Agenda - Council Work Session - 02/23/2021

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Soil composition and slope are important factors affecting the rate and volume of stormwater runoff. The shape and stability of aggregates of soil particles expressed as soil structure influence the permeability, infiltration rate, and erodibility (i.e., potential for erosion) of soils. Slope is important in determining stormwater runoff rates and susceptibility to erosion. Soils present within the watershed generally belong to the following associations, as described in the Anoka County Soil Survey (There are two general soil associations in the watershed (Source: Anoka County Soil Survey), a description of each association follows. The Hubbard-Nymore association is a nearly -level to gently sloping outwash plain which covers most of the watershed. The association is dissected by well-defined drainageways. Because it is sandy throughout, the association is well suited for urban development; in fact, most of the area covered by this soil is already developed. The Zimmerman-Isanti-Lino association is level to undulating and dominated by fine sands. This association generally covers the City of Andover northeast of Round Lake. The sandplain has a naturally occurring high water table. Most of this association is urbanized, although the high water table has limited uses in some areas. Soil infiltration capacity affects the amount of direct runoff resulting from rainfall. Higher infiltration rates result in lower potential for runoff, as more precipitation is able to enter the soil. Conversely, soils with low infiltration rates produce high runoff volumes and high peak discharge rates, as most or all of the rainfall moves as overland flow. The Natural Resources Conservation Service (NRCS — formerly the Soil Conservation Service) has established four general hydrologic soil groups (HSGs). These groups are: Hydrologic Soil Group A —(Low runoff potential): Group A soils have a high infiltration rate and are typically composed of more than 90% sand and gravel. Hydrologic Soil Group B—(Moderately low runoff potential): Group B soils have a moderate infiltration rate and are typically composed of 50-90% sand. Hydrologic Soil Group C—(Moderately high runoff potential): Group C soils have a slow infiltration rate and are composed of less than 50% sand. Hydrologic Soil Group D—(High runoff potential): Group D soils have a very slow infiltration rate and are composed of more than 40% clay. These soils have a combination of high swelling potential, a permanently high water table, and a clay layer at or near the surface. Dual HSGs (types A/D, B/D, and C/D) are soils that are considered D soils primarily because of a high water table. However, if the soil were drained it would be classified into a different group. The second group listed for dual HSG soils is for an undrained condition. For the purpose of evaluating infiltration 2-10