include a grass buffer strip. For example, parking lot island retrofits may not have adequate space to provide a grass buffer. For applications where concentrated (or channelized) runoff enters the bioretention system, such as through a slotted curb opening, a grassed channel with a pea gravel diaphragm is the preferred pre-treatment method.

Accumulated sediment should be removed from the gravel verge (if applicable) and vegetated filter strip approximately quarterly. If the watershed is especially dirty, this frequency may need to be increased to monthly. Trash removal should occur in conjunction with removal of debris from the bioretention cell, at least quarterly. During quarterly maintenance, check for erosion in the filter strip. If it is visible, it should be repaired with topsoil and re-planted. Vegetation of the filter strip should be designed at least 2 inches below the contributing impervious surface. If, over time, the grade of the vegetated filter strip rises above the adjacent impervious surface draining into it, the grade of the vegetated filter strip needs to be lowered to ensure proper drainage.

The type of vegetation in the bioretention cell determines the appropriate flow velocity for which the pre-treatment device should be designed. For tree-shrub-mulch bioretention cells, velocity through the pre-treatment device should not exceed 1 foot per second, which is the velocity that causes incipient motion of mulch. For grassed bioretention cells, flow velocity through the pre-treatment device is not to exceed 3 feet per second. In all cases, appropriate maintenance access should be provided to pre-treatment devices.

In lieu of grass buffer strips, pre-treatment may be accomplished by other methods such as sediment capture in the curb-line entrance areas. Additionally, the parking lot spaces may be used for a temporary storage and pre-treatment area in lieu of a grass buffer strip. If bioretention is used to treat runoff from a parking lot or roadway that is frequently sanded during snow events, there is a high potential for clogging from sand in runoff. Local requirements may allow a street sweeping program as an acceptable pre-treatment practice. It is HIGHLY RECOMMENDED that pre-treatment incorporate as many of the following as are feasible:

• grass filter strip;
• vegetated swale;
• gravel diaphragm;
• mulch layer;
• forebay; and
• up flow inlet for storm drain inflow.

Treatment

The following guidelines are applicable to the actual treatment area of a bioretention practice:

• Space Required: It is RECOMMENDED that approximately 5 to 10 percent of the tributary impervious area be dedicated to the practice footprint; with a minimum 200 square foot area for small sites (equivalent to 10 feet x 20 feet). The surface area of all infiltration designed bioretention practices is a function of MPCA’s 48-hour drawdown requirement and the infiltration capacity of the underlying soils. The surface area of all filtration designed bioretention practices is a function of MPCA’s 48-hour drawdown requirement and the filtration capacity of the soil medium and underdrain.
• Practice Slope: It is RECOMMENDED that the slope of the surface of the bioretention practice not exceed 1 percent, to promote even distribution of flow throughout.
• Side Slopes: It is HIGHLY RECOMMENDED that the maximum side slopes for an infiltration practice is 3:1 (h:v).
• Depth: Ponding design depths have been kept to a minimum to*limit depth and duration of submergence of plants improve plant survivability;*reduce mosquito habitat;*minimize compaction of in-situ soils;*minimize clogging;*maximize contact time;*enhance safety by preventing drowning; and*maintain aesthetic value of the bioretetnion system

When the drawdown time for a bioinfiltration system is 48 hours, the maximum ponding depth is

• 18 inches for Hydrologic Soil Group (HSG) A soils;
• 18 inches for SM (HSG B) soils;

• 14.4 inches for loam, silt loam and HSG MH soils; and
• 9.6 inches for HSG C soils.

If field tested rates for any soil exceed the rate for A soils in the manual (1.63 inches per hour), the maximum ponding depth is 18 inches. When the drawdown time is 24 hours, the above maximum ponding depths are reduced by a factor of 2.

The Construction Stormwater General Permit requires that on-site soil testing be consistent with the Minnesota Stormwater Manual. If the permit requirement is not applicable and the recommended number of soil tests have not been taken within the boundary of the SCM, it is Highly Recommended the maximum ponding depth be 6 inches. Drawdown time is the time until there is no visible standing water in the lowest part of the bioretention system.It is RECOMMENDED that the elevation difference from the inflow to the outflow be approximately 4 to 6 feet when an underdrain is used.

Warning: The REQUIRED drawdown time for bioretention practices is 48 hours or less from the peak water level in the practice.

Caution: It is HIGHLY RECOMMENDED that the drawdown time for bioretention practices is 24 hours or less from the peak water level in the practice when discharges are to a trout stream.

