By Blake Curran - Lead Compliance Manager, Colorado // 2021-09-08
Understanding the soils composition found within your project boundary is an important feature when planning your erosion and sediment control needs. The Natural Resource Conservation Service classifies soils by the Hydrologic Soil Group (HSG) and it represents the soils runoff potential, texture, and physical nature for each of the four respective groups. The four groups represented in the HSG are A, B, C, and D. The HGS characterizes the rate that water infiltrates into and moves through the soil, affecting the volume and rate of runoff leaving the site. Typically, when the rate of infiltration and transmission through the soil is higher, the volume of runoff is lower. The four hydrologic soil groups are described as:
Group A - Soils in this group have a low runoff potential and high infiltration rates even when thoroughly wet. Consists of sand, loamy sand or sandy loam type of soils.
Group B - Soils in this group have moderately low runoff potential and moderate infiltration rates even when thoroughly wet. Consists of silt loam or loam.
Group C - Soils in this group have moderately high runoff potential and low infiltration rates even when thoroughly wet. Consists of sandy clay loam.
Group D - Soils in this group have high runoff potential and very low infiltration rate even when thoroughly wet. Consists of clay loam, sandy clay, silty clay loam, silty clay or clay.
There are dual hydrologic soil groups that are based solely on the presence of a high-water table and once drained they are assigned to dual hydrological soil groups A/D, B/D, and C/D. Meaning the first letter pertains to the drained condition and the second to the undrained condition. The details pertaining to the soils characteristics within your site can be accessed in your stormwater pollution prevention plan or on your engineered plans. Knowing the soils present on your project site will let you choose the appropriate and effective best management practice (BMP) to manage runoff from your project.
Natural Resources Conservation Service (Ed.). (n.d.). National Engineering Handbook [E-book]. In National Engineering Handbook (Chapter 7 Hydrologic Soil Groups ed., p. Chapter 7). https://directives.sc.egov.usda.gov/OpenNonWebContent.aspx?content=17757.wba
Minnesota Stormwater Steering Committee. (n.d.). Soil classification. Minnesota Stormwater Manual. Retrieved June 25, 2021, from https://stormwater.pca.state.mn.us/index.php?title=Soil_classification