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Between 5000 and 10000 square feet of surface area, a total of three soil pit infiltration measurements should be made. 2023 by Minnesota Pollution Control Agency For this reason homogenous clays are generally unsuited to soakaways as they can have very low infiltration rates. By taking the quotient of initial water depth to drawdown time, an estimation of drawdown rate can be obtained. bThe infiltration rates in this table are recommended values for sizing stormwater practices based on information collected from soil borings or pits. Soil Science Society of America Journal. Where regular maintenance is to be relied on, suitable access must be included in the soakaway detailed design to ensure that the Infiltration Rate of the Soil is appropriate to the conditions. Turf had much lower Ksat values than all other SCMs due to soil compaction. (2011). Ahmed, F., Gulliver, J.S. At the other site, the modified Phillip-Dunne test resulted in an infiltration rate 1.6 inches per hour higher than that from the flood test. Over the past 5 to 7 years, several government agencies revised or developed guidance for designing infiltration practices. Monitoring of soakaway performance can be informative about changes in the soil infiltration rate and in warning of soakaway blockage in the long term. Preliminary planning for the location of an infiltration device may be completed using a county soil survey or the NRCS Web Soil Survey (http://websoilsurvey.nrcs.usda.gov/app/HomePage.htm). Some guides specify a minimum infiltration rate of around 1x10-6 m/s. (2012) attempted to determine a relatively rapid, low cost, low effort approach for determining soil infiltration rate. If infiltration rate is overestimated during the infiltration test, system performance suffers once the SCM is constructed. 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. It is included as a case study here but is not specifically recommended and conflicts with guidance provided below. Identifying agricultural practices that enhance water cycling is critical, particularly with increased rainfall variability and greater risks of droughts and floods. 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. Powered by MediaWiki, Guidance for amending soils with rapid or high infiltration rates, CALIFORNIA DEPARTMENT OF TRANSPORTATION (CALTRANS) UNIFIED SOIL CLASSIFICATION SYSTEM, Stormwater post-construction technical standards, Wisconsin Department of Natural Resources Conservation Practice Standards, SWMM Modeling of a Rural Watershed in the Lower Coastal Plains of the United States, https://stormwater.pca.state.mn.us/index.php?title=Design_infiltration_rates&oldid=64100, Level 3 - General information, reference, tables, images, and archives/Tables/Soil and soil properties, Level 3 - General information, reference, tables, images, and archives/Tables/Hydrology and geology, Level 3 - General information, reference, tables, images, and archives/Tables/Models, modeling, model applications and examples, Although a value of 1.63 inches per hour (4.14 centimeters per hour) may be used, it is, GW - Well-graded gravels, fine to coarse gravel. Recommended number of soil borings, pits or permeameter tests for bioretention design. Soil borings should be filled with water. You will need a water bowser, Record Time & Depth until the hole has emptied, Repeat test three times. (2010). Equations for calculating infiltration rates based on field measurements with each device are also provided. Water Resources Research. In the example to the right the standard deviation flattens at about 7 to 10 samples. Brown, R.A., Hunt, W.F., and Skaggs, R.W. ASTM International. Consequences of underestimation of infiltration rate are typically observed in increased construction costs due to the addition of underdrains, deeper bioretention media depths, and reduced ability to utilize internal water storage (submerged) zones as part of the design (Brown et al., 2011). 1978. Groundwater injection: The other way is to construct recharge wells and inject water directly into an aquifer. Rapid-infiltration pits: One way is to spread water over the land in pits, furrows, or ditches, or to erect small dams in stream channels to detain and deflect surface runoff, thereby allowing it to infiltrate to the aquifer. The primary reason for the secondary, outer ring is to reduce edge effects and produce one-dimensional, vertical flow in the inner ring, where measurements of depth as a function of time are made. Usually, these soakaway drainage pipe and trenches are used to intercept flows. If the infiltration rate in the first pit is greater than 2 inches per hour, no additional pits shall be needed. Additional technical review resulted in an update to the table in 2011. NOTE: this technique utilizes soil pits for a specific