• Groundwater Protection: Exfiltration of unfiltered PSH runoff into groundwater should never occur; the CGP specifically prohibits inflow from “designed infiltration systems from industrial areas with exposed significant materials or from vehicle fueling and maintenance areas”.

It is HIGHLY RECOMMENDED that bioretention not be used on sites with a continuous flow from groundwater, sump pumps, or other sources so that constant saturated conditions do not occur.

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

It is HIGHLY RECOMMENDED that soils meet the design criteria outlined later in this section and contain less than 5 percent clay by volume. Elevations must be carefully worked out to ensure that the desired runoff flow enters the facility with no more than the maximum design depth. The bioretention area (A_f) should be sized based on the principles of Darcy’s Law, as follows

\(A_f = V_{wq} d_f / (k (h_f + df) t_f)\)

Where:

A_f = surface area of device (square feet);

d_f = filter bed depth (feet);

k = coefficient of permeability of filter media (k = 0.5 feet/day is appropriate to characterize the planting medium / filter media soil. This value is conservative to account for clogging associated with accumulated sediment (Claytor and Schueler, 1996));

h_f = average height of water above filter bed (feet) (Typically ½ h_max, where h_max is the maximum head on the filter media and is typically ≤6 feet); and

t_f = design filter bed drain time (days)

Warning: It is REQUIRED that the design permeability rate through the planting soil bed be high enough to fully drain the stormwater quality design storm runoff volume within 48 hrs.

It is HIGHLY RECOMMENDED that this permeability rate be determined by field testing.

When using bioretention to treat PSHs, particularly in sensitive watersheds, it is HIGHLY RECOMMENDED that additional practices be incorporated as a treatment train for at least limited treatment during the winter when the bioretention area may be frozen.

Materials specifications - filter media

This site is currently undergoing final review. For more information, open this link.
The anticipated review period for this page is through March 2014

Filter media depth

Research has shown that minimum bioretention soil media depth needed varies depending on the target pollutant(s).

Minimum bioretention soil media depths recommended to target specific stormwater pollutants. From Hunt et al. (2012) and Hathaway et al., (2011). NOTE: The Construction Stormwater permit requires a 3 foot separation from the bottom of an infiltration practice and bedrock or seasonally saturated soils.
Link to this table

Pollutant	Depth of Treatment with upturned elbow or elevated underdrain	Depth of Treatment without underdrain or with underdrain at bottom	Minimum depth
Total suspended solids (TSS)	Top 2 to 3 inches of bioretention soil media	Top 2 to 3 inches of bioretention soil media	Not applicable for TSS because minimum depth needed for plant survival and growth is greater than minimum depth needed for TSS reduction
Metals	Top 8 inches of bioretention soil media	Top 8 inches of bioretention soil media	Not applicable for metals because minimum depth needed for plant survival and growth is greater than minimum depth needed for metals reduction
Hydrocarbons	3 to 4 inch Mulch layer, top 1 inch of bioretention soil media	3 to 4 inches Mulch layer, top 1 inch of bioretention soil media	Not applicable for hydrocarbons because minimum depth needed for plant survival and growth is greater than minimum depth needed for hydrocarbons reduction
Nitrogen	From top to bottom of bioretention soil media; Internal Water Storage Zone (IWS) improves exfiltration, thereby reducing pollutant load to the receiving stream, and also improves nitrogen removal because the longer retention time allows denitrification to occur underanoxic conditions.	From top to bottom of bioretention soil media	Retention time is important, so deeper media is preferred (3 foot minimum)
Particulate phosphorus	Top 2 to 3 inches of bioretention soil media.	Top 2 to 3 inches of bioretention soil media.	Not applicable for particulate phosphorus because minimum depth needed for plant survival and growth is greater than minimum depth needed for particulate phosphorus reduction
Dissolved phosphorus	From top of media to top of submerged zone. Saturated conditions cause P to not be effectively stored in submerged zone.	From top to bottom of bioretention soil media	Minimum 2 feet, but 3 feet recommended as a conservative value; if IWS is included, keep top of submerged zone at least 1.5 to 2 feet from surface of media
Pathogens	From top of soil to top of submerged zone.	From top to bottom of bioretention soil media	Minimum 2 feet; if IWS is included, keep top of submerged zone at least 2 feet from surface of media
Temperature	From top to bottom of bioretention soil media; Internal Water Storage Zone (IWS) improves exfiltration, thereby reducing volume of warm runoff discharged to the receiving stream, and also improves thermal pollution abatement because the longer retention time allows runoff to cool more before discharge.	From top to bottom of bioretention soil media	Minimum 3 feet, with 4 feet preferred

Performance specifications

The following performance specifications are applicable to all bioretention media.

