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: 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 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).

• 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 infiltration designed bioretention 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 infiltration 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 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 cubic feet per second, 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 underdrains be equipped with a minimum 8 inches diameter under-drain in a 1 foot 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.

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, use of a pretreatment device such as a vegetated filter strip, small sedimentation basin, 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 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 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 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 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 intoground 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 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.

This page provides construction details, materials specifications and construction specifications for bioretention systems.

Construction details

Illustration of a cross-section for a bioretention facilities general plan. To access plans and .dwg files, click here.

Illustration of a cross-section of a bioretention BMP with an elevated underdrain. To access cross-sections and .dwg files, click here.

CADD based details for bioretention are contained in the Computer-aided design and drafting (CAD/CADD) drawings section. The following details, with specifications, have been created for bioretention systems:

• Bioretention Facilities General Plan
  • Bioretention plan-offline NEW
  • Bioretention plan-online NEW
  • Biofiltration planter - Plan NEW
  • Bioretention parking median - Plan NEW
• Bioretention Facilities Performance Types Cross-Sections
  • Bioinfiltration NEW
  • Biofiltration with underdrain at the bottom NEW
  • Biofiltration with elevated underdrain NEW
  • Biofiltration with internal water storage NEW
  • Biofiltration with liner NEW
  • Biofiltration planter - Section NEW
  • Bioretention parking median - Section NEW
  • Cleanout NEW
  • Underdrain valve NEW
  • Biofiltration with elevated underdrain NEW
  • Biofiltration with internal water storage NEW
  • Biofiltration with underdrain at bottom NEW
  • Bioinfiltration NEW
  • Biofiltration with liner NEW
  • Infiltration / Recharge Facility
  • Filtration / Partial Recharge Facility
  • Infiltration / Filtration / Recharge Facility
  • Filtration Only Facility

Information: Information on bioretention media mixes, previously included on this page, has been moved to the section on design criteria.

Construction specifications

Information: Off-line construction is preferred to allow for establishment of vegetation

Proper construction techniques are critical to achieve long-term functionality of bioretention systems. Construction sequencing is imperative; infiltration BMPs need to be installed as close to the end of construction as possible. Preferably, site slopes have been stabilized and the asphalt has been installed before the bioretention construction begins. However, this is not always possible in bioretention cells that are surrounded by a parking lot and are the only outlet for stormwater. In this case, excavate the bioretention cell during a period of dry weather. Install the underdrains, the outlet structure, and the media up to the proposed final elevation. Once the construction of the parking lot is complete, scrape off any silty or clayey confining layer that has accumulated on top of the media and remove it from the site and de-compact soil. Top up media to desired finished elevation. Mulch and plants (or sod if it is a grassed bioretention cell) may then be installed.

During construction it is critical to keep sediment out of the infiltration device as much as practicable. Utilizing sediment and erosion control measures, such as compost logs, check dams, and sediment basins, will help to keep bioretention cells from clogging. As soon as grading is complete, slopes should be stabilized to reduce erosion of native soils. If vegetated filter strips are used as pre-treatment, they must be vegetated as soon as possible following the completion of grading.

If the bioretention cell has sod specified for its cover (as opposed to tree/shrub/mulch systems), the sod must be either 1) grown on sandy underlying soils or 2) be washed sod. Washed sod has had all soil removed from the roots, which prevents the sod layer from restricting infiltration into the underlying media. It also typically roots more quickly than standard sod.

Demonstration of the rake technique with a bucket with teeth (top) and scoop technique using a bucket with a smooth blade (right). During the final pass of excavation, the rake technique should be used to break up the soil and promote exfiltration. Source: Dr. Robert Brown, ORISE Research Fellow, US EPA, Edison, NJ.

