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==Major design elements== | ==Major design elements== | ||
===Physical feasibility initial check=== | ===Physical feasibility initial check=== | ||
− | Before deciding to use a | + | Before deciding to use a <span title="Bioretention is a terrestrial-based (up-land as opposed to wetland) water quality and water quantity control process. Bioretention employs a simplistic, site-integrated design that provides opportunity for runoff infiltration, filtration, storage, and water uptake by vegetation. Bioretention areas are suitable stormwater treatment practices for all land uses, as long as the contributing drainage area is appropriate for the size of the facility. Common bioretention opportunities include landscaping islands, cul-de-sacs, parking lot margins, commercial setbacks, open space, rooftop drainage and street-scapes (i.e., between the curb and sidewalk). Bioretention, when designed with an underdrain and liner, is also a good design option for treating Potential stormwater hotspots. Bioretention is extremely versatile because of its ability to be incorporated into landscaped areas. The versatility of the practice also allows for bioretention areas to be frequently employed as stormwater retrofits."> '''bioretention practice'''</span> 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. |
− | <span title="Bioretention is a terrestrial-based (up-land as opposed to wetland) water quality and water quantity control process. Bioretention employs a simplistic, site-integrated design that provides opportunity for runoff infiltration, filtration, storage, and water uptake by vegetation. Bioretention areas are suitable stormwater treatment practices for all land uses, as long as the contributing drainage area is appropriate for the size of the facility. Common bioretention opportunities include landscaping islands, cul-de-sacs, parking lot margins, commercial setbacks, open space, rooftop drainage and street-scapes (i.e., between the curb and sidewalk). Bioretention, when designed with an underdrain and liner, is also a good design option for treating Potential stormwater hotspots. Bioretention is extremely versatile because of its ability to be incorporated into landscaped areas. The versatility of the practice also allows for bioretention areas to be frequently employed as stormwater retrofits."> '''bioretention practice'''</span> 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. | ||
− | *<span title="Contributing area is defined as the total area, including pervious and impervious surfaces, contributing to a BMP. It is assumed that in most cases, with the exception of green roofs and many permeable pavement systems, impervious surfaces will constitute more than 50 percent of the contributing area to the BMP and that most of this impervious is directly connected."> '''Drainage Area'''</span>: The ''RECOMMENDED'' maximum drainage area is typically 5 acres, but can be greater if the discharge to the basin has received adequate | + | *<span title="Contributing area is defined as the total area, including pervious and impervious surfaces, contributing to a BMP. It is assumed that in most cases, with the exception of green roofs and many permeable pavement systems, impervious surfaces will constitute more than 50 percent of the contributing area to the BMP and that most of this impervious is directly connected."> '''Drainage Area'''</span>: The ''RECOMMENDED'' maximum drainage area is typically 5 acres, but can be greater if the discharge to the basin has received adequate <span title="Pretreatment reduces maintenance and prolongs the lifespan of structural stormwater BMPs by removing trash, debris, organic materials, coarse sediments, and associated pollutants prior to entering structural stormwater BMPs. Implementing pretreatment devices also improves aesthetics by capturing debris in focused or hidden areas. Pretreatment practices include settling devices, screens, and pretreatment vegetated filter strips."> [https://stormwater.pca.state.mn.us/index.php?title=Pretreatment '''pretreatment''']</span> and the basin is properly designed, [[Construction specifications for bioretention|constructed]], and [[Operation and maintenance of bioretention|maintained]]. For larger sites, multiple bioretention areas can be used to treat site runoff provided appropriate grading is present to convey flows. |
− | <span title="Pretreatment reduces maintenance and prolongs the lifespan of structural stormwater BMPs by removing trash, debris, organic materials, coarse sediments, and associated pollutants prior to entering structural stormwater BMPs. Implementing pretreatment devices also improves aesthetics by capturing debris in focused or hidden areas. Pretreatment practices include settling devices, screens, and pretreatment vegetated filter strips."> [https://stormwater.pca.state.mn.us/index.php?title=Pretreatment '''pretreatment''']</span> and the basin is properly designed, [[Construction specifications for bioretention|constructed]], and [[Operation and maintenance of bioretention|maintained]]. For larger sites, multiple bioretention areas can be used to treat site runoff provided appropriate grading is present to convey flows. | ||
For more information on contributing area, see | For more information on contributing area, see |
The following terminology is used throughout this "Design Section":
HIGHLY RECOMMENDED - Indicates design guidance that is extremely beneficial or necessary for proper functioning of the bioretention practice, but not specifically required by the MPCA CGP.