• Growing media must be suitable for supporting vigorous growth of selected plant species.
• The pH range (Soil/Water 1:1) is 6.0 to 8.5
• Soluble salts (soil/Water 1:2) should not to exceed 500 parts per million
• All bioretention growing media must have a field tested infiltration rate between 1 and 8 inches per hour. Growing media with slower infiltration rates could clog over time and may not meet drawdown requirements. Target infiltration rates should be no more than 8 inches per hour to allow for adequate water retention for vegetation as well as adequate retention time for pollutant removal. The following infiltration rates should be achieved if specific pollutants are targeted in a watershed.
  • Total suspended solids: Any rate is sufficient, 2 to 6 inches recommended
  • Pathogens: Any rate is sufficient, 2 to 6 inches recommended
  • Metals: Any rate is sufficient, 2 to 6 inches recommended
  • Temperature: slower rates are preferable (less than 2 inches per hour)
  • Total nitrogen (TN): 1 to 2 inches per hour, with 1 inch per hour recommended
  • Total phosphorus (TP): 2 inches per hour

The following additional bioretention growing media performance specifications are required to receive P reduction credit.

• Option A - use bioretention soil with phosphorus content between 12 and 36 mg/kg per Mehlich III test
• Option B - include a soil amendment that facilitates adsorption of phosphorus

Guidance for bioretention media composition

Mix A: Water quality blend

A well blended, homogenous mixture of

• 60 to 70 percent construction sand;
• 15 to 25 percent top soil; and
• 15 to 25 percent organic matter.

Sand: Provide clean construction sand, free of deleterious materials. AASHTO M-6 or ASTM C-33 washed sand.

Top Soil: Sandy loam, loamy sand, or loam texture per USDA textural triangle with less than 5 percent clay content

Organic Matter: MnDOT Grade 2 compost is recommended. (see also the section on Using Compost as a Soil Amendment

It is assumed this mix will leach phosphorus. When an underdrain is utilized a soil phosphorus test is needed to receive water quality credits for the portion of stormwater captured by the underdrain. The phosphorus index (P-index) for the soil must be low, between 10 and 30 milligrams per kilogram when using the Mehlich-3 test. This is enough phosphorus to support plant growth without exporting phosphorus from the cell.

Mix B: Enhanced filtration blend

A well-blended, homogenous mixture of

• 70 to 85 percent construction sand; and
• 15 to 30 percent organic matter.

Sand: Provide clean construction sand, free of deleterious materials. AASHTO M-6 or ASTM C-33 washed sand.

Top Soil in the mix will help with some nutrient removal, especially nutrients, but extra care must be taken during construction to inspect the soils before installation and to avoid compaction.

Organic Matter: MnDOT Grade 2 compost is recommended. (see also the section on Using Compost as a Soil Amendment

It is assumed this mix will leach phosphorus. When an underdrain is utilized a soil phosphorus test is needed to receive water quality credits for the portion of stormwater captured by the underdrain. The phosphorus index (P-index) for the soil must be low, between 10 and 30 milligrams per kilogram when using the Mehlich-3 test. This is enough phosphorus to support plant growth without exporting phosphorus from the cell.

Mix C: North Carolina State University water quality blend (North Carolina Department of Environment and Natural Resources. 2009)

This mix is a homogenous soil mix of

• 85 to 88 percent by volume sand (USDA Soil Textural Classification);
• 8 to 12 percent fines by volume (silt and clay); and
• 3 to 5 percent organic matter by weight (ASTM D 2974 Method C) MnDOT Grade 2 compost is recommended.

A higher concentration of fines (12 percent) should be reserved for areas where nitrogen is the target pollutant. In areas where phosphorus is the target pollutant, a lower concentration of fines (8 percent) should be used. A soil phosphorus test using the Mehlich-3 method is recommended but not required to receive water quality credits. The phosphorus index (P-index) for the soil must be low, between 10 and 30 milligrams per kilogram. This is enough phosphorus to support plant growth without exporting phosphorus from the cell. It is assumed this mix will not exceed the upper range of recommended values (30 milligrams per kilogram), although at lower concentrations of organic matter a soil test may be needed to confirm there is adequate phosphorus for plant growth.