Preventing and alleviating compaction are crucial during construction of infiltration practices, as compaction can reduce infiltration rates in sandy soils by an order of magnitude and in clayey soils by a factor of 50 (Pitt et al. , 2008). Therefore, it is critical to keep heavy construction equipment from compacting or smearing soils at the bottom of the excavation. Excavate the soil from the perimeter of the infiltration device. Tracked vehicles should be used to reduce the pressure placed on the soil. It is highly recommended to excavate during dry conditions to prevent smearing of the soil, which has been shown to reduce the soil infiltration rate (Brown and Hunt, 2010). Driveable mats can be used for backfill and grading to minimize compaction. During the final pass with the excavator bucket (i.e. bottom of excavation), it is highly recommended to rake the soil with the teeth of the bucket to loosen any compaction (Brown and Hunt, 2010). Smooth bucket blades smear soils and restrict infiltration rates. Soil ripping has also been shown to increase infiltration and reduce compaction from construction activities (Tyner et al., 2009; Wardynski et al., 2013).

Given that the construction of bioretention practices incorporates techniques or steps which may be considered non-traditional, it is recommended that the construction specifications include the format and information discussed below.

Temporary erosion control

Warning:

• It is REQUIRED that future bioretention locations not be used as temporary sedimentation basins unless 3 feet of cover is left in place during construction.
• If the bioretention area is excavated to final grade (or within 3 feet of) it is REQUIRED that rigorous erosion prevention and sediment controls (e.g. diversion berms) are used to keep sediment and runoff completetly away from the bioretention area.

• Install prior to site disturbance
• Protect catch basin/inlet
• Cover erodible surfaces, for example with plastic covers, that can be re-used.

Excavation, backfill and grading

• It is HIGHLY RECOMMENDED that prior to beginning the installation, sufficient material quantities shall be onsite to complete the installation and stabilize exposed soil areas without delay.
• It is HIGHLY RECOMMENDED that excavation, soil placement and rapid stabilization of perimeter slopes be completed before the next precipitation event
• Timing of grading of infiltration practices relative to total site development
• Use of low-impact, earth moving equipment (wide track or marsh track equipment, or light equipment with turf-type tires)
• Do not over-excavate
• Restoration in the event of sediment accumulation during construction of practice
• Alleviate any compacted soil (compaction can be alleviated at the base of the practice by using a primary tilling operation such as a chisel plow, ripper or sub-soiler to a minimum 12 inch depth
• Gravel backfill specifications
• Gravel filter specifications
• Filter fabric specifications===Alleviating compaction resulting from construction===

Alleviation of compaction of disturbed soil is crucial to the installation of successful vegetated stormwater infiltration practices. Typical bulk densities for non-compacted soils are in the range of 1.0 to 1.50 grams per cubic centimeter. Urban soils typically have bulk densities greater than this. Construction activities can increase bulk densities by 20 percent or more. The compaction can extend up to two feet into the soil profile, often resulting in bulk densities that do not readily support healthy plant growth.

Comparison of bulk densities for undisturbed soils and common urban conditions. (Source: Schueler, T. 2000. The Compaction of Urban Soils: Technical Note #107 from Watershed Protection Techniques. 3(2): 661-665. Center for Watershed Protection, Ellicott City, MD.)
For information on alleviating soil compaction, see Alleviating compaction from construction activities
Link to this table

Undisturbed soil type or urban condition	Surface bulk density (grams / cubic centimeter
Peat	0.2 to 0.3
Compost	1.0
Sandy soil	1.1 to 1.3
Silty sands	1.4
Silt	1.3 to 1.4
Silt loams	1.2 to 1.5
Organic silts / clays	1.0 to 1.2
Glacial till	1.6 to 2.0
Urban lawns	1.5 to 1.9
Crushed rock parking lot	1.5 to 2.0
Urban fill soils	1.8 to 2.0
Athletic fields	1.8 to 2.0
Rights of way and building pads (85% compaction)	1.5 to 1.8
Rights of way and building pads (95% compaction)	1.6 to 2.1
Concrete pavement	2.2
Quartzite (rock)	2.65

