This page provides a discussion of design elements and design steps for infiltration practices. These practices include infiltration trench, infiltration basin, dry wells, and underground infiltration practices, although many of the design guidelines can be applied to other infiltration practices.
The following terminology is used throughout this design page.
HIGHLY RECOMMENDED - Indicates design guidance that is extremely beneficial or necessary for proper functioning of the infiltration practice, but not specifically required by the MPCA CGP.
RECOMMENDED - Indicates design guidance that is helpful for infiltration practice performance but not critical to the design.
Before deciding to use an infiltration 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. This section describes considerations in making an initial judgment as to whether or not an infiltration practice is the appropriate BMP for the site. The following links provide additional information on specific constraints to infiltration.
It is HIGHLY RECOMMENDED that the following infiltration practices be designed with the indicated maximum drainage areas. See the table below for recommended contributing drainage areas for all infiltration BMPs.
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
Unless slope stability calculations demonstrate otherwise, it is HIGHLY RECOMMENDED that infiltration practices be located a minimum horizontal distance of 200 feet from down-gradient slopes greater than 20 percent, and that slopes in contributing drainage areas be limited to 15 percent.
It is HIGHLY RECOMMENDED that native soils in proposed infiltration areas have a minimum infiltration rate of 0.2 inches per hour (typically Hydrologic Soil Group A, B and C soils). Initially, soil infiltration rates can be estimated from NRCS soil data, and confirmed with an on-site infiltration evaluation or geotechnical investigation (see Step 5 of the Design procedures section for investigation procedures). It is HIGHLY RECOMMENDED that native soils have silt/clay contents less than 40 percent and clay content less than 20 percent, and that infiltration practices not be situated in fill soils.
Note that if underlying soils are ripped to alleviate compaction, the requirement is a 2 foot minimum between the bottom of the ripped zone and a 3 foot minimum from the bottom of the infiltration practice. If there is only a 3 foot separation distance between the bottom of the infiltration practice and the elevation of the seasonally high water table or bedrock, limit ripping depth to 12 inches. See the alleviating compaction and soil ripping webpages for more detail on compaction prevention.
The following table summarizes horizontal and vertical setback distances for required and recommended minimum distances from an infiltration practice to an above-ground or underground structure. It will be necessary to consult local ordinances for further guidance on siting infiltration practices.
Required and recommended minimum vertical and horizontal separation distances. This represents the minimum distance from the infiltration practice to the structure of concern. If the structure is above-ground, the distance is measured from the edge of the BMP to the structure. If the structure is underground, the vertical separation distance represents the distance from the point of infiltration through the bottom of the system to the structure, while the horizontal separation (often called setback) distance is the shortest distance from the edge of the system to the structure.
Link to this table
Structure | Distance (feet) | Requirement or recommendation | Note(s) | |
---|---|---|---|---|
Vertical | Saturated soil | 3 | Requirement1 | |
Bedrock | 3 | Requirement1 | ||
Horizontal | Public supply well | 100 for sensitive wells; 50 for others3 | Requirement | |
Building/structure/property line2 | 10 | Recommended | ||
Surface water | none unless local requirements exist | If nearby stream is impaired for chloride, see [1] | ||
Septic system | 35 | Recommended | ||
Contaminated soil/groundwater | No specific distance. Infiltration must not mobilize contaminants. | |||
Slope | 200 | Recommended | from toe of slope >= 20% | |
Karst | 1000 up-gradient 100 down-gradient | Requirement1 | active karst |
1 Required under the Construction Stormwater General Permit
2 Minimum with slopes directed away from the building
3If treating an average of 10,000 gallons per day; otherwise separation distance is 300 feet
It is HIGHLY RECOMMENDED that infiltration practices not be used in active karst formations without adequate geotechnical testing.
See stormwater and wellhead protection for guidance and recommendations for determining the appropriateness of infiltrating stormwater in a Drinking Water Supply Management Area (DWSMA). For more information on source water protection see Minnesota Department of Health.