Mix D

Bioretention Soil Mix D soil shall be a mixture of coarse sand, compost and topsoil in proportions which meet the following:

• silt plus sand (combined): 25 to 40 percent, by dry weight
• total sand: 60 to 75 percent, by dry weight
• total coarse and medium sand: minimum of 55 percent of total sand, by dry weight
• fine gravel less than 5 millimeters: up to 12 percent by dry weight (calculated separately from sand/silt/ clay total)
• organic matter content: 2 to 5 percent, percent loss on ignition by dry weight; MnDOT Grade 2 compost is recommended.
• saturated hydraulic conductivity: 1 to 4 inches per hour ASTM F1815. Note that although this infiltration rate is generally applicable at 85 percent compaction, Standard Proctor ASTM D968, this is an infiltration rate standard and not a compaction standard. Therefore, this infiltration rate may be met at lower levels of compaction.

Suggested mix ratio ranges are

• Coarse sand: 50 to 65 percent
• Topsoil: 25 to 35 percent
• Compost (assuming MnDOT Grade 2 compost is being used): 10 to 15 percent

A soil phosphorus test using the Mehlich-3 method is recommended but not required to receive water quality credits. The phosphorus index (P-index) for the soil must be low, between 10 and 30 milligrams per kilogram. This is enough phosphorus to support plant growth without exporting phosphorus from the cell. It is assumed this mix will not exceed the upper range of recommended values (30 milligrams per kilogram), although at lower concentrations of organic matter a soil test may be needed to confirm there is adequate phosphorus for plant growth.

Comparison of pros and cons of bioretention soil mixes
Link to this table.

Mix	Composition in original Manual	Proposed updated composition	Pros	Cons
A	55-65% construction sand 10-20% top soil 25-35% organic matter2	60-70% construction sand 15-25% top soil 15-25% organic matter2 to receive P credit for water captured by underdrain the P content must be less than 30 mg/kg (ppm) per Mehlich III (or equivalent) test; NOTE a minimum P concentration of 12 mg/kg is recommended for plant growth.	Likely to sorb more dissolved P and metals than mix B because it contains some fines; best for growth of most plants	Likely to leach P; if topsoil exceeds maximum allowed clay content, higher fines content could result in poor hydraulic performance and long drawdown times
B	50-70% construction sand 30-50% organic matter	70-85% construction sand 15-30% organic matter to receive P credit for water captured by underdrain the P content must be less than 30 mg/kg per Mehlich III (or equivalent) test; NOTE a minimum P concentration of 12 mg/kg is recommended for plant growth.	Easy to mix; least likely to clog	Likely to leach P, lack of fines in mix results in less dissolved pollutant removal; harder on most plants than mix A because it dries out very quickly
C	Not in original MN Stormwater Manual	85-88 percent by volume sand and 8 to 12 percent fines by volume, 3 to 5 percent organic matter by volume recommended P content between 12 and 30 mg/kg per Mehlich III (or equivalent) test	Likely to sorb more dissolved P and metals than mix B because it contains some fines; less likely to leach P than mix B because of low P content	Harder on most plants than mix A because it dries out very quickly. Research in Wisconsin indicates that in cold climates, excess of Na ions can promote displacement of Mg and Ca in the soil, which breaks down soil structure and decreases infiltration rate, and can also cause nutrient imbalances1
D	Not in original MN Stormwater Manual	All components below by dry weight: 60-75% sand Min. 55% total coarse and medium sand as a % of total sand Less than 12% fine gravel less than 5 mm (Calculated separately from sand/silt/ clay total) 2 to 5 % organic matter recommended P content between 12 and 30 mg/kg per Mehlich III (or equivalent) test	Best for pollutant removal, moisture retention, and growth of most plants; less likely to leach P than mix B because of low P content	Harder to find. Research in Wisconsin indicates that in cold climates, excess of Na ions can promote displacement of Mg and Ca in the soil, which breaks down soil structure and decreases infiltration rate, and can also cause nutrient imbalances
E	Not in original manual	60-80% sand meeting gradation requirements of MnDOT 3126, ―Fine Aggregate for Portland Cement Concrete 20-40% MnDOT 3890 Grade 2 Compost 30% organic leaf compost	High infiltration rates, relatively inexpensive	As compost breaks down, nutrients available for plants decreases
F	Not in original manual	75% loamy sand by volume: Upper Limit: 85-90% sand with %Silt + 1.5 times %Clay > 15%. Lower Limit: 70-85% sand with %Silt + 2 times %Clay < 30%. Maximum particle size < 1-inch 25% MnDOT 3890 Grade 2 Compost	Finer particles in loamy sand holds moisture for better plant growth	Lower infiltration rates, requires careful soil placement to avoid compaction, requires custom mixing

¹This problem can be avoided by minimizing salt use. Sodium absorption ratio (SAR) can be tested; if the SAR becomes too high, additions of gypsum (calcium sulfate) can be added to the soil to free the Na and allow it to be leached from the soil (Pitt et al in press).
²MnDOT Grade 2 compost is recommended.