Increase in soil bulk density as a result of different land uses or activities.
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Land use or activity	Increase in bulk density (grams / cubic centimeter	Source (link to Reference list)
Grazing	0.12 to 0.20	Smith, 1999
Crops	0.25 to 0.35	Smith, 1999
Construction, mass grading	0.34 to 0.35	Randrup, 1998; Lichter and Lindsey, 1994
Construction, no grading	0.20	Lichter and Lindsey, 1994
Construction traffic	0.17 to 0.40	Lichter and Lindsey, 1994; Smith, 1999; Friedman, 1998
Athletic fields	0.38 to 0.54	Smith, 1999
Urban lawn and turf	0.30 to 0.40	Various sources

While natural processes can alleviate soil compaction, additional techniques to alleviate soil compaction are often desirable because

• it can take many years for natural processes to loosen up soil;
• natural processes operate primarily within the first foot or so of soil, and compaction from development can extend to two feet deep; and
• once soil compaction becomes so severe that plants and soil microbes can no longer thrive, natural processes are no longer able to reduce soil compaction.

The most effective method for alleviating compaction is to add compost amendment. Other amendments, such as sand (in clay soils), can significantly reduce compaction since sandy soils are more resistant to compaction. An additional technique for alleviating compaction is subsoiling or soil ripping, although ripping by itself appears to have limitations. Ripping is most effective when used in conjunction with compost and/or sand amendment.

Schueler (Technical Note 108) concludes that “Based on current research, it appears that the best construction techniques are only capable of preventing about a third of the expected increase in bulk density during construction.”

Reported Activities that Restore or Decrease Soil Bulk Density
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Land use or activity	Decrease in bulk density (gms/cc)	Source
Tilling of soil	0.00 to 0.02	Randrup, 1918. Patterson and Bates, 1994
Spedialized soil loosening	0.05 to 0.15	Rolf, 1998
Selective grading	0.00	Randrup, 1998 and Lichter and Linsy, 1994
Soil amendments	0.17	Patterson and Bates, 1994
Compost amendments	0.25 to 0.35	Kolsti et al. 1995
Time	0.20	Legg et al, 1996
Reforestration	0.25 to 0.35	Article 36

Soil ripping

The goal of soil ripping or subsoiling is to fracture compacted soil without adversely disturbing plant life, topsoil, and surface residue. Soil compaction occurs most frequently with soils having a high clay content. Fracturing compacted soil promotes root penetration by reducing soil density and strength, improving moisture infiltration and retention, and increasing air spaces in the soil. Compacted layers typically develop 12 to 22 inches below the surface when heavy equipment is used. Conventional cultivators cannot reach deep enough to break up this compaction. Subsoilers (rippers) can break up the compacted layer without destroying soil aggregate structure, surface vegetation, or mixing soil layers (Kees, 2008).

How effectively compacted layers are fractured depends on the soil's moisture, structure, texture, type, composition, porosity, density, and clay content. Success depends on the type of equipment selected, its configuration, and the speed with which it is pulled through the ground. No one piece of equipment or configuration works best for all situations and soil conditions, making it difficult to define exact specifications for subsoiling equipment and operation.

Example of different shank designs commonly used for agricultural tillage (Kees, 2008).

Subsoilers are available with a wide variety of shank designs. Shank design affects subsoiler performance, shank strength, surface and residue disturbance, effectiveness in fracturing soil, and the horsepower required to pull the subsoiler. According to Kees (2008), “Parabolic shanks require the least amount of horsepower to pull. In some forest applications, parabolic shanks may lift too many stumps and rocks, disturb surface materials, or expose excess subsoil. Swept shanks tend to push materials into the soil and sever them. They may help keep the subsoiler from plugging up, especially in brush, stumps, and slash. Straight or "L" shaped shanks have characteristics that fall somewhere between those of the parabolic and swept shanks.”