method developed in Washington State. Salvucci and Entekhabi, 1994). Soil type is one of several variables that will ensure accurate application of water to foster plant health. SW - Well-graded sand, fine to coarse sand, clay loam NOTE: this technique utilizes soil pits for a specific method developed in Washington State. The infiltration rate is dependent on the physical properties of the soil. 0000010771 00000 n Soil Science Society of America Journal. Because of the potential for 3-dimensional flow in a bore hole percolation test (wherein water is flooded into a bore hole and flow rate measured), this method should not be used when designing a stormwater infiltration device. The values shown in this table are for uncompacted soils. To allow for this a factor of safety is included with the Infiltration Rate of Soils. silty clay loam The device may be either flooded with water from a fire hydrant or other source, or a natural rainfall event which fills the bowl entirely may also be utilized for this purpose. Annual book of ASTM standards, vol. In one rain garden, Asleson (2007) estimated an infiltration rate of 0.72 in/hr from 40 mini-disk measurements. 2014. C372; Rawls, W.J., D. Gimenez, and R. Grossman. Soils with a clay/silt content greater than 40% are likely to be unsuitable. OL - Organic silt This page was last edited on 25 January 2023, at 23:33. As the permeameter drains, measurements of stage and time are taken. The inspection access should provide a clear view to the base of the soakaway, even for filled-type soakaways. There do not appear to be any very large or small values and the geomean is recommended. Asleson, (2007) summarizes pros and cons of five types of infiltration measurement devices (mini-disk infiltrometer, tension infiltrometer, Guelph permeameter, modified Phillip-Dunne permeameter, and double-ring infiltrometer). 0000005436 00000 n Field testing is recommended for gravelly soils (HSG A; GW and GP soils; gravel and sandy gravel soils). Fig 1 - Runoff Coefficients vs land use and . Coefficients of variation were typically greater than 1 and arithmetic means were consistently greater than geometric means, often by a factor of 2 or more. The process is as follow: The soil infiltration rate from the time taken for the water level to fall from 75% to 25% effective storage depth in the soakage trial pit, using the lowest f value of the three test results for design. interface of gravel drainage layer and in situ soil). 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). Depth to groundwater. (2007). ASTM D3385. Link to this table, GM - Silty gravel Trench-type soakaways should have at least two inspection access points, one at each end of a straight trench, with a horizontal perforated or porous distributor pipe linking the ends along the top of the granular fill. 2005. The authors contest that the most accurate measurement of infiltration rate is post-installation flood testing. This page provides a literature review, case studies, and recommendations for field measurement of soil infiltration. Journal of Water Management Modeling. Any fractures or fissures in the rock formations can have a large effect on the infiltration rates achieved. There are two very large values (25.1 and 31.1 inches per hour) so the median is recommended, although the geomean and median are close. sandy clay 43 41 This table can be used as a guide to determine if a soil is compacted. You must not discharge raw sewage. When use as an infiltration system, they trend to be very long or draining small section of impermeable areas. Runoff is likely to be exacerbated by water repellence, as it decreases infiltration rates, enhances overland flow and increases the risk of soil erosion (Doerr et al., 2000). The soakaway problems are in relation to increase sediment within it. no standing water in the bowl) should be recorded. 41(4): 983-988; Saxton, K.E., and W. J. Rawls. The arithmetic mean should not be used to estimate the infiltration rate. A literature review suggests the values in the design infiltration rate table are not appropriate for soils with very high infiltration rates. clay, GC - Clayey gravel Lets understand the legislation, guidance, permits and standards underpinning the soakaway design. 0000010238 00000 n If there are a few extreme values in the data (i.e. no standing water in the bowl) should be recorded. After installing the crates. For more detailed descriptions, see the following links: The Unified Soil Classification System, CALIFORNIA DEPARTMENT OF TRANSPORTATION (CALTRANS) UNIFIED SOIL CLASSIFICATION SYSTEM. As the sediments are likely to be a similar particle size to the silts, the sediment does not have a big impact on the soil infiltration rate. In general, these are the prices within the south east with a medium permeability soil. This would lead to a much lower effective infiltration rate than that expected of a large layer of similar material. These publications provide HSG information for soils across Minnesota. At the other site, the modified Phillip-Dunne test resulted in an infiltration rate 1.6 inches per hour higher than that from the flood test. A literature review suggests the values in the design infiltration rate table are not appropriate for soils with very high infiltration rates. Tests should be run until a constant intake rate (inches per hour) into the soil is achieved. 