RECOMMENDED - Indicates design guidance that is helpful for bioretention practice performance but not critical to the design.
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.
For more information on contributing area, see Contributing drainage area to stormwater BMPs.
It is HIGHLY RECOMMENDED that bioinfiltration practices not be hydraulically connected to structure foundations or pavement to avoid seepage and frost heave concerns, respectively. If groundwater contamination is a concern, it is RECOMMENDED that groundwater mapping be conducted to determine possible connections to adjacent groundwater wells. The table below provides the minimum recommended setbacks for the design and location of bioretention practices.
Recommended minimum setback requirements. This represents the minimum distance from the infiltration practice to the structure of concern. If the structure is aboveground, the distance is measured from the edge of the permeable pavement to the structure. If the structure is underground, the setback distance represents the distance from the point of infiltration through the bottom of the permeable pavement system to the structure.
Link to this table
Setback from | Minimum Distance [feet] |
---|---|
Property Line | 10 |
Building Foundation* | 10 |
Private Well | 50 |
Septic System Tank/Leach Field | 35 |
* Minimum with slopes directed away from the building. |
It is Highly Recommended that the designer provides non-erosive flow velocities at the outlet point to reduce downstream erosion. During the 10-year or 25-year storm (depending on local drainage criteria), discharge velocity should be kept below 4 feet per second for established grassed channels. Erosion control matting or rock should be specified if higher velocities are expected.
Common overflow systems within the structure consist of a yard drain inlet, where the top of the yard drain inlet is placed at the elevation of the shallow ponding area. A stone drop of about 12 inches or small stilling basin could be provided at the inlet of bioretention areas where flow enters the practice through curb cuts or other concentrated flow inlets. In cases with significant drop in grade this erosion protection should be extended to the bottom of the facility.
The following are RECOMMENDED for infiltration practices with underdrains.
The procedure to size underdrains is typically determined by the project engineer. An example for sizing underdrains is found in the North Carolina Department of Environment and Natural Resources Stormwater BMP Manual. Underdrain spacing can be calculated using the following spreadsheet, which utilizes the vanSchilfgaarde Equation. The spreadsheet includes an example calculation. File:Underdrain spacing calculation.xlsx
Pretreatment refers to features of a bioretention area that capture and remove coarse sediment particles.
For applications where runoff enters the bioretention system through sheet flow, such as from parking lots, or residential back yards, a grass filter strip with a pea gravel diaphragm is the preferred pre-treatment method. The width of the filter strip depends on the drainage area, imperviousness and the filter strip slope. The minimum RECOMMENDED vegetated filter strip width is 3 feet. The width should increase with increasing slope of the filter strip. Slopes should not exceed 8 percent. Pretreatment filter strips greater than 15 feet in width will provide diminishing marginal utility on the installation cost.
For retrofit projects and sites with tight green space constraints, it may not be possible to include a grass buffer strip. For example, parking lot island retrofits may not have adequate space to provide a grass buffer. For applications where concentrated (or channelized) runoff enters the bioretention system, such as through a slotted curb opening, a grassed channel with a pea gravel diaphragm is the preferred pre-treatment method.
The bioretention practice should be inspected semi-annually to determine if accumulated sediment needs to be removed. Accumulated sediment should be removed from the gravel verge (if applicable) and vegetated filter strip as needed. If the watershed runoff is especially dirty, this frequency may need to be monthly or quarterly. Trash removal should occur in conjunction with removal of debris from the bioretention cell. During maintenance, check for erosion in the filter strip. If it is visible, it should be repaired with topsoil and re-planted. Vegetation of the filter strip should be designed at least 2 inches below the contributing impervious surface. If, over time, the grade of the vegetated filter strip rises above the adjacent impervious surface draining into it, the grade of the vegetated filter strip needs to be lowered to ensure proper drainage.
The type of vegetation in the bioretention cell determines the appropriate flow velocity for which the pre-treatment device should be designed. For tree-shrub-mulch bioretention cells, velocity through the pre-treatment device should not exceed 1 foot per second, which is the velocity that causes incipient motion of mulch. For grassed bioretention cells, flow velocity through the pre-treatment device should not exceed 3 feet per second. In all cases, appropriate maintenance access should be provided to pre-treatment devices.