Photos of wing tip and conventional tip subsoilers (Kees, 2008).

Comparison of soil disturbance from a winged tip versus a conventional tip: winged tips can typically be spaced farther apart because they fracture more of the soil than conventional tips (Kees, 2008).

Researchers have found that there is a “critical depth”, and according to Spoor and Godwin (1978) this “critical depth is dependent upon the width, inclination and lift height of the tine foot and on the moisture and density status of the soil.” Spoor and Godwin (1978) explainthat tine depth is crucial because “At shallow working depths the soil is displaced forwards, side-ways and upwards (crescent failure), failing along well defined rupture planes which radiate from just above the tine tip to the surface at angles of approximately 45” to the horizontal. Crescent failure continues with increasing working depth until, at a certain depth, the critical depth, the soil at the tine base begins to flow forwards and sideways only (lateral failure) creating compaction at depth.” They found that below the critical depth “compaction occurs rather than effective soil loosening.” They also found that “The wetter and more plastic a soil is, the shallower is thecritical depth.” An approach developed by Silsoe College, Cranfield University, in collaboration with Transco UK, for use on pipeline sites, was to work progressively deeper with repeated passes, up to 5 or 6 under extreme conditions, with the tractor operating on the same tramline/traffic lane on each pass (Spoor & Foot, 1998).”

Shanks are available with winged tips and conventional tips. Winged tips cost more than conventional tips and require more horsepower, but can often be spaced farther apart. Increasing wing width also increases critical depth – the depth below which little soil loosening occurs (Owen 1987, Spoor 1978). Using shallow leading tines ahead of deeper tines also increases required shank spacing (Spoor 1978). According to Kees (2008), the shank’s tip should run to a depth of 1 to 2 inches below the compacted layer. Kees (2008) also recommends making sure that the shanks on the subsoiler are spaced so that they run in the tracks of the tow vehicle, because the equipment used to pull subsoilers is heavy enough to create compaction itself. Ideal shank spacing will depend on soil moisture, soil type,degree of compaction, and the depth of the compacted layer. Spacing should be adjustable so the worked area can be fractured most efficiently. Because ideal shank configuration will vary with varying soil textures and moisture, shank spacing and height should be adjustable in the field (Kees 2008).

Impacts of having subsoiler shanks spaced correctly (top left) versus spaced too widely apart (bottom left) and having shanks at correct depth (top right) versus too deep (bottom right) (Image from Kees, 2008). Note: compaction on a construction site can be much more severe than just the plow layer shown in the above agricultural or forestry images.

Kees (2008) recommends following ground contours whenever possible when subsoiling to “increase water capture, protect water quality, and reduce soil erosion.” He also states that “in some cases, two passes at an angle to each other may be required to completely fracture compacted soil.” Spoor and Godwin (1978) also found that “Relatively closely spaced tines, staggered to prevent blockage, are more efficient at producing complete loosening than repeated passes with tines at wider spacings.”

Travel speed of the subsoiler also affects subsoiling disturbance. “Travel speed that is too high can cause excessive surface disturbance, bring subsoil materials to the surface, create furrows, and bury surface residues. Travel speed that is too slow may not lift and fracture the soil adequately” (Kees2008).

Soils should be mostly dry and friable. Urban (2008) describes ideal conditions for compaction reduction as follows: “soil moisture must be between field capacity and wilt point during compaction reduction for maximum effectiveness."

Always know where utilities are buried prior to subsoiling. Avoid subsoiling in area that have buried utilities, wires, pipes, culverts, or diversion channels (Kees 2008, Urban 2008).

Soil ripping will generally be more effective with the addition of an amendment. This can be either sand or compost. Tilling in compost amendment may not be desirable on sites with steep slopes, a high water table, wet saturated soils, or downhill slope toward a house foundation (Schueler Technical Note #108) or where there are tree roots or utilities, or where nutrients leaching from compost would pose a problem. Since soil restoration techniques will need to be tailored to site conditions, a prescriptive soil restoration specification is not recommended. However, Pennsylvania, Virginia, and Washington State have specifications for soil amendment and restoration and these may be used as guidance in determining how to amend a compacted soil.