If physical attributes of a site do not prohibit infiltration, there are several considerations for the infiltration practice and site.
It is HIGHLY RECOMMENDED that a flow splitter or diversion structure be provided to divert the Vwq to the infiltration practice and allow larger flows to bypass the practice, unless the infiltration practice is sized to retain Vcp, Vp10 or Vp100. Where a flow splitter is not used, it is HIGHLY RECOMMENDED that contributing drainage areas be limited to the appropriate size given the BMP and an overflow be provided within the practice to pass part of the Vwq to a stabilized watercourse or storm drain. It is also HIGHLY RECOMMENDED that overflow associated with the Vp10 or Vp100 storm (depending on local drainage criteria) be controlled such that velocities are non-erosive at the outlet point (to prevent downstream slope erosion), and that when discharge flows exceed 3 cubic feet per second, the designer evaluate the potential for erosion to stabilized areas and infiltration facilities.
An infiltration device can be designed to accommodate a concentrated influent flow; however, an energy dissipater and/or level spreader may be needed. See the pretreatment section for more information on pretreatment devices.
Perforated elevated underdrains are sometimes used to facilitate infiltration. If an infiltration system does not have an underdrain, it should be designed with dewatering provisions in the event of failure. This can be done with underdrain pipe systems that can be pumped out or allowed to gravity drain to the surface.
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
It is HIGHLY RECOMMENDED that the following pretreatment sizing guidelines be followed:
It is HIGHLY RECOMMENDED that pretreatment practices be designed such that exit velocities from the pretreatment systems are non-erosive (less than 3 feet per second) and flows are evenly distributed across the width of the practice (e.g., by using a level spreader).
For additional information, see the pretreatment section in the manual.
Space varies depending on the depth of the practice. Typically, infiltration trenches are 3 to 12 feet deep with a width less than 25 feet. A dry well is essentially a smaller version of an infiltration trench, consistent with the fact that the drainage area to an infiltration trench is typically five times greater (or larger) than that of a dry well. Underground infiltration systems are larger practices that range in depth from approximately 2 to 12 feet. Permeable Pavement systems have an average depth of 2 to 5 feet depending on the pavement and media thickness. The surface area of all infiltration practices is a function of MPCA’s 48-hour drawdown requirement and the infiltration capacity of the underlying soils.
The maximum storage volume statically stored within the infiltration practice must completely drawdown within 48 hours. An emergency spillway and/or backup underdrain should be constructed if the infiltration device is unable to dewater within 48 hours.
It is RECOMMENDED that the bottom of all infiltration practices be flat, in order to enable even distribution and infiltration of stormwater. It is RECOMMENDED that the longitudinal slope range only from the ideal 0 percent up to 1 percent, and that lateral slopes be held at 0 percent.
It is HIGHLY RECOMMENDED that the maximum side slopes for an infiltration practice is 3:1 (h:v).
The depth of an infiltration practice is a function of the maximum drawdown time and the design infiltration rate. When the drawdown time for an infiltration system is 48 hours, the total drawdown depth is 78.2 inches for GW and GP Hydrologic Soil Group (HSG) A soils, 38.4 inches for GM and SW (HSG A) soils; 21.6 inches for SM (HSG B) soils; 14.4 inches for loam, silt loam and MH (HSG B) soils; and 9.6 inches for HSG C soils. If field tested rates for any soil exceeds the rate for A soils in the manual (1.63 inches per hour), the total drawdown depth (water quality volume depth + bounce) must not exceed 6.5 feet. When the drawdown time is 24 hours, the above water quality volume depths are reduced by a factor of 2. Any captured depth (bounce) beyond the water quality volume needs to be removed from the BMP within 48 hours via an emergency spillway designed to overflow at the top of the storage volume and/or a control structure used to limit peak discharge rates.
Infiltration basins can be effectively integrated into the site planning process, and aesthetically designed as attractive green spaces planted with native vegetation. If vegetation is used, the infiltration practice becomes a bioinfiltration practice. See the Design Criteria for bioinfiltration practices webpage for more information. Infiltration trenches are less conducive to site aesthetics, but the surface of trenches can be designed with turf cover crops if desired.