In lieu of grass buffer strips, pre-treatment may be accomplished by other methods such as sediment capture in the curb-line entrance areas. Additionally, the parking lot spaces may be used for a temporary storage and pre-treatment area in lieu of a grass buffer strip. If bioretention is used to treat runoff from a parking lot or roadway that is frequently sanded during snow events, there is a high potential for clogging from sand in runoff. Local requirements may allow a street sweeping program as an acceptable pre-treatment practice. It is HIGHLY RECOMMENDED that pre-treatment incorporate as many of the following as are feasible:
The following guidelines are applicable to the actual treatment area of a bioretention practice:
When the drawdown time for a bioinfiltration system is 48 hours, the maximum ponding depth is
If field tested rates for any soil exceed the rate for A soils in the manual (1.63 inches per hour), the maximum ponding depth is 18 inches. When the drawdown time is 24 hours, the above maximum ponding depths are reduced by a factor of 2.
The Construction Stormwater General Permit requires that on-site soil testing be consistent with the Minnesota Stormwater Manual. If the permit requirement is not applicable and the recommended number of soil tests have not been taken within the boundary of the SCM, it is Highly Recommended the maximum ponding depth be 6 inches. Drawdown time is the time from the high water level in the practice to 1 to 2 inches above the bottom of the facility at the lowest part of the bioretention system. It is RECOMMENDED that the elevation difference from the inflow to the outflow be approximately 4 to 6 feet when an underdrain is used.
It is HIGHLY RECOMMENDED that the soil permeability rate be determined by field testing.
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.
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 (Af) should be sized based on the principles discussed below.
Landscaping is critical to the performance and function of vegetated areas of infiltration practices. Therefore, a landscaping plan is HIGHLY RECOMMENDED for vegetated infiltration practices. RECOMMENDED planting guidelines for vegetated practices are as follows.
Operation and maintenance of vegetated practices is critical to meeting these landscape recommendations and targets. For more information on operation and maintenance, see the section on operation and maintenance of stormwater infiltration practices.
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.
Flow path length is important only if high flows are not bypassed. Below are recommendations from other states or localities.
In comparison to multiple cells, one large bioretention or infiltration cell will often perform just as well as multiple smaller cells if sized and designed appropriately. One large cell is generally less costly than multiple smaller cells. This is due to the simpler geometry and grading requirements of one large cell, as well as a reduction in piping and outlet structures. Multiple smaller cells do however provide greater redundancy, i.e. if one large cell fails, more function is lost than if just one of multiple cells fail. Multiple cells are also more feasible than one large cell in steep terrain (slopes greater than 5 percent), where they can be terraced to match the existing grade. Provided access is maintained to each cell, multiple cells typically results in less and easier maintenance.
Considering management of snow, the following are recommended.
Research has shown that minimum bioretention soil media depth needed varies depending on the target pollutant(s). The table below summarizes the relationship between media depth and pollutant attenuation. In general, the Recommended filter media depth is 2.5 feet or more to allow adequate filtration processes to occur.
Minimum bioretention soil media depths recommended to target specific stormwater pollutants. From Hunt et al. (2012) and Hathaway et al., (2011). NOTE: The Construction Stormwater permit requires a 3 foot separation from the bottom of an infiltration practice and bedrock or seasonally saturated soils.
Link to this table
Pollutant | Depth of Treatment with upturned elbow or elevated underdrain | Depth of Treatment without underdrain or with underdrain at bottom | Minimum depth |
---|---|---|---|
Total suspended solids (TSS) | Top 2 to 3 inches of bioretention soil media | Top 2 to 3 inches of bioretention soil media | Not applicable for TSS because minimum depth needed for plant survival and growth is greater than minimum depth needed for TSS reduction |
Metals | Top 8 inches of bioretention soil media | Top 8 inches of bioretention soil media | Not applicable for metals because minimum depth needed for plant survival and growth is greater than minimum depth needed for metals reduction |
Hydrocarbons | 3 to 4 inch Mulch layer, top 1 inch of bioretention soil media | 3 to 4 inches Mulch layer, top 1 inch of bioretention soil media | Not applicable for hydrocarbons because minimum depth needed for plant survival and growth is greater than minimum depth needed for hydrocarbons reduction |
Nitrogen | From top to bottom of bioretention soil media; Internal Water Storage Zone (IWS) improves exfiltration, thereby reducing pollutant load to the receiving stream, and also improves nitrogen removal because the longer retention time allows denitrification to occur underanoxic conditions. | From top to bottom of bioretention soil media | Retention time is important, so deeper media is preferred (3 foot minimum) |
Particulate phosphorus | Top 2 to 3 inches of bioretention soil media. | Top 2 to 3 inches of bioretention soil media. | Not applicable for particulate phosphorus because minimum depth needed for plant survival and growth is greater than minimum depth needed for particulate phosphorus reduction |
Dissolved phosphorus | From top of media to top of submerged zone. Saturated conditions cause P to not be effectively stored in submerged zone. | From top to bottom of bioretention soil media | Minimum 2 feet, but 3 feet recommended as a conservative value; if IWS is included, keep top of submerged zone at least 1.5 to 2 feet from surface of media |
Pathogens | From top of soil to top of submerged zone. | From top to bottom of bioretention soil media | Minimum 2 feet; if IWS is included, keep top of submerged zone at least 2 feet from surface of media |
Temperature | From top to bottom of bioretention soil media; Internal Water Storage Zone (IWS) improves exfiltration, thereby reducing volume of warm runoff discharged to the receiving stream, and also improves thermal pollution abatement because the longer retention time allows runoff to cool more before discharge. | From top to bottom of bioretention soil media | Minimum 3 feet, with 4 feet preferred |
The following performance specifications are applicable to all bioretention media.
