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City of Pleasanton—Hidden Canyon Residences and Preserve Geology, Soils, and Seismicity Draft EIR 3.6-28 FirstCarbon Solutions https://adecinnovations.sharepoint.com/sites/PublicationsSite/Shared Documents/Publications/Client (PN-JN)/2148/21480016/EIR/3 - Draft EIR/21480016 Sec03-06 Geology.docx Level of Significance After Mitigation Less than significant impact. Soil Erosion or Topsoil Loss Impact GEO-2: The proposed project would not result in substantial soil erosion or the loss of topsoil. Impact Analysis Construction The proposed project would involve grading, building construction, paving, and utility installation activities that would expose soils and could result in erosion and sedimentation. Left unabated, the accumulation of sediment in downstream waterways such as Devany Creek or the Municipal Separate Storm Sewer System (MS4) could result in the blockage of flows, potentially causing increased localized ponding or flooding, and may increase the turbidity of receiving waters. However, the proposed project would be required to comply with Municipal Code Section 9.14 which requires construction projects to conform to the C.6 Construction Site Control Requirements in the San Francisco Bay Regional Water Quality Control Board’s (RWQCB) Municipal Regional Stormwater NPDES permit (MRP) (Order No. R2-2022-1108; NPDES Permit No. CAS612008). The C.6 Construction Site Control Requirements mandates construction projects that disturb more than one acre of land to file a Notice of Intent (NOI) and prepare an erosion/pollution control plan or SWPPP to obtain a Construction Stormwater General Permit. The SWPPP or erosion control plan will identify site- specific BMPs that will implement erosion, run-on and runoff, and sediment control measures to prevent the discharge of sediments. Under Municipal Code Section 9.14 and the C.6 Construction Site Control Requirements, no construction permits shall be issued, or any construction activities shall begin until the submittal of a NOI, City review of the SWPPP or applicable erosion control plan to ensure compliance with local regulations, and installation of site-specific BMPs. Since the proposed project would be required to comply with Municipal Code Section 9.14 which conforms to the C.6 Construction Site Control Requirements under the RWQCB NPDES permit before any construction activity can be implemented, impacts from construction activities on the project site would not result in substantial soil erosion or the loss of topsoil. Therefore, constructed-related impacts to soil erosion and loss of topsoil would be less than significant. Operation The proposed project would introduce new impervious surfaces and would involve the cutting of existing surfaces on the project site for the development of the proposed structures. The introduction of new impervious surfaces and exposure of hillsides could alter the natural drainage patterns on the project site, potentially exacerbating soil erosion during operation of the proposed project. However, the proposed project would be required to comply with Municipal Code Section 9.14 where new development projects are required to conform to MRP Provision C.3, New and Redevelopment Standards. Under MRP Provision C.3, new development projects that create 10,000 square feet or more of impervious surfaces must implement Low Impact Development (LID) source control, site design, and stormwater treatment on-site or at a joint stormwater treatment facility to