3 - The Bowery_PUBLIC COMMENT_RAMSEY

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9.0 EIR Subject Areas Requiring No Change in Analysis Avion Project SEIR Page 9-3 proposes to introduce new drainage facilities consisting of culverts, brow ditches, curb gutters, storm drain inlets, and pipes that would convey flows to a new biofiltration basin to be constructed in the northeast corner of the project site. The biofiltration basin would treat and detain stormwater flows before discharging them into the existing channel on the eastern side of the project site. The biofiltration basin would be needed to reduce post-project stormwater flows. Introduction of new impervious surfaces associated with the project would increase the 100-year storm runoff rate from the existing 20.6 cubic feet per second (cfs) to 25.7 cfs in the post-project condition. However, the proposed biofiltration basin includes design features that would reduce the 100-year storm runoff rate to less than or equal to the 20.6 cfs present in the existing condition. The biofiltration basin would include an aboveground storage component with a series of flow orifices that would detain stormwater and slowly release treated stormwater. Additionally, the biofiltration basin would include riprap energy dissipaters to reduce flow velocities both entering and exiting the basin. Therefore, no new significant or substantially increased adverse impacts relative to hydrology would result from implementing the project. 9.3.2 Water Quality The 1998 EIR also concluded that implementation of the Subarea Plan has the potential to significantly impact water quality (both directly and cumulatively) in the San Dieguito River and Lagoon. The EIR MMRP recommends measures to reduce levels of erosion, sedimentation, and runoff and requires that the recommended measures or the equivalent thereof would be conditions of future tentative maps for the Southeast Perimeter properties. Since the certification of the 1998 EIR, the regulatory framework relative to water quality and drainage has changed. The project would be required to comply with new regulatory standards. To ensure that the project would comply with new state and local regulations, a site-specific SWQMP was prepared for the project (PDC 2019c). The project would be consistent with the land use for the site as designated by the Subarea Plan. Therefore, pollutants of concern would be the same as those addressed in previous documents, and the BMPs for the project will comply with the City’s requirements for stormwater treatment. In addition to the proposed biofiltration BMP attenuating flows to provide detention benefits for peak flows, the proposed biofiltration basin is also designed to treat the water quality flows (the “85th percentile runoff”) and mitigate for hydromodification impacts. The project would construct a biofiltration basin in the northeast corner of the project site that would meet City pollutant control and volume retention requirements and also control post- developed flow rates to within 10 percent of the pre-developed flow durations across the range of hydromodification mitigation flows, which include flow frequencies ranging from a fraction of the 2- year flow (Q2) to the 10-year flow (Q10). The proposed biofiltration basin would be lined, would not allow for infiltration, and would include an 18-inch engineered soil mix on top of a 3-inch washed sand layer. Additionally, the lower portion of the biofiltration basin includes a 3-inch choking stone layer above the gravel layer. Based on the design components described above, the Preliminary Hydromodification Management Study completed for the project determined that the biofiltration basin would satisfy the Preliminary Hydromodification Management Study requirements of the San Diego Regional Water Quality Control Board (PDC 2019d). Furthermore, the project would include other source control BMPs such as storm drain stenciling/signage and prohibition of illicit discharges into the MS4. In addition to the permanent BMPs, temporary BMPs will be employed during