Other media

Several other media are currently being tested. A few examples are listed below.

Wisconsin peat moss replacement (Bannerman, 2013)

The following mix utilizes peat moss instead of compost.

• 12 percent peat moss*2 percent Imbrium Sorptive®MEDIA*86 percent sand

This mix aims to maximize phosphorus removal in 2 ways:

• substituting peat moss for compost, since peat moss has lower phosphorus content than compost and does not leach phosphorus; and
• including Sorptive®MEDIA to sorb phosphorus and minimize phosphorus in effluent

Wisconsin layered system

This layered system is designed to minimize phosphorus in bioretention effluent.

Landscaping

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

Landscaping is critical to the performance and function of bioretention areas. Therefore, a landscaping plan is HIGHLY RECOMMENDED for bioretention areas. RECOMMENDED planting guidelines for bioretention facilities are as follows:

• Vegetation should be selected based on a specified zone of hydric tolerance. Plants for Stormwater Design by the Minnesota Pollution Control Agency is a good resource.
• Native plant species should be specified over non-native species. Hardy native species that thrive in our ecosystem without chemical fertilizers and pesticides are the best choices.
• Many bioretention facilities feature wild flowers and grasses as well as shrubs and some trees.
• Woody vegetation should not be specified at inflow locations.
• Trees should not be planted directly overtop of under-drains and may be best located along the perimeter of the practice.
• Salt resistant vegetation should be used in locations with probable adjacent salt application, i.e. roadside, parking lot, etc.
• Fluctuating water levels following seeding (prior to germination) can cause seed to float and be transported. Seed is also difficult to establish through mulch, a common surface component of Bioretention. It may take up to two growing seasons to establish the function and desired aesthetic of mature vegetation via seeding. Therefore mature plantings are recommended over seed.
• If a minimum coverage of 50 percent is not achieved after the first growing season, a reinforcement planting is required
• Bioretention area locations should be integrated into the site planning process, and aesthetic considerations should be taken into account in their siting and design.

Safety

Bioretention practices do not pose any major safety hazards. Trees and the screening they provide may be the most significant consideration of a designer and landscape architect. Where inlets exist, they should have grates that either have locks or are sufficiently heavy that they cannot be removed easily. Standard inlets and grates used by Mn/DOT and local jurisdictions should be adequate. Fencing of bioretention facilities is generally not desirable

Design procedure

The following steps outline a recommended design procedure for bioretention practices in compliance with the MPCA Construction General Permit for new construction. Design recommendations beyond those specifically required by the permit are also included and marked accordingly.

Design steps

Step 1: Make a preliminary judgment

Make a preliminary judgment as to whether site conditions are appropriate for the use of a bioretention practice, and identify the function of the practice in the overall treatment system

A. Consider basic issues for initial suitability screening:

• Site drainage area
• Site topography and slopes
• Soil infiltration capacity
• Regional or local depth to ground water and bedrock
• Site location/minimum setbacks
• Presence of active karst

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

• Decide whether the bioretention 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 bioretention practice is most likely to be located.

Step 2: Confirm design criteria and applicability

• Determine whether the bioretention practice must comply with the MPCA Construction Stormwater General (CSW) Permit.
• Check with local officials, WMOs, and other agencies to determine if there are any additional restrictions and/or surface water or watershed requirements that may apply.

Warning: If the bioretention practice must comply with the CSW permit, the following prohibitions apply:

• areas that receive discharges from vehicle fueling and maintenance;
• areas with less than three (3) feet of separation distance from the bottom of the infiltration system to the elevation of the seasonally saturated soils or the top of bedrock;
• areas that receive discharges from industrial facilities which are not authorized to infiltrate industrial stormwater under an NPDES/SDS Industrial Stormwater Permit issued by the MPCA;
• areas where high levels of contaminants in soil or groundwater will be mobilized by the infiltrating stormwater;
• areas of predominately Hydrologic Soil Group D (clay) soils unless allowed by a local unit of government with a current MS4 permit;
• areas within 1,000 feet up‐gradient, or 100 feet down‐gradient of active karst features unless allowed by a local unit of government with a current MS4 permit;
• areas within a Drinking Water Supply Management Area (DWSMA) as defined in Minn. R. 4720.5100, subp. 13., unless allowed by a local unit of government with a current MS4 permit; and
• areas where soil infiltration rates are more than 8.3 inches per hour unless soils are amended to slow the infiltration rate below 8.3 inches per hour or as allowed by a local unit of government with a current MS4 permit.

