For instance, ASTM D3385 (2003) describes field measurement of infiltration rate using the double ring infiltrometer method. Measurements may be taken as a function of time to determine the design infiltration rate of the soil. Typically, these measurements are made with an infiltrometer – a circular device which allows water to be ponded on the soil surface. A method which mirrors the one-dimensional exfiltration from a bioretention cell or infiltration device is desired. Soil infiltration rate measurement techniquesīecause 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. Internal water storage is typically not recommended in HSG D soils or soils with infiltration rates less than 0.05 inches per hour. 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. If infiltration rate is overestimated during the infiltration test, system performance suffers once the SCM is constructed. There are consequences for both overestimation and underestimation of infiltration rate beneath a stormwater control measure (SCM). Since infiltration rate varies both spatially and temporally as a result of being dependent on multiple soil properties, such as texture, compaction, and presence of vegetation, it is useful to have a large number of samples to estimate the median value for design of an infiltration practice ( Warrick and Nielson, 1980 Chowdary et al., 2006). 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. During an infiltration test, infiltration rate decreases over time as the soil becomes saturated. Infiltration testing is meant to mimic these natural processes and informs the design of stormwater infiltration practices. A transmission zone between the saturated zone and the wetting front conveys water deeper into the soil profile. 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). This page includes example calculations and links to related pages in this manual.ĭuring rainfall, a saturated zone forms quickly at the soil surface as rainfall causes surface ponding.
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Properly conducted infiltration tests are preferred to estimates of soil infiltration based on borings. This page provides a literature review, case studies, and recommendations for field measurement of soil infiltration.
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Caution: It is Highly Recommended that soil borings be extended a minimum of 5 feet below the bottom of the proposed infiltration practice and that the most restrictive soil layer be considered in estimating infiltration rate.