According to Spoor and Godwin (1978) “The number of variables involved and soil variation make the accurate prediction of the critical depth for field conditions impractical. Simple field modifications are available, however, such as increasing tine foot width and lift height or loosening the surface layers, to allow rapid implement adjustment to satisfy a range of field conditions.”

If subsoiling was effective, “The ground should be lifted slightly and remain relatively even behind the subsoiler, without major disruption of surface residues and plants. No more than a little subsoil and a few rocks should be pulled to the surface. If large furrows form behind the subsoiler, the shanks may not be deep enough, the angle on winged tips may be too aggressive, or the travel speed may be too high” (Kees 2008).

Cost for subsoiling varies by project. The Pennsylvania Stormwater Best Management Practices Manual estimates the cost of tilling soils ranges from $800 to $1000 per acre, while the cost of compost amending soil is about the same.

Schematic illustrating separation distance from bottom of infiltration BMP and soil ripped zones to water table or top of bedrock

An extensive literature review of the effects of soil ripping can be found in File:Bioretention task 6 soil ripping.docx.

Recommendations for soil ripping to alleviate compaction

For basins larger than 1000 square feet, if compaction is above ideal bulk density indicated in the table below the soil should be remediated as follows:

• Rip to a depth of 18 inches where feasible
• For clay subsoil, incorporate 2 inches of sand. For bioretention without an underdrain, MnDOT Type 2 compost may be incorporated instead of sand.
• Maintain a 3 foot minimum separation distance between the bottom of the infiltration practice and the seasonally high water table or bedrock. If soil ripping is utilized, 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 to the water table or top of bedrock. 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 1 foot (12 inches).

General relationship of soil bulk density to root growth based on soil texture
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Soil texture	Ideal bulk densities (g/cm3)	Bulk densities that may affect plantgrowth (g/cm3)	Bulk densities that restrict root growth (g/cm3)
sands, loamy sands	<1.60	1.69	>1.80
sandy loams, loams	<1.40	1.63	>1.80
sandy clay loams, loams, clay loams	<1.40	1.60	>1.75
silts, silt loams	<1.30	1.60	>1.75
silt loams, silty clay loams	<1.40	1.55	>1.65
sandy clays, silty clays, clay loams with 35-45% clay	<1.10	1.49	>1.58
clays (>45% clay)	<1.10	1.39	>1.47

Effect of ripping plus compost amendment on soil compaction

Compost aggregates soil particles (sand, silt, and clay) into larger particles (Cogger, 2005). Aggregation of soil particles creates additional porosity, which reduces the bulk density of the soil (Cogger, 2005). Compost can also reduce the bulk density of a soil by dilution of the mineral matter in the soil (Cogger, 2005). When the porosity of the soil increases and the particle surface area increases, water holding capacity is also increased (Cogger, 2005). Increases in macropore continuity have been found as well (Harrison et al., 1998). Studies have cited numerous beneficial abilities of compost: increased water drainage, increased water holding capacity, increased plant production, increased root penetrability, reduction of soil diseases, reduction of heavy metals, and the ability to treat many chemical pollutants (EPA, 1997; Harrison et al., 1998; WDOE Stormwater Management Manual, 2007; Olson 2010).

Studies show that tilling in compost is an effective technique to alleviate soil compaction. Two of these studies are summarized below.

• Olson (2010) found that plots where soil was ripped and amended with compost showed reduced soil strength, bulk densities were 18 to 37 percent lower on compost plots compared to controls, and the geometric mean of K_sat on the compost plots was 2.7 to 5.7 times that of the control plot.
• A study at Virginia Tech shows soils with compost incorporated into the soil to a depth of 2 feet has decreased bulk density in the subsoil and is accelerating the process of soil formation and long-term carbon storage compared to other treatments in the study. The result is that trees growing in the compost-amended soil have increased height, canopy diameter, and trunk diameter compred to trees in other treatments.