Step 3: Perform field verification of site suitability

If the initial evaluation indicates that a bioretention practice would be a good BMP for the site, it is RECOMMENDED that soil borings or pits be dug (in the same location as the proposed bioretention practice) to verify soil types and infiltration capacity characteristics and to determine the depth to groundwater and bedrock. The number of soil borings should be selected as needed to determine local soil conditions.

It is RECOMMENDED that the minimum depth of the soil borings or pits be five feet below the bottom elevation of the proposed bioretention 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 consistence, root abundance and size
• Soil boundary
• Occurrence of saturated soil, impermeable layers/lenses, ground water, bedrock or disturbed soil

Warning: A separation distance of 3 feet is REQUIRED between the bottom of the bioretention 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).

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 (V_wq), Channel Protection Volume (V_cp), Overbank Flood Protection Volume (V_p10), and the Extreme Flood Volume (V_p100). See the Unified sizing criteria section for details.

Warning: If the bioretention 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 V_wq is to be treated by other BMPs, subtract that portion from the V_wq to determine the part of the V_wq to be treated by the bioretention 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 (V_cp), Overbank Flood Protection Volume (V_p10), and the Extreme Flood Volume (V_p100) may also be managed in the bioretention practice (see Step 7).

Step 5: Determine bioretention type and size practice

(Note: Steps 5, 6, 7 and 8 are iterative)

A. Select Design Variant

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 layers, and the contributing drainage area. While the first step in sizing a bioretention practice is selecting the type of design variant for the site, the basic design procedures for each type of bioretention practice are similar.

After determining the water quality volume for the entire site (Step 1), determine the portion of the total volume that will be treated by the bioretention practice. Based on the known V_wq, infiltration rates of the underlying soils and the known existing potential pollutant loading from proposed/existing landuse select the appropriate bioretention practice from the table below. Note: the determination for underdrain is an iterative sizing process.

Warning: 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.

Caution: It is HIGHLY RECOMMENDED that the drawdown time for bioretention practices is 24 hours or less from the peak water level in the practice when discharges are to a trout stream.

Summary of Bioretention Variants for Permeability of Native Soils and Potential Land use Pollutant Loading
(Link to this table)

Bioretention Type1	Variant	Underlying Soil Performance Criteria
Bioinfiltration (Infiltration/Recharge Facility)	No underdrain	Higher recharge potential (facility drain time without underdrain is 48 hours or less)
Biofiltration with underdrain at the bottom (Filtration/Partial Recharge Facility)	Underdrain	Lower recharge potential (facility drain time without underdrain is > 48 hours)
Biofiltration with internal water storage	Underdrain	Lower recharge potential (facility drain time without underdrain is >48 hours)
Biofiltration with elevated underdrain (Infiltration/Filtration/Recharge Facility)	Elevated underdrain	Higher nutrient loadings and/or quantity control
Biofiltration with liner (Filtration Only Facility)	Underdrain with liner	Hot Spot Treatment

¹The terminology has been changed from the original manual. The original Manual terminology is shown in parenthesis. For more information, see Bioretention terminology

Information collected during the Physical Suitability Evaluation (see Step 2) should be used to explore the potential for multiple bioretention practices versus relying on a single bioretention practice. Bioretention is best employed close to the source of runoff generation and is often located in the upstream portion of the stormwater treatment train, with additional stormwater BMPs following downstream.

B. Determine Site Infiltration Rates (for facilities with infiltration and/or recharge)

If the infiltration rate is not measured, use the table below to estimate an infiltration rate 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.

Warning: Select the design infiltration rate from the table based on the least permeable soil horizon within the first 3 feet below the bottom elevation of the proposed infiltration practice

.

Caution: The table for design infiltration rates has been modified. Field testing is recommended for gravelly soils (HSG A; GW and GP soils; gravel and sandy gravel soils). If field-measured soil infiltration rates exceed 8.3 inches per hour, the Construction Stormwater permit requires the soils be amended. Guidance on amending these soils can be found here.

