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Administrative Draft Environmental Impact Report Gill Medical Center Project Erosion Erosion naturally occurs on the surface of the earth as surface materials (i.e. rock, soil, debris) are loosened, dissolved, or worn away, and transported from one place to another by gravity.Two common types of soil erosion include wind erosion and water erosion.The steepness of a slope is an important factor that affects soil erosion. Erosion potential in soils is influenced primarily by loose soil texture and steep slopes. Loose soils can be eroded by water or wind forces,whereas soils with high clay content are generally susceptible only to water erosion. The potential for erosion generally increases as a result of human activity, primarily through the development of facilities and impervious surfaces and the removal of vegetative cover. The NRCS Web Soil Survey identified the erosion potential for the soils in the Project site. Soil property data for each map unit component includes the hydrologic soil group, erosion factors (Kf) for the surface horizon, erosion factor(T), and the representative percentage of sand, silt, and clay in the surface horizon. Erosion factor (K) indicates the susceptibility of a soil to sheet and rill erosion by water.Values of K range from 0.02 to 0.69. Other factors being equal, the higher the value, the more susceptible the soil is to sheet and rill erosion by water.Within the Project site, the erosion factor(Kf) of the soil is 0.20, which is considered a low potential for erosion. Furthermore, the Proposed Project would be required to obtain coverage under the SWRCB General Permit for Storm Water Discharges Associated with Construction Activity because Project activities would result in ground disturbance of more than 1 acre.As a result, the Proposed Project would prepare and implement a SWPPP to prevent construction-related erosion, sediment runoff, and discharge of pollutants into waterways or onto neighboring properties. The SWPPP would require implementation of temporary Best Management Practices for erosion control measures to control erosion from disturbed areas, sedimentation control measures, and post-construction restoration and sediment stabilization measures. Because the Project is required to prepare and implement a SWPPP, potential erosion impacts are less than significant. Mitigation Measures None required. Impact 4.9-3: The project could be located on a geologic unit, expansive soils, or soil that is unstable or would become unstable. Impact Determination: less than significant Threshold: Location of the project on a geologic unit, expansive soils, or soil that is unstable, or that would become unstable as a result of the project, and potentially result in on- or off-site landslide, lateral spreading, subsidence, liquefaction, or collapse. Threshold: Location of the project on expansive soil, as defined in Table 18-1-B of the Uniform Building Code (1994), creating substantial risks to life or property. Expansive Soils Expansive soils can undergo significant volume change with changes in moisture content.They shrink and harden when dried and expand and soften when wet. If structures are underlain by expansive soils, it is Geology and Soils 4.9-9 October 2021