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. Booltink, and J. Bouma. A transmission zone between the saturated zone and the wetting front conveys water deeper into the soil profile. The PIT test involved digging a large hole (10 ft by 15 ft bottom dimensions) to the proposed infiltrative surface (bottom of infiltration basin or top of in-situ soil for bioretention cell). Use of soil texture, bulk density and slope of the water retention curve to predict saturated hydraulic conductivity Transactions of the ASAE. Soil Infiltration Rates. Ahmed et al. For all sites, the estimated infiltration rates of saturated soils ranged from 9.8 to 115 centime-ters per hour in flatwoods, 3.4 to 66 centimeters per hour in rock, and 2.5 to 55 centimeters per hour in slough. Various devices are available for measurement of infiltration within a proposed SCM, including the single ring infiltrometer, double ring infiltrometer, Philip-Dunne permeameter, Guelph permeameter, and tension infiltrometer. Ideally you prefer a soakaway rate of 1 x 10-5m/s in order to achieve the half emptying time, The water table is 1m below the base of the soakaway, The distance from the soakaway to a building is 5m or more, Within 10m to 15m away of a building if soakaway is within Chalk, In areas where ground stability is a problem, In contaminated soil or near landfills in which gas accumulation can be an issue, In the proximity to drainage fields. Soil type Steady-state infiltration rate (in/hr) Sand >0.8 Sandy and silty soils 0.4-0.8 Loam 0.2-0.4 Depending on the flows the test can take 1 to 3 days, Test to building regulation standards: 1,500.00. For small, filled soakaways, a 225 mm perforated pipe provides a suitable inspection well. 0000104037 00000 n You can use this method only if your house is being approved by NHBC and tests are not required by the Lead Local Planning Authority. Typically, these measurements are made with an infiltrometer a circular device which allows water to be ponded on the soil surface. 14:4:601604; Moynihan, K., and Vasconcelos, J. Additional technical review resulted in an update to the table in 2011. Gulliver, J.S., A.J. This spreadsheet calculates the requirements for a attenuation system and assists the user to design a suitable system. Philips and Kitch (2011) compare in-situ methods of infiltration rate measurement at three sites in southern California. Dakota County Soil and Water Conservation District. Anderson (eds). An infiltration rate of 15 mm/hour means that a water layer of 15 mm on the soil surface, will take one hour to infiltrate. Infiltration rate tends to asymptotically approach the saturated hydraulic conductivity (the maximum water transmission rate of the soil) after long periods of infiltration, though entrapped air in the soil never results in perfectly saturated flow in the vadose zone. So if youre ready to start calculating your soakaway, this chapter is for you. Gravel trenches use a shingle with a perforated drainage pipe at the base. This a detail of a rainwater garden, there are more rainwater garden details on this page from RHS. 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). If a soil is compacted, reduce the soil infiltration rate by one level (e.g. 41(4): 983-988; Saxton, K.E., and W. J. Rawls. SC - Clayey sand A soakaway is a subsurface structure for the temporary storage of water before it soaks in to the ground. Gravel layers are however seldom homogenous and variable particle sizes and sand horizons can significantly reduce the infiltration rate. There may be situations where fewer permeameter tests may be used (5 is the minimum) . 0000005001 00000 n Philips and Kitch (2011) compare in-situ methods of infiltration rate measurement at three sites in southern California. The infiltration rate is the velocity or speed at which water enters into the soil. There may be situations where fewer permeameter tests may be used (5 is the minimum) . The other method uses the typical infiltration rate of the most restrictive underlying soil (determined during soil borings). Various devices are available for measurement of infiltration within a proposed SCM, including the single ring infiltrometer, double ring infiltrometer, Philip-Dunne permeameter, Guelph permeameter, and tension infiltrometer. The wetting front advances downward into the soil as infiltration continues, causing an increase in the moisture content as a function of depth (Hillel, 1982). Download resource These can have a big impact on the infiltration rate but require detailed site investigation to determine and may change significantly over small distances. 