The following additional bioretention growing media performance specifications are required to receive P reduction credit.
In general, Bioretention Mixes A and B will not be suitable for achieving reductions in phosphorus loading for bioretention systems having an underdrain unless an amendment is added to the bioretention soil. For guidance on adding an amendment to a bioretention soil, see Soil amendments to enhance phosphorus sorption.
Biofiltration practices (bioretention systems with an underdrain) return treated water to the stormwater discharge system. Bioretention media with high concentrations of organic matter can export soluble phosphorus in higher concentrations than the incoming stormwater runoff, thus contributing to increased phosphorus loading to receiving waters. The International Stormwater BMP Database (2016), for example, shows statistically higher concentrations of dissolved phosphorus in effluent from bioretention systems compared to influent.
If biofiltration practices are implemented to reduce phosphorus loads to receiving waters, we recommend implementing one of the following recommendations.
A well blended, homogenous mixture of
It is assumed this mix will leach phosphorus. When an underdrain is utilized a soil phosphorus test is needed to receive water quality credits for the portion of stormwater captured by the underdrain. The phosphorus index (P-index) for the soil must be low, between 10 and 30 milligrams per kilogram when using the Mehlich-3 (or equivalent) test. This is enough phosphorus to support plant growth without exporting phosphorus from the cell.
A well-blended, homogenous mixture of
It is assumed this mix will leach phosphorus. When an underdrain is utilized a soil phosphorus test is needed to receive water quality credits for the portion of stormwater captured by the underdrain. The phosphorus index (P-index) for the soil must be low, between 10 and 30 milligrams per kilogram when using the Mehlich-3 (or equivalent) test. This is enough phosphorus to support plant growth without exporting phosphorus from the cell.
Source: North Carolina Department of Environment and Natural Resources, 2009. See Section 12.3.4
This mix is a homogenous soil mix of
A higher concentration of fines (12 percent) should be reserved for areas where nitrogen is the target pollutant. In areas where phosphorus is the target pollutant, a lower concentration of fines (8 percent) should be used. A soil phosphorus test using the Mehlich-3 (or equivalent) method is recommended but not required to receive water quality credits. The phosphorus index (P-index) for the soil must be low, between 10 and 30 milligrams per kilogram. This is enough phosphorus to support plant growth without exporting phosphorus from the cell. It is assumed this mix will not exceed the upper range of recommended values (30 milligrams per kilogram), although at lower concentrations of organic matter a soil test may be needed to confirm there is adequate phosphorus for plant growth.
Bioretention Soil Mix D soil shall be a mixture of coarse sand, compost and topsoil in proportions which meet the following:
Suggested mix ratio ranges, by volume, are
Note that the above mix ratios are on a volume basis rather than a weight basis. See specific guidance on these.
A soil phosphorus test using the Mehlich-3 (or equivalent) method is recommended but not required to receive water quality credits. The phosphorus index (P-index) for the soil must be low, between 10 and 30 milligrams per kilogram. This is enough phosphorus to support plant growth without exporting phosphorus from the cell. It is assumed this mix will not exceed the upper range of recommended values (30 milligrams per kilogram), although at lower concentrations of organic matter a soil test may be needed to confirm there is adequate phosphorus for plant growth.