Precedents for soil restoration specifications

Since soil restoration techniques will need to be tailored to site conditions, a prescriptive soil restoration specification is not recommended. The 2006 Pennsylvania Stormwater Best Management Practices Manual’s chapter on soil amendment and restoration provides a sample specification for soil restoration. Their specification is not prescriptive, but does provide guiding principles, compost material specifications, and performance requirements. They require sub-soiling to loosen soil to less than 1400 kPa (200 psi) to a depth of 20 inches below final topsoil grade to reduce soil compaction in all areas where plant establishment is planned in areas where subsoil has become compacted by equipment operation, or has become dried out andcrusted, or where necessary to obliterate erosion hills.

The Virginia Tech Rehabilitation study website also provides a Soil Profile Rebuilding Specification based on their research. The basic steps in their specification are described below.

• Spread mature, stable compost to a 4 inch depth over compacted subsoil.
• Subsoil to a depth of 24 inches.
• Replace topsoil to 4 inches (6 to 8 inches if severely disturbed).
• Rototill topsoil to a depth of 6 to 8 inches.
• Plant with woody plants.

Washington State’s Department of Ecology’s Stormwater Management for Western Washington (Volume V: Runoff Treatment BMPs, Chapter 5, pages 5-7 to 5-10) also includes a very detailed soil restoration specification that includes the following.

• The topsoil layer has a minimum organic matter content of 10 percent dry weight in planting beds, 5 percent organic matter content in turf areas, and a pH from 6.0 to 8.0 or matching the pH of the undisturbed soil. The topsoil layer shall have a minimum depth of 8 inches except where tree roots limit the depth of incorporation of amendments needed to meet the criteria. Subsoils below the topsoil layer should be scarified at least 4 inches with some incorporation of the upper material to avoid stratified layers, where feasible.
• Mulch planting beds with 2 inches of organic material.
• Use compost and other materials that meet these organic content requirements.
  • The organic content for pre-approved amendment rates can be met only using compost that meets the definition of composted materials in WAC 173-350-100. The compost must also have an organic matter content of 40 to 65 percent and a carbon to nitrogen ratio below 25:1. The carbon to nitrogen ratio may be as high as 35:1 for plantings composed entirely of plants native to the Puget Sound Lowlands region.
  • Calculated amendment rates may be met through use of composted materials meeting above conditions, or other organic materials amended to meet the carbon to nitrogen ratio requirements and meeting the contaminant standards of Grade A Compost. The resulting soil should be conducive to the type of vegetation to be established.
• The soil quality design guidelines listed above can be met by using one of the methods listed below.
  • Leave undisturbed native vegetation and soil, and protect from compaction during construction.
  • Amend existing site topsoil or subsoil either at default pre-approved rates, or at custom calculated rates based on tests of the soil and amendment.
  • Stockpile existing topsoil during grading, and replace it prior to planting. Stockpiled topsoil must also be amended if needed to meet the organic matter or depth requirements, either at a default pre-approved rate or at a custom calculated rate.
  • Import topsoil mix of sufficient organic content and depth to meet the requirements. More than one method may be used on different portions of the same site. Soil that already meets the depth and organic matter quality standards, and is not compacted, does not need to be amended.For a general guide that can be used in plant selection, see Plants for stormwater design by Shaw and Schmidt (2003).

Vigorous plants are crucial to the bioretention system’s long term stormwater performance. Plant roots help soil particles form stable aggregates, improve soil structure, maintain and increase water storage and infiltration capacity, as well as improve stormwater pollutant removal. Specific stormwater benefits include the following.