7(4), 168-176. The graph in Fig 1, taken from Kaipara District Council's May 2000 "Engineering Code of Practice for Land Subdivision and Development" illustrates the way in which soil Runoff Coefficients may vary according to (i) ground cover and (ii) rainfall intensity (mm/h). CL - Lean clay 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. A detailed description of each technique for infiltration rate measurement, including methods and required post-processing, may be found in Gulliver et al. 0000139869 00000 n prior to the commencement of construction) to determine how extensive the soil testing will be during construction. Hydrologic Soil Groups A and B are more conducive to stormwater infiltration than "C" soils, so care must be exercised when designing stormwater infiltration system in "C" soils. Infiltration is only allowed at the sides, Design is based on a Soakaway tests as per BRE365, A factor of safety should be included as part of the design, Excavate a hole to the base of the potential soakaway, Dig a 300mm x 300mx30mm deep at the base of the excavation, If the above to steps are difficult, use a 300mm earth auger, Fill in the holes with 300mm of water and leave to drain overnight, The following day, repeat the test by infilling the hole up to 300mm height, Bore a hole 150mm dia to 1m below ground level, Pour water to 300mm in depth, measure from the bottom, Take the time that water takes to soakaway, Once the tests are completed then bore the hole 1 m lower (2m bgl), The average figure is used for the design of the soakaway, Excavate a hole 1m x 1m x Depth of potential soakaway, Pour in min 1m3 of water (Instantaneously). In example 2, shown in the upper right, the arithmetic mean is 1.29 inches per hour, the geomean is 1.06, and the median is 1.02. Soil infiltration rates can vary widely over short distances, even in soils that appear to be homogeneous. Soil infiltration rate (IR) is defined as the volume flux of water flowing into the profile per unit of soil surface area under any set of circumstances. Note that filling a pit with water is potentially subject to errors resulting from three-dimensional flow from the pit into the underlying soil. At two of three sites with data reported, the modified Phillip-Dunne test under-predicted the infiltration rate of the flood test by as much as a factor of sixteen. Guidance for the number of permeameter tests required based on bioretention surface area is given in the table below. Powered by MediaWiki, Soil infiltration rate measurement techniques, Infiltration rate measurement case studies, Types of infiltration measurement devices, Soil property determination for infiltration SCM design, Calculating soil infiltration rate and example calculations, University of Minnesota, St. Anthony Falls Laboratory, http://websoilsurvey.nrcs.usda.gov/app/HomePage.htm, Dakota County Soil and Water Conservation District, Guidance for amending soils with rapid or high infiltration rates, CALIFORNIA DEPARTMENT OF TRANSPORTATION (CALTRANS) UNIFIED SOIL CLASSIFICATION SYSTEM, Stormwater post-construction technical standards, Wisconsin Department of Natural Resources Conservation Practice Standards, SWMM Modeling of a Rural Watershed in the Lower Coastal Plains of the United States, Stormwater infiltration Best Management Practices, Understanding and interpreting soils and soil boring reports for infiltration BMPs, A New Technique to Measure Infiltration Rate for Assessing Infiltration of BMPs, Low Impact Development Standards for Dakota County, Minnesota, Assessment of Stormwater Best Management Practices, A review of methods for characterization of site infiltration with design recommendations, https://stormwater.pca.state.mn.us/index.php?title=Determining_soil_infiltration_rates&oldid=65982, Level 2 - Technical and specific topic information/infiltration, Level 2 - Technical and specific topic information/soils and media, Although a value of 1.63 inches per hour (4.14 centimeters per hour) may be used, it is, GW - Well-graded gravels, fine to coarse gravel. They performed infiltration measurements on six rain gardens, three turf areas, three swales, an infiltration basin, and proposed rain garden/infiltration areas. To ensure long-term performance, however, field soil measurements are desired to provide site-specific data. Note that A and B soils have two infiltration rates that are a function of soil texture. The test procedure is outlined here. Booltink, and J. Bouma. 