Design infiltration rates, in inches per hour, for A, B, C, and D soil groups. Corresponding USDA soil classification and Unified soil Classifications are included. Note that A and B soils have two infiltration rates that are a function of soil texture.*
The values shown in this table are for uncompacted soils. This table can be used as a guide to determine if a soil is compacted. For information on alleviating compacted soils, link here. If a soil is compacted, reduce the soil infiltration rate by one level (e.g. for a compacted B(SM) use the infiltration rate for a B(MH) soil).
Link to this table

Hydrologic soil group	Infiltration rate (inches/hour)	Infiltration rate (centimeters/hour)	Soil textures	Corresponding Unified Soil ClassificationSuperscript text
A	Although a value of 1.63 inches per hour (4.14 centimeters per hour) may be used, it is Highly recommended that you conduct field infiltration tests or amend soils.b See Guidance for amending soils with rapid or high infiltration rates and Determining soil infiltration rates.		gravel sandy gravel	GW - Well-graded gravels, fine to coarse gravel GP - Poorly graded gravel
	1.63a	4.14	silty gravels gravelly sands sand	GM - Silty gravel SW - Well-graded sand, fine to coarse sand
	0.8	2.03	sand loamy sand sandy loam	SP - Poorly graded sand
B	0.45	1.14	silty sands	SM - Silty sand
	0.3	0.76	loam, silt loam	MH - Elastic silt
C	0.2	0.51	Sandy clay loam, silts	ML - Silt
D	0.06	0.15	clay loam silty clay loam sandy clay silty clay clay	GC - Clayey gravel SC - Clayey sand CL - Lean clay OL - Organic silt CH - Fat clay OH - Organic clay, organic silt

¹For Unified Soil Classification, we show the basic text for each soil type. For more detailed descriptions, see the following links: The Unified Soil Classification System, CALIFORNIA DEPARTMENT OF TRANSPORTATION (CALTRANS) UNIFIED SOIL CLASSIFICATION SYSTEM

• NOTE that this table has been updated from Version 2.X of the Minnesota Stormwater Manual. The higher infiltration rate for B soils was decreased from 0.6 inches per hour to 0.45 inches per hour and a value of 0.06 is used for D soils (instead of < 0.2 in/hr).
  

Source: Thirty guidance manuals and many other stormwater references were reviewed to compile recommended infiltration rates. All of these sources use the following studies as the basis for their recommended infiltration rates: (1) Rawls, Brakensiek and Saxton (1982); (2) Rawls, Gimenez and Grossman (1998); (3) Bouwer and Rice (1984); and (4) Urban Hydrology for Small Watersheds (NRCS). SWWD, 2005, provides field documented data that supports the proposed infiltration rates. (view reference list)
^aThis rate is consistent with the infiltration rate provided for the lower end of the Hydrologic Soil Group A soils in the Stormwater post-construction technical standards, Wisconsin Department of Natural Resources Conservation Practice Standards.
^bThe infiltration rates in this table are recommended values for sizing stormwater practices based on information collected from soil borings or pits. A group of technical experts developed the table for the original Minnesota Stormwater Manual in 2005. Additional technical review resulted in an update to the table in 2011. Over the past 5 to 7 years, several government agencies revised or developed guidance for designing infiltration practices. Several states now require or strongly recommend field infiltration tests. Examples include North Carolina, New York, Georgia, and the City of Philadelphia. The states of Washington and Maine strongly recommend field testing for infiltration rates, but both states allow grain size analyses in the determination of infiltration rates. The Minnesota Stormwater Manual strongly recommends field testing for infiltration rate, but allows information from soil borings or pits to be used in determining infiltration rate. A literature review suggests the values in the design infiltration rate table are not appropriate for soils with very high infiltration rates. This includes gravels, sandy gravels, and uniformly graded sands. Infiltration rates for these geologic materials are higher than indicated in the table.
References: Clapp, R. B., and George M. Hornberger. 1978. Empirical equations for some soil hydraulic properties. Water Resources Research. 14:4:601–604; Moynihan, K., and Vasconcelos, J. 2014. SWMM Modeling of a Rural Watershed in the Lower Coastal Plains of the United States. Journal of Water Management Modeling. C372; Rawls, W.J., D. Gimenez, and R. Grossman. 1998. Use of soil texture, bulk density and slope of the water retention curve to predict saturated hydraulic conductivity Transactions of the ASAE. VOL. 41(4): 983-988; Saxton, K.E., and W. J. Rawls. 2005. Soil Water Characteristic Estimates by Texture and Organic Matter for Hydrologic Solutions. Soil Science Society of America Journal. 70:5:1569-1578.

The infiltration capacity and existing hydrologic regime of natural basins are inherently different than constructed practices and may not meet MPCA 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 per 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 D3385 should 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 safety factor also accounts for the long-term infiltration capacity of the stormwater management facility.

C. Size bioretention area

Without An UnderDrain: The bioretention surface area, A_f, is computed using the following equation, for those practices that are designed without an underdrain

\(A_f = V_{wq} d_f / (i (h_f + d_f) t_f)\)

Where:

A_f = surface area of filter bed (square feet);

d_f = filter bed depth (feet);

i = infiltration rate of underlying soils (feet per day);

h_f = average height of water above filter bed (feet); and

t_f = design filter bed drain time (days)

Warning: 48 hours is the REQUIRED maximum t_f for bioretention under the CGP

Use the table below to determine the infiltration rate of the underlying soils. Note that these numbers are intentionally conservative based on experience gained from Minnesota infiltration sites.

