2019-040 - Mainplace Mall Transformation

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City of Santa Ana Addendum to the MainPlace Mall Transformation Proiect Fashion Square Commercial Center Final EIR 5.3 HYDROLOGY Summary of Impacts Identified in the 1983 EIR The 1983 EIR described the site as basically flat, and almost entirely covered with buildings and parking lots. The EIR describes that runoff from the site drains in a southwesterly direction to a 36-inch storm drain in Arnett Drive, which discharges to Santiago Creek. In addition, a 42-inch storm drain in Main Street discharges into Santiago Creek. Also, the northwestern portion of the site drains to an open channel within the Caltrans right-of-way. The 1983 EIR determined that redevelopment of the site would not result in a significant amount of additional drainage and that the existing drainage facilities would be adequate to accommodate the Originally Approved Plan. Overall, the 1983 EIR determined that impacts would be less than significant, and no mitigation measures were required. Summary of Impacts Identified in the 1996 Addendum The 1996 Addendum described that incorporation of new structures and parking areas would not result in increased amounts of runoff and that the existing drainage facilities would be adequate to serve the site modifications. No mitigation was required. Impacts Associated with the Proposed Project No New Impact. A Preliminary Hydrology and Hydraulics Study and a Water Quality Management Plan (WQMP 2019) were prepared for the proposed Project by Kimley Horn in 2019 and are included as Appendix C. Construction Construction of the Project would require activities such as demolition of existing structures, parking areas, and subterranean infrastructure; excavation, construction of new structures, and landscaping activities that would expose and loosen sediment and could temporarily alter drainage on the Project site. Also, construction would require the use of heavy equipment and construction -related materials and chemicals, such as concrete, cement, asphalt, fuels, oils, antifreeze, transmission fluid, grease, solvents, and paints that could be accidentally spilled or improperly disposed of during construction. This could pollute surface waters or groundwater and result in impacts to water quality. However, the National Pollutant Discharge Elimination System (NPDES) Construction General Permit requires preparation and implementation of a Stormwater Pollution Prevention Plan (SWPPP) by a Qualified SWPPP Developer (QSD) for projects that would disturb greater than 1 acre of soil, such as the proposed Project. The SWPPP is required to include a site map, description of stormwater discharge activities, and site -specific BMPs that would be employed to prevent water pollution. The QSD-prepared SWPPP would address site -specific conditions related to construction, identify the sources of sediment and other pollutants that may affect the 57