• Crucial to Long Term Infiltration: As plant roots grow and then decay, they restore and/or enhance soil porosity and infiltration rates. Deeper rooted plants yield higher infiltration rates than shallow rooted plants. Bioretention practices with native prairie plants will typically have greater infiltration rates, deeper rooting depths, greater biological activity of flora and fauna, and deeper drainage compared to turf systems.
• Crucial to Water Quality Benefits: Plants in bioretention systems have been shown to improve dissolved nutrient removal, improve hydrocarbon removal and aid TSS sequestration.
• Interception and Evapotranspiration: Woody vegetation typically intercepts and evapotranspires significantly more water than herbaceous vegetation, and large trees intercept and evapotranspire significantly more rain than small trees

Construction considerations include the following.

• timing of native seeding and native planting;
• weed control; and
• watering of plant material.

Caution: Seeding is generally not recommended for bioretention systems unless the bioretention system can be kept off line until the vegetation is mature, which is several years for native plants, for example, and generally not practical for bioretention projects.

Caution: It is critical to ensure that plants are watered until the plants are adequately rooted. Determine if there are nearby available sources of water for irrigation.

Construction sequence scheduling

• Temporary construction access
• Location of temporary sediment and erosion control practices to protect BMPs and downstream receiving waters
• Removal and storage of excavated material
• Installation of underground utilities
• Rough grading
• Seeding and mulching disturbed areas
• Road construction
• Final grading
• Site stabilization
• Installation of semi-permanent and permanent erosion control measures
• Removal of temporary sediment and erosion control devices

Construction observation

• Adherence to construction documents
• Verification of physical site conditions
• Erosion control measures installed appropriately

Minnesota Department of Transportation example construction protocols

Preliminary analysis and selection

Recommended number of soil borings, pits or permeameter tests for bioretention design. Designers select one of these methods.
Link to this table

Surface area of stormwater control measure (BMP)(ft2)	Borings	Pits	Permeameter tests
< 1000	1	1	5
1000 to 5000	2	2	10
5000 to 10000	3	3	15
>10000	41	41	202

¹an additional soil boring or pit should be completed for each additional 2,500 ft² above 12,500 ft^2
2an additional five permeameter tests should be completed for each additional 5,000 ft² above 15,000 ft²

Field verification testing prior to pond construction

• Soil hydraulic group represent what is stated in SWPPP (Stormwater Pollution Prevention Plan)
• Seasonally high water table not discovered within 3 feet of the excavated pond base within a test pit
• Commonly will test bottom of proposed pond for soil compaction (subsequent subsoil ripping) prior to media placement
• Commonly will test bottom of proposed pond for insitu infiltration rate by test pit or water filled barrel placed on pond base surface

Filter media and material testing

• Existing soil (option 1 below) or Washed sand (option 2 below), and compost certification
• Washed course aggregate choker certification
• Other treatment material certification of iron filings, activated charcoal, pH buffers, minerals, etc.
• Geotextile separation fabric certification
• Drain-tile certification (if filtration is specified)
• Seed source certification
• Barrel test verification of infiltration rate using 2.5 feet of imported 3877 Type G media

Field verification testing/inspection/verification during construction

• Water drains away in 48 hours
• Infiltration drainage rate does not exceed 8.3 inches per hour
• No tracking/equipment in pond bottom
• No sediment deposits from ongoing construction activity, media perimeter controls kept functional
• Forebay is trapping settleable solids, floating materials, and oil/grease
• Area staked off