Design Infiltration Rates

With An UnderDrain:

The bioretention surface area is computed using the following equation, for those practices that are designed with an underdrain

\(A_f = (V_{wq} x d_f) / k (h_f + d_f) t_f\)

Where:

A_f = surface area of filter bed (square feet);

d_f = filter bed depth (feet);

k = coefficient of permeability of filter media (feet per day);

h_f = average height of water above filter bed (feet); and

t_f = design filter bed drain time (days)

Warning: 48 hours is the REQUIRED maximum t_f for bioretention under the CGP

All bioretention growing media should have a field tested infiltration rate between 1 and 8 inches per hour. Growing media with slower infiltration rates could clog over time and may not meet drawdown requirements. Target infiltration rates should be no more than 8 inches per hour to allow for adequate water retention for vegetation as well as adequate retention time for pollutant removal. Slower rates (2 inches per hour or less) are recommended if the primary pollutant(s) of concern are temperature, total nitrogen or total phosphorus. If the infiltration rate of the growing media has not been field tested, the coefficients of permeability recommended for the Planting Medium / Filter Media Soil is 0.5 feet per day (Claytor and Schueler, 1996). Note: the value is conservative to account for clogging associated with accumulated sediment.

Step 6. Size outlet structure and/or flow diversion structure, if needed

(Note: Steps 5, 6, 7 and 8 are iterative)

Warning: It is REQUIRED that an outlet be incorporated into the design of a bioretention practice to safely convey excess stormwater.

Step 7. Perform groundwater mounding analysis

(Note: Steps 5, 6, 7 and 8 are iterative) Groundwater mounding, the process by which a mound forms on the water table as a result of recharge at the surface, can be a limiting factor in the design and performance of bioretention practices where infiltration is a major design component. A minimum of 3 feet of separation between the bottom of the bioretention practice and seasonally saturated soils (or from the top of 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 designed bioretention practices.

The most widely known and accepted analytical methods to solve for groundwater mounding is 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 8. Determine pre-treatment volume and design pre-treatment measures

Warning: Some form of dry or wet pre-treatment is REQUIRED prior to the discharge of stormwater into the bioretention practice, to remove any sediment and fines that may result in clogging of the soils in the sediment basin area.

If a grass filter strip is used, it is HIGHLY RECOMMENDED that it be sized using the guidelines in the table below.

Guidelines for filter strip pre-treatment sizing
Link to this table

Parameter	Impervious Parking Lots				Residential Lawns
Maximum Inflow Approach Length (ft)	35		75		75		150
Filter Strip Slope	=<2%	>2%	=<2%	2%	=<2%	2%	=<2%	2%
Filter Strip Minimum Length	10'	15'	20'	25'	10'	12'	15'	18'

Grass channel sizing

It is HIGHLY RECOMMENDED that grass channel pre-treatment for bioretention be a minimum of 20 feet in length and be designed according to the following guidelines:

• Parabolic or trapezoidal cross-section with bottom widths between 2 and 8 feet
• Channel side slopes no steeper than 3:1 (horizontal:vertical).
• Flow velocities limited to 1 foot per second or less for peak flow associated with the water quality event storm (i.e., 0.5 or 1.0 inches depending on watershed designation).
• Flow depth of 4 inches or less for peak flow associated with the water quality event storm.

Step 9. Check volume, peak discharge rates and period of inundation against State, local and watershed management organization requirements

(Note: Steps 5, 6, 7 and 8 are iterative)

Follow the design procedures identified in the Unified sizing criteria section to determine the volume control and peak discharge recommendations for water quality, recharge, 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 (see also Introduction to stormwater modeling). This includes defining the parameters of the bioretention practice defined above: sedimentation basin elevation and area (defines the pond volume), infiltration/permeability rate, 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 2).

Step 10. Prepare vegetation and landscaping plan

See Major Design Elements for guidance on preparing vegetation and landscaping management plan.

Step 11. Prepare operations and maintenance (O&M) plan

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

Step 12. Prepare cost estimate

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

Related pages

• Overview for bioretention
• Types of bioretention
• Design criteria for bioretention
• Construction specifications for bioretention
• Operation and maintenance of bioretention
• Cost-benefit considerations for bioretention
• External resources for bioretention
• References for bioretention
• Requirements, recommendations and information for using bioretention BMPs in the MIDS calculator