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City of Pleasanton—Hidden Canyon Residences and Preserve Draft EIR Geology, Soils, and Seismicity FirstCarbon Solutions 3.6-29 https://adecinnovations.sharepoint.com/sites/PublicationsSite/Shared Documents/Publications/Client (PN-JN)/2148/21480016/EIR/3 - Draft EIR/21480016 Sec03-06 Geology.docx minimize runoff and maintain a site’s predevelopment hydrology through minimizing disturbed areas and then infiltrating, storing, detaining, or biotreating stormwater runoff close to the source. In compliance with Municipal Code Section 9.14 which conforms to MRP Provision C.3 requirements, the proposed project would develop on-site bioretention areas where runoff from introduced impervious surfaces from driveways, sidewalks, roadways, and roofs would flow into proposed storm drains that would connect to vegetated areas consisting of proposed bioretention areas to reduce the rate of runoff and to treat stormwater before flowing into the City’s existing storm drainage system. In addition, the proposed project would limit the disturbance of natural drainage systems through preserving natural areas by dedicating approximately 116.8 acres for open space use managed by the future Homeowner’s Association (HOA) and EBRPD. Since the proposed project would comply with Municipal Code Section 9.14 through the development of on-site bioretention areas and would dedicate land use for open space preventing alteration of existing natural drainage features, operation of the proposed project would not result in substantial soil erosion or the loss of topsoil, and impacts would be less than significant. Level of Significance Less than significant impact. Unstable Geologic Location Impact GEO-3: The proposed project would not be located on a geologic unit 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. Impact Analysis A majority of the project site, including the development area identified within the limits of grading, is underlain by a thin layer of colluvium and residual soil with stiff clays and no groundwater indicating a low potential for liquefaction in the main development area. However, as previously mentioned above, some areas within the northern and western portions of the project site are designated as Landslide Zones of Required Investigation by the CGS. Additionally, the northwestern and southeastern portions of the project site, especially along Devany Creek, have higher potential for liquefaction and are within a CGS Zone of Required Investigation for liquefaction. Since the northern and southeastern sections of the project site have topographic and subsurface conditions indicating potential for landslides and liquefaction, additional loading from the development of structures along the northern and southeastern portions of the project site may result in soils becoming unstable and could induce landslides or liquefaction. As such, impacts related to new developments being situated on unstable geologic units or soils that could engender landslides or liquefaction may be potentially significant. Areas susceptible to lateral spreading are situated on near horizontal slopes with subsurface conditions comprising of a cohesive layer on the upper portions of the soil profile overlaying loose and saturated sediments. The thin colluvium and residual soils associated with the main development area are situated above marine sedimentary rocks comprised of sandstone, siltstone, and siliceous shale. As such, the subsurface conditions in the main development area have a low