A well-blended, homogenous mixture of
Provide topsoil borrow containing two blended components of sand and compost for water quality, plant growing medium, and filtration medium with a filtration rate of at least 4 inches per hour [10 centimeters per hour].
See page 672 of MnDOT Standard Specifications for Construction
This mix is a homogenous soil mix of
Loamy sand as determined by the USDA soil texture classification based on grain size. Loamy sand is defined as soil material that contains at the upper limit 85 to 90 percent sand, and the percentage of silt plus 1.5 times the percentage of clay is not less than 15. At the lower limit it contains not less than 70 to 85 percent sand, and the percentage of silt plus twice the percentage of clay does not exceed 30. In addition, the maximum particle size shall be less than 1-inch.
Comparison of pros and cons of bioretention soil mixes
Link to this table.
Mix | Composition in original Manual | Proposed updated composition | Pros | Cons |
---|---|---|---|---|
A |
|
|
Likely to sorb more dissolved P and metals than mix B because it contains some fines; best for growth of most plants | Likely to leach P; if topsoil exceeds maximum allowed clay content, higher fines content could result in poor hydraulic performance and long drawdown times |
B |
|
|
Easy to mix; least likely to clog | Likely to leach P, lack of fines in mix results in less dissolved pollutant removal; harder on most plants than mix A because it dries out very quickly |
C | Not in original MN Stormwater Manual |
|
Likely to sorb more dissolved P and metals than mix B because it contains some fines; less likely to leach P than mix B because of low P content | Harder on most plants than mix A because it dries out very quickly. Research in Wisconsin indicates that in cold climates, excess of Na ions can promote displacement of Mg and Ca in the soil, which breaks down soil structure and decreases infiltration rate, and can also cause nutrient imbalances1 |
D | Not in original MN Stormwater Manual |
|
Best for pollutant removal, moisture retention, and growth of most plants; less likely to leach P than mix B because of low P content | Harder to find. Research in Wisconsin indicates that in cold climates, excess of Na ions can promote displacement of Mg and Ca in the soil, which breaks down soil structure and decreases infiltration rate, and can also cause nutrient imbalances |
E | Not in original manual |
|
High infiltration rates, relatively inexpensive | As compost breaks down, nutrients available for plants decreases |
F | Not in original manual |
|
Finer particles in loamy sand holds moisture for better plant growth | Lower infiltration rates, requires careful soil placement to avoid compaction, requires custom mixing |
1This problem can be avoided by minimizing salt use. Sodium absorption ratio (SAR) can be tested; if the SAR becomes too high, additions of gypsum (calcium sulfate) can be added to the soil to free the Na and allow it to be leached from the soil (Pitt et al in press).
2MnDOT Grade 2 compost is recommended.
The Mehlich III phosphorus test is specified throughout the Manual, with the stipulation that other soil P tests may be acceptable. Other common P tests used by soil testing laboratories are the Bray and Olsen tests. These tests are acceptable substitutes for the Mehlich III test, with the exception that the Bray test should not be used in calcareous soils or those with a pH greater than 7.3. If in doubt, ask your soil testing laboratory to recommend the appropriate test.
If the pH and non-calcareous conditions are met, the numerical results of the Bray test can be considered equal to those of the Mehlich III test for the purposes of assessing bioretention mixes and other recommendations or requirements stated in the Manual (e.g., if less than 30 milligrams per kilogram by the Mehlich III test is specified to receive the P credit, then the Bray test result should be less than 30 milligrams per kilogram if the Bray test is substituted). The equivalent Olsen test result is lower, such that if 30 milligrams per kilogram or less by the Mehlich test is specified, then an Olsen test result of 20 milligrams per kilogram or less is necessary to receive the P credit. In general, most guidance interprets Olsen test results at a ratio of approximately 2:3 of those of Bray and Mehlich III, with Bray and Mehlich III being roughly equivalent to each other. For example, a Mehlich III result of 9 milligrams per kilogram would be equivalent to 9 milligrams per kilogram by Bray (as long as pH is less than 7.3) and equivalent to 6 milligrams per kilogram by Olsen.
For more information on the relationships between these P tests, see "Differentiating and Understanding the Mehlich 3, Bray, and Olsen Soil Phosphorus Tests", by Sawyer and Mallarino (1999).
Several other media are currently being tested. A few examples are listed below.
The following mix utilizes peat moss instead of compost.
