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 (or other consistently applicable regulations)

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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.

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: Less than 1 acre maximum and ½ acre impervious maximum per infiltration design practice is RECOMMENDED. 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% but greater than 1%, to promote positive flow towards the practice.

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

• Depth to Ground Water and Bedrock: A separation distance of 3 feet is REQUIRED between the bottom of the bioretention practice and the elevation of the seasonally high water table or bedrock.

Karst: Under-drains and an impermeable liner may be desirable in some karst areas; specific site geotechnical assessment RECOMMENDED

• Site Location / Minimum Setbacks: It is HIGHLY RECOMMENDED that infiltration designed bioretention practices not be hydraulically connected to structure foundations or pavement, to avoid seepage and frost heave concerns, respectively. If ground-water contamination is a concern, it is RECOMMENDED that ground-water mapping be conducted to determine possible connections to adjacent ground-water wells. Table (12.BIO.4.) provides the minimum setbacks REQUIRED by the Minnesota Department of Health for the design and location of bioretention practices.

Warning: Incorporating pre-treatment helps to reduce the maintenance burden of bioretention, and reduces the likelihood that the soil bed will clog over time. Adequate pre-treatment for bioretention systems is REQUIRED

Conveyance

It is HIGHLY RECOMMENDED that overflow associated with the 10-year or 25-year 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 cfs, the designer evaluate the potential for erosion to stabilized areas and bioretention facilities.

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 twelve 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.

It is HIGHLY RECOMMENDED that bioretention areas with under-drains be equipped with a minimum 8” diameter under-drain in a 1’ deep gravel bed. Increasing the diameter of the underdrain makes freezing less likely, and provides a greater capacity to drain standing water from the filter. The porous gravel bed prevents standing water in the system by promoting drainage. Gravel is also less susceptible to frost heaving than finer grained media. It is also HIGHLY RECOMMENDED that a pea gravel diaphragm and/or permeable filter fabric be placed between the gravel layer and the filter media.

Pre-treatment

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

Warning: Incorporating pre-treatment helps to reduce the maintenance burden of bioretention, and reduces the likelihood that the soil bed will clog over time. Adequate pre-treatment for bioretention systems 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 length of the filter strip depends on the drainage area, imperviousness, and the filter strip slope. 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.

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. It is HIGHLY RECOMMENDED that grass filter strips or grass channels at least 10 or 20 feet long, respectively, convey flow to the system in these situations. 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
• Gravel diaphragm
• Mulch layer
• Forebay
• 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 5-10% 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 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 under-drain.
• Practice Slope: It is RECOMMENDED that the slope of the surface of the bioretention practice not exceed 1%, 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 reduce hydraulic overload of in-situ soils/soil medium and to maximize the surface area to facility depth ratio, where space allows. Where feasible ponding depths should be no greater than 6 inches. The maximum allowable pooling depth is 18 inches. It is RECOMMENDED that the elevation difference from the inflow to the outflow be approximately 4-6 feet when an under-drain is used.

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

• Ground water Protection: Exfiltration of unfiltered PSH runoff into ground water 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 ground water, sump pumps, or other sources so that constant saturated conditions do not occur.

Information: 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% 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 should be sized based on the principles of Darcy’s Law.

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

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.

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% 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.

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'

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 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.

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 ground water 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

Information: A minimum of 3 feet of separation between the bottom of the bioretention practice and 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.

• 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

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). Details on the Unified Stormwater Sizing Criteria are found in Chapter 10.

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 ½ inch of runoff from the new impervious surfaces created from the project (or 1 inch for certain protected waterbodies). 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 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 (Vcp), Overbank Flood Protection Volume (Vp10), and the Extreme Flood Volume (Vp100) 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.

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.

Water Quality Volume (V_wq): 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 Vwq, infiltration rates of the underlying soils and the known existing potential pollutant loading from proposed/existing landuse select the appropriate bioretention practice from Table 12.BIO.6. Note: the determination for under-drain is an iterative sizing process.

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 BMP following downstream.

B. Determine Site Infiltration Rates (for facilities with infiltration and/or recharge). If the infiltration rate is not measured: Table 12.BIO.7 provides 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 from the table based on the least permeable soil horizon within the first five feet below the bottom elevation of the proposed infiltration practice.

The infiltration capacity and existing hydrologic regime of natural basins are inheritably 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/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 should be used for the field test.

C. Size bioretention area Without An Under-Drain: The bioretention surface area is computed using the following equation, for those practices that are designed without an under-drain\[A_f = V_{wq} d_f / i (h_f + d_f) t_f\]

Where:

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

d_f = filter bed depth (feet);

i = infiltration rate of underlying soils (ft/day);

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

t_f = design filter bed drain time (days)

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

Use Table 12.BIO.7 to determine the infiltration rate of the underlying soils. Note that these numbers are intentionally conservative based on experience gained from Minnesota infiltration sites.

With An Under-Drain: The bioretention surface area is computed using the following equation, for those practices that are designed with an under-drain\[A_f = (V_{wq} x d_f) / k (h_f + d_f) t_f]\]

Where:

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

d_f = filter bed depth (feet);

k = coefficient of permeability of filter media (feet/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 tf for bioretention under the CGP

The coefficients of permeability recommended for the Planting Medium / Filter Media Soil is 0.5 ft/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) It is REQUIRED that a secondary outlet be incorporated into the design of a bioretention practice to safely convey excess stormwater.

Step 7. Perform ground water mounding analysis

(Note: Steps 5, 6, 7 and 8 are iterative) Ground water 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 bedrock) is REQUIRED (5 feet RECOMMENDED) to maintain the hydraulic capacity of the practice and provide adequate water quality treatment. A ground water mounding analysis is RECOMMENDED to verify this separation for infiltration designed bioretention practices.

The most widely known and accepted analytical methods to solve for ground water mounding is based on the work by Hantush (1967) and Glover (1960). The maximum ground water mounding potential should be determined through the use of available analytical and numerical methods. Detailed ground water 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

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 Table 12.BIO.8.

A. 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 of the Manual (Chapter 10) 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 Chapter 8 and Appendix B of the manual). 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:

A. Volume

Infiltration or filtration systems shall be sufficient to infiltrate or filter a water quality volume of ½ inch of runoff (1” is required for discharge to protected waters) 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.

B. 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.

C. 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.