Notice of Termination (NOT) verification

• Option 1. Amending existing HSG soils with compost or other treatment material. Test the infiltration rate of each infiltration basin using a double ring infiltrometer prior to completion of the basin. Conduct the test at the finished grade of the basin bottom, prior to blending the compost with the in-situ soils or sand. Ensure infiltration rates meet or exceed greater of two times the designed infiltration rate or 2 inches per hour. Conduct a minimum of five tests per representative acre of basin area and a minimum of five tests per basin. Conduct double ring infiltrometer tests in accordance with ASTM standards. Thoroughly wet test areas prior to conducting infiltrometer tests.
• Option 2. Importing 3877 Type G Filter Topsoil Borrow (may be amended with other treatment material). Ensure infiltration rates meet or exceed greater of two times the designed infiltration rate or 2 inches per hour, or rate specified in the plan. Conduct a minimum of five tests per representative acre of basin area and a minimum of five tests per basin. Conduct double ring infiltrometer tests in accordance with ASTM standards. Thoroughly wet test areas prior to conducting infiltrometer tests. Amend soils with additional washed sand if rates less than specified in the contract, or compost if rates exceed 8.3 inches per hour.

The permanent stormwater management system must meet all requirements in sections 15, 16, and 17 of the CSW permit and must operate as designed. Temporary or permanent sedimentation basins that are to be used as permanent water quality management basins have been cleaned of any accumulated sediment. All sediment has been removed from conveyance systems and ditches are stabilized with permanent cover.

References

• Brown, R.A. and Hunt, W.F. (2010). Impacts of construction activity on bioretention performance. Journal of Hydrologic Engineering. 15(6), 386-394.
• Chaplin, Jonathan, Min Min, and Reid Pulley. 2008. Compaction Remediation for Construction Sites. Final Report. Department of Bioproducts and Biosystems Engineering, University of Minnesota, St. Paul,Minnesota.
• Cogger, C. 2005. Potential Compost Benefits for Restoration of Soils Disturbed by Urban Development. Compost Science & Utilization. 13.4:243-251.
• Hanks, Dallas, and A. Lewandowski, 2003. Protecting Urban Soil Quality: Examples for Landscape Codes and Specifications. USDA Natural Resources Conservation Services.
• Kees, Gary. 2008. Using Subsoiling To Reduce Soil Compaction. U.S. Forest Service Technology & Development Publication 3400 Forest Health Protection 0834-2828-MTDC.
• NRCS. 1998. Soil Quality Test Kit Guide.
• Olson, Nicholas Charles. 2010. Quantifying the Effectiveness of Soil Remediation Techniques in Compact Urban Soils. University Of Minnesota Master Of Science Thesis.
• Owen, Gordon T. 1987. Soil disturbance associated with deep subsoiling in compact soils. Canadian Agricultural Engineering: 33-37.
• Pennsylvania department of Environmental Protection. 2006. Pennsylvania Stormwater Best Management Practices Manual. BMP 6.7.3: Soil Amendment & Restoration. 2006.
• Pitt, R., Chen, S., Clark, S.E., Swenson, J., and Ong, C.K. 2008. Compaction’s impact on urban storm-water infiltration. Journal of Irrigation and Drainage Engineering. 134(5):652-658.
• Schueler, T. 2000. The Compaction of Urban Soil: The Practice of Watershed Protection. Center for Watershed Protection, Ellicott City, MD. pp. 210-214.
• Schueler, T. R. Can Urban Soil Compaction Be Reversed? Technical Note #108 from Watershed Protection Techniques. 1(4): 666-669.
• Selbig, W.R., and N. Balster. 2010. Evaluation of turf-grass and prairie-vegetated rain gardens in a clay and sand soil: Madison, Wisconsin, water years 2004–08. U.S. Geological Survey, Scientific Investigation Report 2010–5077, 75 p.
• Spoor, G. & Godwin, R.J. 1978. An experimental investigation into the deep loosening of soil by rigid tines. Journal of Agricultural Engineering Research, 23, 243–258.
• Spoor, G., Tijink, F.G.J. & Weisskopf, P. 2003. Subsoil compaction: risk, avoidance, identification and alleviation. Soil and Tillage Research,73, 175–182.
• Spoor, G. 2006. Alleviation of Soil Compaction: Requirements, Equipment and Techniques. Soil Use and Management 22:113-122.

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