Concerning infiltration practices with exposed filter media, keep adjacent vegetation from forming an overhead canopy above infiltration practices, in order to keep leaf litter, fruits, and other vegetative materials from clogging the filter media.
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.
Dry wells, infiltration trenches and subsurface infiltration systems do not pose any major safety hazards. Infiltration basins should have similar side slope considerations as ponds and wetlands.
Additional information on safety for construction sites is available from OSHA.
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.
Specific specifications for infiltration basin, infiltration trench, dry well, and underground infiltration practices are provided below.
The following table shows a comparison of different material specifications for infiltration practices.
Infiltration material specifications
Link to this table. To open this table in Excel format, link here.
Component | Infiltration basin | Bioinfiltration basin | Infiltration trench | Underground infiltration | Dry well | Dry swale with check dams | Permeable pavement | Tree trench/box/planter |
---|---|---|---|---|---|---|---|---|
Observation well |
|
|
|
|||||
Emergency overflow structure |
|
NA | A surcharge pipe should be constructed as an emergency overflow device for Dry Wells which receive runoff directly from a roof leader. [2] | NA | See Design criteria for permeable pavement | Tree box filters should be designed with an emergency overflow pipe | ||
High flow bypass structure |
|
|
NA | NA | NA | NA | ||
Buffer vegetation |
|
|
Keep adjacent vegetation from forming an overhead canopy above infiltration practices. This keeps leaf litter, fruits, and other vegetative material from clogging stone. |
|
NA |
|
||
Surface cover | For stone cover: apply a two inch layer of pea gravel or river stone |
|
3-inch layer of river stone or pea gravel with filter fabric and additional aggregate on top |
|
Covered by a minimum of 12 inches of topsoil |
|
|
|
Intermediate layer | NA | NA | NA | NA | NA | NA | Choker Layer: A 2 to 8 inch bedding coarse of Medium Filter Aggregate (MNDOT Section 3149 J.1) is usually placed over the top of the base material to help stabilize the irregular surface | NA |
Filter bed (stone) |
|
Bioinfiltration engineered soil mix |
|
|
20-30 inch layer of permeable manufactured soil mixture meeting the bioinfiltration soil mix |
|
3' of bioinfiltration Soil Media | |
Filter fabric |
|
|
|
|
|
|
|
|
Bottom |
|
|
|
|
Bottom Layer: 4" min depth of .75" crushed stone MNDOT Coarse Aggregate Bedding (Section 3149 G.2) | 2' layer of clean, washed angular gravel 0.75 to 1.5 inch diameter | ||
in-situ soils | NRCS Type A and B soils are the most efficient soils for proper infiltration. Type C soils are acceptable of the infiltration practice meets the required 48 hour drawdown period | |||||||
Miscellaneous | All material must be placed such that compaction is avoided. See the Construction Specifications webpage for more details | To increase the runoff capture storage volume of trenches, plastic, aluminum or concrete gallery frames can be inserted All material must be placed such that compaction is avoided. See the Construction Specifications webpage for more details |
|
1Thickness will vary depending on traffic conditions.Typically, thicker configurations are needed for heavier traffic loads.
Infiltration practices do not typically use engineered filter media. However, when designing a bioinfiltration basin, all media should meet the specifications listed in the design criteria for bioretention basin webpage. Links are provided below.
The following steps outline a recommended design procedure for infiltration practices in compliance with the MPCA Permit for new construction. Design recommendations beyond those specifically required by the permit are also included and marked accordingly.
When riser pipe outlets are used in infiltration basins, it is HIGHLY RECOMMENDED that they be constructed with manholes that either have locks or are sufficiently heavy to prevent easy removal.
Fencing of dry wells and infiltration trenches is neither necessary nor desirable. Infiltration basins may warrant fencing in some situations.
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 |
---|---|---|---|
< 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 infiltration practice.