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 bioretention SCM (Dakota County Soil and Water Conservation District, 2012) per ASTM D1586 (ASTM, 2011). bThe infiltration rates in this table are recommended values for sizing stormwater practices based on information collected from soil borings or pits. 0000006955 00000 n The drainage should conform with the below ground drainage building regulations. The designer may use this fact to justify a smaller factor of safety. 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. This suggests that methodologies for soil infiltration rate measurement need to be refined, and we recommend that this be an area that is focused on for future research. Methods typically use either a constant or falling head of water above the infiltrative surface. This spreadsheet uses the Manning formula to calculate the flow conditions in a circular pipe acting under gravity only. OL - Organic silt Infiltration is the process by which water ponded or flowing over a soil surface is absorbed into the soil profile. A group of technical experts developed the table for the original Minnesota Stormwater Manual in 2005. 0000038659 00000 n This is because each test takes longer to complete. tests in a range of soil types indicated as suitable for infiltration drainage.1 The model domain will be expanded to include greater depths to groundwater to allow thresholds to be identified for each soil type. 0000000016 00000 n This spreadsheet calculates the requirements for a attenuation system and assists the user to design a suitable system. 43 0 obj <> endobj Typical infiltration rates for four soil types are listed in Table 1. (view reference list) Now its time to show you how to calculate a basic soakaway. VOL. By taking the quotient of initial water depth to drawdown time, an estimation of drawdown rate can be obtained. Borings should be completed using continuous split spoon sampling, with blow counts being recorded to determine the level of compaction of the soil. For instance, ASTM D3385 (2003) describes field measurement of infiltration rate using the double ring infiltrometer method. Infiltration and Soil Properties Testing Table for Input into the Caltrans . Soil borings for building structural analysis will not be acceptable. Soil profiles should be recorded for each layer encountered in the boring, including information on soil texture, Munsell soil color,redoximorphic features, soil structure, occurrence and abundance of roots, and occurrence of saturated soils or lenses. * Walsh, E., and McDonnell, K.P. However, note that there are four very high infiltration rates in the upper right portion of the schematic, compared to much lower rates in the remainder of the schematic. Properly conducted infiltration tests are preferred to estimates of soil infiltration based on borings. 0000003393 00000 n Link to this table, GM - Silty gravel Annual book of ASTM standards, vol. 2014. The life of a soakaways depends on the amount of silt or sediment that enter it and the maintenance of the structures that protected it. The higher infiltration rate for B soils was decreased from 0.6 inches per hour to 0.45 inches per hour and a value of 0.06 is used for D soils (instead of < 0.2 in/hr). For example, small pockets of sand or gravelly soils will simply become saturated before the soakaway can fully empty. The first, and preferred method, is to field-test the soil infiltration rate using appropriate methods described below. III and Table 7.1 in Vol. The results suggest there may be two distinct infiltration regimes and borings should be conducted to confirm this. (2012). Standard 3: Adoption Design and Construction Guidance. Measurements of infiltrated volume as a function of time are made, with design infiltration rate or intake rate taken as the infiltration rate once the measurement has approached a horizontal asymptote. The most reliable method is the BRE365. In example 3, shown on the lower right, the arithmetic mean is 2.80 inches per hour, the geomean is 0.76, and the median is 0.63. The results for several different soil types are shown in Figures 2, 3, and 4. The key standards for the design of the soakaway under this process are: The first step to design of a soakaway is undertaking a soakaway test. Standard test method for infiltration rate of soils in field using double-ring infiltrometer. Designers should evaluate soil properties during preliminary site layout with the intent of installing bioretention or bioinfiltration practices on soils with the highest infiltration rates (HSG A and B). The primary reason for the secondary, outer ring is to reduce edge effects and produce one-dimensional, vertical flow in the inner ring, where measurements of depth as a function of time are made. 0000007958 00000 n 0000011197 00000 n Therefore, the resulting infiltration rates are 1.0 inches per hour for example 1, 0.38 inches per hour for example 2, and for the high infiltration area in example 3 the calculated rate is 5.6 inches per hour. Similar to sands, in the UK gravelly layers often include significant quantities of clay which can reduce their infiltration rate significantly. This includes installation of 100mm base gravel and levelling of the soakaway to avoid movement. This spreadsheet calculates the maximum flow from a specified linear drainage channel and checks whether the channel is sufficient for the specified site and storm conditions. 