This mix aims to maximize phosphorus removal in 2 ways:
Several researchers are currently testing layered systems designed to minimize phosphorus in bioretention effluent. The Wisconsin layered system utilizes a 5 inch surface layer containing 20 percent compost, a 10 inch sand layer below the top layer, and a 10 inch lower layer containing 5 percent iron filings. Advantages of this system include
Disadvantages include
Dakota County developed a layered system with compost only in top six inches, 20 percent coir pith, and 5 percent iron filings in the bottom layer (Isensee 2013). Advantages of this mix include:
Disadvantages include:
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.
Make a preliminary judgment as to whether site conditions are appropriate for the use of an infiltration practice, and identify the function of the practice in the overall treatment system.
A. Consider basic issues for initial suitability screening, including:
B. Determine how the infiltration practice will fit into the overall stormwater treatment system.
Stormwater infiltration BMPs - contributing drainage area
Link to this table
Stormwater BMP | Recommended contributing area | Notes |
---|---|---|
Infiltration Basin | 50 acres or less | A natural or constructed impoundment that captures, temporarily stores and infiltrates the design volume of water into the surrounding naturally permeable soil over several days. In the case of a constructed basin, the impoundment is created by excavation or embankment. |
Bioinfiltration Basin | 5 acres or less | Bioinfiltration basins must meet the required 48 hour drawdown time and must be sized in order to allow for adequate maintenance. It is HIGHLY RECOMMENDED that bioinfiltration basins be designed to prevent high levels of bounce as submerging vegetation may inhibit plant growth. A maximum wet storage depth of 1.5 feet is HIGHLY RECOMMENDED. |
Infiltration Trench | 5 acres or less | |
Dry Well Synonym: Infiltration Tube, French Drain, Soak‐Away Pits, Soak Holes | 1 acre or less (rooftop only) | |
Underground Infiltration | 10 acres or less | Though feasible, larger underground infiltration systems may cause groundwater contamination as water is not able to infiltrate through a surface cover. In addition, wind flocculation, UV degradation, and bacterial degradation, which provide additional treatment in surface systems, do not occur in underground systems. Because performance research is lacking for larger features, it is HIGHLY RECOMMENDED that the contributing drainage area to a single device not exceed 10 acres. |
Dry Swale with Check Dams | 5 acres or less | |
Permeable Pavement | It is RECOMMENDED that external contributing drainage area not exceed the surface area of the permeable pavement. It is HIGHLY RECOMMENDED that external contributing drainage area not exceed twice the surface area of the permeable pavement | It is RECOMMENDED that external drainage area be as close to 100% impervious as possible. Field experience has shown that drainage area (pervious or impervious) can contribute particulates to the permeable pavement and lead to clogging. Therefore, sediment source control and/or pretreatment should be used to control sediment run-on to the permeable pavement section. |
Tree Trench/Tree Box | up to 0.25 acres per tree |
References: Virginia, North Carolina, West Virginia, Maine, Lake Tahoe, Connecticut, Massachusetts, New York, Wisconsin, Vermont, New Hampshire, Ontario, Pennsylvania
Determine whether the infiltration practice must comply with the MPCA Construction Stormwater General (CSW) Permit. Check with local officials, Watershed management Organizations (WMOs), and other agencies to determine if there are any additional restrictions and/or surface water or watershed requirements that may apply.
16.10. Permittees must provide at least one soil boring, test pit or infiltrometer test in the location of the infiltration practice for determining infiltration rates.
Designers should evaluate soil properties during preliminary site layout with the intent of installing infiltration practices on soils with the highest infiltration rates ( hydrologic soil group A and B). Preliminary planning for the location of an infiltration device may be completed using a county soil survey or the NRCS Web Soil Survey. These publications provide HSG information for soils across Minnesota. To ensure long-term performance, however, field soil measurements are desired to provide site-specific data.
If the initial evaluation indicates that an infiltration practice would be a good BMP for the site, it is RECOMMENDED that soil borings or pits be dug within the proposed boundary of the infiltration practice to verify soil types and infiltration capacity characteristics and to determine the depth to groundwater and bedrock. Soil borings for building structural analysis are not acceptable. In all design scenarios, a minimum of one soil boring (two are recommended) shall be completed to a depth 5 feet below the bottom of the proposed infiltration Stormwater Control Measure (SCM or BMP) (Dakota County Soil and Water Conservation District, 2012) per ASTM D1586 (ASTM, 2011). For infiltration SCMs with surface area between 1000 and 5000 square feet, two borings shall be made. Between 5000 and 10000 square feet, three borings are needed, and for systems with greater than 10000 square feet in surface area, 4 or more borings are needed. For each additional 2500 square feet beyond 12,500 square feet, an additional soil boring should be made. Soil borings must be undertaken during the design phase (i.e. prior to the commencement of construction) to determine how extensive the soil testing will be during construction. Borings should be completed using continuous split spoon sampling, with blow counts being recorded to determine the level of compaction of the soil. Soil borings are needed to understand soil types, seasonally high groundwater table elevation, depth to karst, and bedrock elevations.