Once the physical suitability evaluation is complete (Step 3), it is HIGHLY RECOMMENDED that the designer apply the better site design principles in sizing and locating the infiltration practice(s) on the development site.
After following the steps outlined above, the designer will presumably know the location of naturally occurring permeable soils, the depth to the water table, bedrock or other impermeable layer, and the contributing drainage area. Given the steps performed in the physical suitability evaluation (Step 3), identify the most suitable location for the infiltration practice. Given the water quality volume and the drainage area, select the appropriate infiltration practice for the first iteration of the design process. See the section on BMPs for stormwater infiltration for more information.
Note: Information collected during the site suitability evaluation (see Steps 1 and 3) should be used to explore the potential for multiple infiltration practices versus relying on a single infiltration facility. The use of smaller infiltration practices dispersed around a development is usually more sustainable than a single regional facility that is more likely to have maintenance and groundwater mounding problems (Source: Site Evaluation for Stormwater Infiltration, Wisconsin Department of Natural Resources Conservation Practice Standards, 2004). See Using the treatment train approach to BMP selection for more information on selecting multiple BMPs at a site.
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. For information on conducting soil borings see Understanding and interpreting soils and soil boring reports for infiltration BMPs.
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 an infiltration 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 infiltration practice.
For a 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.
Given the assumed infiltration rate for the practice, determine the maximum depth using the following equation
\(D = I_R DDT_{calc}\)
where
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. Note the numbers in the table are intentionally conservative based on experience gained from Minnesota infiltration sites. Two example calculations are provided below.
Assume a 5 acre watershed is 20 percent impervious. Runoff from this watershed will be routed to an infiltration practice that has an underlying loam soil.
The dimensions of the infiltration practice can be determined to accommodate this area. For example, a square practice will be 55 feet wide by 55 feet long.
Assume a 7 acre watershed is 15 percent impervious. Runoff from this watershed will be routed to an infiltration practice where the underlying soil has a field-measured infiltration rate of 2 inches per hour.
Note:
The dimensions of the infiltration practice can be determined to accommodate this volume. For example, a square practice will be 30.9 feet wide by 30.9 feet long.
If the infiltration practice does not require meeting the Construction Stormwater General Permit, methods other than the instantaneous volume method may be used. For example, as an infiltration basin fills during a rain event, water infiltrates the media. The infiltration 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.
Infiltration 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.
It is HIGHLY RECOMMENDED that the outlet for the infiltration practice shall safely convey stormwater using all of the following mechanisms (Stormwater post-construction technical standards, Wisconsin Department of Natural Resources Conservation Practice Standard).
Groundwater mounding, the process by which a mound of water forms on the water table as a result of recharge at the surface, can be a limiting factor in the design and performance of infiltration practices. A groundwater mounding analysis is RECOMMENDED to verify separation distances required for infiltration practices. For more information on groundwater mounding, see the following sections in this manual.
See the section on pretreatment for specific pretreatment design guidance
Follow the design procedures identified in the unified sizing criteria section of the Manual to determine the volume control and peak discharge requirements for water quality, recharge, channel protection, overbank flood and extreme storm.
Perform hand calculations or model the proposed development scenario using a surface water model appropriate for the hydrologic and hydraulic design considerations specific to the site (see also the section on stormwater modeling). This includes defining the parameters of the infiltration practice defined above: elevation and area (defines the storage volume), infiltration rate and method of application (effective infiltration area), and outlet structure and/or flow diversion information. The results of this analysis can be used to determine whether or not the proposed design meets the applicable requirements. If not, the design will have to be re-evaluated (back to Step 5).
The following items are specifically REQUIRED by the MPCA Permit
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.
Other design requirements may apply to a particular site. The applicant should confirm local design criteria and applicability (see Step 2).
A landscaping plan for an infiltration trench should be prepared to indicate how the enhanced swale system will be stabilized and established with vegetation. Landscape design should specify proper grass species and wetland plants based on specific site, soils and hydric conditions present along the channel. Further information on plant selection and use occurs in the Minnesota plant lists section.
See Operation and Maintenance section 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.