Determining soil infiltration rates for your infiltration practice – a few tips

Stormwater control measures (SCMs, also called BMPs or best management practices), such as bioinfiltration (rain gardens), permeable pavement, and tree trenches, are designed to capture and infiltrate a specified amount of stormwater runoff, usually over a period of 48 hours. To properly design these practices we must know the rate at which water infiltrates into the underlying soil. If the infiltration rate is overestimated, volume reduction, mitigation of peak flow rate, and groundwater recharge will all be overestimated during the design phase, resulting in poorer hydrologic function and pollutant mitigation than expected. Underestimating the infiltration rate will likely result in increased construction costs due to the addition of underdrains, deeper bioretention media depths, or reduced ability to utilize internal water storage as part of the design (Brown et al., 2011).

As an example we designed an infiltration basin using the MIDS calculator. For a practice designed to retain runoff from 1 acre of impervious on a Hydrologic group B soil with an estimated infiltration rate of 0.45 inches per hour, the practice could be 1.8 feet deep and have a surface area of 2016 square feet. If the actual infiltration rate is 0.6 inches per hour, the practice could be 2.4 feet deep and we would need a surface area of only 1512 square feet, meaning the surface area of our system was 25% greater than necessary, assuming we could have made the practice deeper.

Soil infiltration rates are often estimated based on soil texture (e.g. sand, silty sand, sandy loam). Determining soil texture is relatively easy and can be determined by collecting samples from soil borings. In a previous post we discussed the importance of properly collecting and interpreting soil borings. The Minnesota Stormwater Manual provides estimated infiltration rates for different soil textures.

However, soil texture may be a poor proxy for hydrologic performance of in situ soils (Lee et al., 2016). This is a result of natural variability in soil properties, even over very short distances. It is not uncommon for soil infiltration rates to vary by 2 to 3 orders of magnitude over a distance of 10 meters or less (Ahmed, 2011). Field measurement of soil infiltration rate is recommended for determining hydrologic properties of soil. Unfortunately, field measurement of infiltration rate can be challenging and may require several measurements.



The preferred method for measuring infiltration rate for an infiltration practice should mirror the one-dimensional exfiltration from a bioretention cell or infiltration device. Typically, these measurements are made with an infiltrometer – a circular device which allows water to be ponded on the soil surface. Measurements may be taken as a function of time to determine the design infiltration rate of the soil. Another method for estimating the soil infiltration rate is the Modified Phillip-Dunne Permeameter, which was developed by the University of Minnesota (Ahmed et al., 2014). For more information on determining soil infiltration rates, see the Minnesota Stormwater Manual.



The recommended number of permeameter tests is provided in the table below. The median of all measurements should be used to estimate the infiltration rate at the site. A useful calculation is to plot standard deviation and median against the number of samples. If a minimum of 5 samples have been collected measurements may be halted when the standard deviation becomes relatively constant from one sample to the next.  As an example see the figure to the right, which indicates 7 to 10 samples are sufficient to characterize the infiltration rate at the site.

 

 
References

Brown, R.A., Hunt, W.F., and Skaggs, R.W. (2011). Long-term modeling of bioretention hydrology with DRAINMOD. Final Report for Water Resources Research Institute of the University of North Carolina. Project #70255.

Lee, Ryan S.; Robert G. Traver, F.; and Andrea L. Welker. 2016. Evaluation of Soil Class Proxies for Hydrologic Performance of In Situ Bioinfiltration Systems. J. Sustainable Water Built Environ. Volume 2 Issue 4.

Ahmed, F. 2011. Spatial Variation of Infiltration in LID practices. Stormwater Research at St. Anthony Falls Laboratory. University of Minnesota, St. Anthony Falls Laboratory. Minneapolis, MN. Vol 6:2.

Ahmed, F.; R. Nestingen; J.L. Nieber; J.S. Gulliver; and R.M. Hozalski. 2014. A Modified Philip–Dunne Infiltrometer for Measuring the Field-Saturated Hydraulic Conductivity of Surface Soil. Vadose Zone Journal. 13:10.