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 |
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< 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 ft2
2an additional five permeameter tests should be completed for each additional 5,000 ft2 above 15,000 ft2
It is HIGHLY RECOMMENDED that soil profile descriptions be recorded and include the following information for each soil horizon or layer (Source: Site Evaluation for Stormwater Infiltration, Wisconsin Department of Natural Resources Conservation Practice Standards 2004):
It is RECOMMENDED that a standard soil boring form be used. A good example is File:Boring Pit Log form.docx. The NRCS Field Book for Describing and Sampling Soils provide detailed information for identifying soil characteristics. Munsell color charts can be found here.
It is HIGHLY RECOMMENDED that the field verification be conducted by a qualified geotechnical professional.
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.
(Note: Steps 5, 6, 7 and 8 are iterative)
After following the steps outlined above, the designer will presumably know the location of naturally occurring permeable soils, the depth to the water table, bedrock or other impermeable 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 4), 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 the table below. Note: the determination for underdrain is an iterative sizing process.
Experience has demonstrated that, although the drawdown period is 48 hours, there is often some residual water pooled in the infiltration practice after 48 hours. This residual water may be associated with reduced head, water gathered in depressions within the practice, water trapped by vegetation, and so on. The drawdown period is therefore 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. This time period has also been called the period of inundation.
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 |
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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 3) 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.
For design purposes, there are two ways of determining the soil infiltration rate. The first, and preferred method, is to field-test the soil infiltration rate using appropriate methods described below. The other method uses the typical infiltration rate of the most restrictive underlying soil (determined during soil borings).
If infiltration rate measurements are made, a minimum of one infiltration test in a soil pit must be completed at the elevation from which exfiltration would occur (i.e. interface of gravel drainage layer and in situ soil). When the SCM surface area is between 1000 and 5000 square feet, two soil pit measurements are needed. Between 5000 and 10000 square feet of surface area, a total of three soil pit infiltration measurements should be made. Each additional 5000 square feet of surface area triggers an additional soil pit.
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 ft2
2an additional five permeameter tests should be completed for each additional 5,000 ft2 above 15,000 ft2
The median measured infiltration rate should be utilized for design. Soil pits should be dug during the design phase and should be a minimum of two feet in diameter for measurement of infiltration rate. Infiltration testing in the soil pit can be completed with a double-ring infiltrometer or by filling the pit with water and measuring stage versus time. If the infiltration rate in the first pit is greater than 2 inches per hour, no additional pits shall be needed.
Alternatively, a Modified Philip-Dunne permeameter can be used to field test infiltration rate. Modified Philip-Dunne permeameter tests may be made in conjunction with soil borings or may be completed using a handheld soil auger. Borings should be lined with a plastic sleeve to prevent infiltration from the sides of the borehole (i.e. restrict flow to vertical infiltration). Soil borings should be filled with water. The time for the borehole to drain should be recorded and divided by the initial ponding depth in the borehole to provide an infiltration rate measurement. The design infiltration rate should be the lower of the median soil pit infiltration rate or the median borehole method infiltration rate.
NOTE: In the table above, the recommended number of permeameter tests increases by 5 tests per each additional 5000 square feet of surface area. For larger sites, this can result in a very large number of samples. There may be situations where fewer permeameter tests may be used (5 is the minimum) . For example, in situations where the variability in saturated hydraulic conductivity between measurements is not great, fewer samples may be taken. One method for determining the number of samples is to plot standard deviation versus number of samples. Measurements may be halted when the standard deviation becomes relatively constant from one sample to the next. In the example to the right the standard deviation flattens at about 7 to 10 samples. Therefore, 7 to 10 samples would be an appropriate number of samples for this situation.
For information on conducting soil infiltration rate measurements, see Determining soil infiltration rates.
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.