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. Flood test had an average Ksat nearly four times higher, at 2.71 in/hr. References: Clapp, R. B., and George M. Hornberger. Design infiltration rates, in inches per hour, for A, B, C, and D soil groups. The soil gradings values are shown on the graph below. The adjustment factors can also be used to compensate 0000015225 00000 n However, for very small catchments they could still be used. For design purposes, there are two ways of determining the soil infiltration rate. Water repellence is a transient soil . If field-measured soil infiltration rates exceed 8.3 inches per hour, the Construction Stormwater permit requires the soils be amended. 70:5:1569-1578. References: Clapp, R. B., and George M. Hornberger. This is a common problem with newer soakaways in which the pre-treatment systems are not in place. Properly conducted infiltration tests are preferred to estimates of soil infiltration based on borings. Consequences of underestimation of infiltration rate are typically observed in increased construction costs due to the addition of underdrains, deeper bioretention media depths, and reduced ability to utilize internal water storage (submerged) zones as part of the design (Brown et al., 2011). Volume reduction, mitigation of peak flow rate, and groundwater recharge will all be overestimated during the design phase, resulting in a lack of hydrologic function and pollutant mitigation. Infiltration takes place on the sides and base of the trench. For . 14:4:601604; Moynihan, K., and Vasconcelos, J. One of the main risks of a rainwater soakaway is silting that causes clogging of the surrounding soil. I focus on delivering reliable flood risk information to protect livelihoods and the environment. A transmission zone between the saturated zone and the wetting front conveys water deeper into the soil profile. The building regulations soakaway calculations are given in section 3.27 of their regulations. Detailed investigation work will be required to estimate the infiltration rate at design stage. 0000001784 00000 n Coefficients of variation were typically greater than 1 and arithmetic means were consistently greater than geometric means, often by a factor of 2 or more. Chowdary, V.M., Rao, M.D., and Jaiswal, C.S. Soil borings should be filled with water. As the variation tends to be greater at lower infiltration rates the levels of precision are defined as a pro-portion of the mean viz 1, 0.4 and 0.2 times the mean. For more detailed descriptions, see the following links: The Unified Soil Classification System, CALIFORNIA DEPARTMENT OF TRANSPORTATION (CALTRANS) UNIFIED SOIL CLASSIFICATION SYSTEM. There are also typical infiltration rates which have been compiled for the USCS soil classification system. Powered by MediaWiki, Soil infiltration rate measurement techniques, Infiltration rate measurement case studies, Types of infiltration measurement devices, Soil property determination for infiltration SCM design, Calculating soil infiltration rate and example calculations, University of Minnesota, St. Anthony Falls Laboratory, http://websoilsurvey.nrcs.usda.gov/app/HomePage.htm, Dakota County Soil and Water Conservation District, Guidance for amending soils with rapid or high infiltration rates, CALIFORNIA DEPARTMENT OF TRANSPORTATION (CALTRANS) UNIFIED SOIL CLASSIFICATION SYSTEM, Stormwater post-construction technical standards, Wisconsin Department of Natural Resources Conservation Practice Standards, SWMM Modeling of a Rural Watershed in the Lower Coastal Plains of the United States, Stormwater infiltration Best Management Practices, Understanding and interpreting soils and soil boring reports for infiltration BMPs, A New Technique to Measure Infiltration Rate for Assessing Infiltration of BMPs, Low Impact Development Standards for Dakota County, Minnesota, Assessment of Stormwater Best Management Practices, A review of methods for characterization of site infiltration with design recommendations, https://stormwater.pca.state.mn.us/index.php?title=Determining_soil_infiltration_rates&oldid=65982, Level 2 - Technical and specific topic information/infiltration, Level 2 - Technical and specific topic information/soils and media, Although a value of 1.63 inches per hour (4.14 centimeters per hour) may be used, it is, GW - Well-graded gravels, fine to coarse gravel.