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 |
---|---|---|---|---|
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 |
GW - Well-graded gravels, fine to coarse gravel GP - Poorly graded gravel |
||
1.63a | 4.14 |
silty gravels |
GM - Silty gravel |
|
0.8 | 2.03 |
sand |
SP - Poorly graded sand |
|
0.45 | 1.14 | silty sands | SM - Silty sand | |
0.3 | 0.76 | loam, silt loam | MH - Elastic silt | |
0.2 | 0.51 | Sandy clay loam, silts | ML - Silt | |
0.06 | 0.15 |
clay loam |
GC - Clayey gravel |
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
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:
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 (Standard test method for infiltration rate of soils in field using double-ring infiltrometer) should be used for the field test.
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.
To meet requirements of the Stormwater General Permit (CSW permit), the surface area (As, in square feet) of a bioinfiltration practice is given by
\(A_s = V_w / D_o\)
The water treatment volume is given by
\(V_w = 0.0833 A_c\)
The entire water quality treatment volume is assumed to be instantaneously ponded in the bioinfiltration practice.
For a bioretention BMP with sloped sides, the surface area (As) of an infiltration practice is the average area of the BMP, given by
\( A_s = (A_o + A_M)/2 \)
The water treatment volume must drain with 48 hours (24 hours is RECOMMENDED if discharges from the practice are to a trout stream). The ponding depth can therefore be calculated knowing the infiltration rate of the soils underlying the practice. Field-measured infiltration rates are preferred. If the infiltration rate has not been measured, use the table below to determine the infiltration rate of the underlying soils. The ponded depth must not exceed 18 inches (1.5 feet) regardless of the soil infiltration rate. Note the numbers in the table are intentionally conservative based on experience gained from Minnesota infiltration sites. Two example calculations are provided below.
Example 1 Assume a 5 acre watershed is 20 percent impervious. Runoff from this watershed will be routed to a bioinfiltration practice that has an underlying loam soil.
The dimensions of the bioinfiltration practice can be determined to accommodate this area. For example, a square practice will be 55 feet wide by 55 feet long. Note that the depth of 1.2 feet meets the requirement that the ponded depth be 1.5 feet or less.
Example 2 Assume a 7 acre watershed is 15 percent impervious. Runoff from this watershed will be routed to a bioinfiltration practice where the underlying soil has a field-measured infiltration rate of 2 inches per hour.
Note:
The dimensions of the bioinfiltration practice can be determined to accommodate this volume. For example, a square practice will be 50.4 feet wide by 50.4 feet long.
If the bioinfiltration practice does not require meeting the Construction Stormwater General Permit, methods other than the instantaneous volume method may be used. For example, as a bioinfiltration basin fills during a rain event, water infiltrates the media. The bioinfiltration area could be sized as follows
\(A_s = V_{wq} / (D_o + (I_R * t))\)
The time during which runoff continues to be delivered to the BMP varies with each event. As an example, for a 1 hour event on a B (SM) soil with an infiltration rate of 0.45 inches per hour, 1 acre of contributing impervious area, and a 1.5 foot ponding depth, As is 2361 square feet, compared to 2420 square feet considering only an instantaneous volume, or a decrease of 2.4 percent in the size of the basin. On an A soil with and infiltration rate of 1.6 inches per hour, As is 2222 square feet, or a decrease of 8.2 percent in the needed size of the basin. The area of the basin can also be decreased by increasing the ponded depth to greater than the 1.5 foot recommended. However, increased ponding depths increase the inundation time for plants in the bioretention basin.
Bioinfiltration practices may also be sized using different treatment goals. For example, the performance goal for Minimal Impact Design Standards (MIDS) is 1.1 inches, compared to 1 inch for the CSW permit. The MIDS performance goal was also based on initial modeling that included infiltration during the rain event.
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.
(Note: Steps 5, 6, 7 and 8 are iterative)
(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.
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) | ||||||||
Filter Strip Slope | =<2% | >2% | =<2% | 2% | =<2% | 2% | =<2% | 2% |
Filter Strip Minimum Length | 10' | 15' | 20' | 25' | 10' | 12' | 15' | 18' |
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:
(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).
Experience has demonstrated that, although the drawdown period is 48 hours, there is often some residual water pooled in the infiltration practice after 48 hours. This residual water may be associated with reduced head, water gathered in depressions within the practice, water trapped by vegetation, and so on. The drawdown period is therefore 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. This time period has also been called the period of inundation.
See Major Design Elements for guidance on preparing vegetation and landscaping management plan.
See Operations and Maintenance for guidance on preparing an O&M plan.
See Cost Considerations section for guidance on preparing a cost estimate that includes both construction and maintenance costs.