06 16 20 PWA_02_02 Exh2 - IS-MND

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INITIAL STUDY/MITIGATED NEGATIVE DECLARATION FAIRVIEw BRIDGE REPLACEMENT AND STREET IMPROVEMENTS w MAY 2020 (9TH STREET TO 16TH STREET) PROJECT J" SANTA ANA. CALIFORNIA into receiving waters. Each of these pollutants on its own or in combination with other pollutants can have a detrimental effect on water quality. Drainage patterns would be temporarily altered during grading and other construction activities, and construction -related pollutants could be spilled, leaked, or transported via storm runoff into adjacent drainages and downstream receiving waters. However, as specified in Mitigation Measure HYDRO-1, the proposed Project would be required to comply with the requirements set forth by the CGP and SWPPP, which would specify BMPs to be implemented to control the discharge of pollutants in storm water runoff as a result of construction activities. Therefore, construction of the proposed Project would not create or contribute runoff water that would exceed the capacity of existing or planned storm water drainage systems or provide substantial additional sources of polluted runoff. Impacts would be less than significant with mitigation incorporated. In addition, as discussed above in Response 3.10.2(a), expected pollutants of concern during operation of the proposed Project include suspended solids/sediment, nutrients, heavy metals, pathogens (bacteria/viruses), pesticides, oil and grease, toxic organic compounds, and trash and debris. The pollutants of concern for the Project are metals and oil and grease. The proposed Project would increase impervious area by approximately 8,500 sf (approximately 0.2 ac), which would increase the volume of runoff during a storm and more effectively transport pollutants to receiving waters. In addition, an increase in impervious surface would increase the total amount of pollutants in the storm water runoff, which would increase the amount of pollutants discharged to downstream receiving waters. As required by Mitigation Measure HYDRO-3, a final WQMP would be prepared for the proposed Project that would require implementation of operational BMPs to reduce pollutants of concern in storm water runoff. With implementation of operational BMPs, no substantial additional sources of polluted runoff would be discharged to the storm drain system. As such, when operational BMPs are implemented in accordance with Mitigation Measure HYDRO-3, operation of the proposed Project would not create or contribute runoff water that would exceed the capacity of existing or planned storm water drainage systems or provide substantial additional sources of polluted runoff. Impacts would be less than significant with mitigation incorporated. iv. Impede or redirect flood flows? Less Than Significant Impact. As discussed previously, once operational, the proposed Project would improve the hydraulics of the Santa Ana River. As part of the bridge replacement, the proposed Project would replace eight existing pier walls within the Santa Ana River (totaling an area of 0.09 ac) with four new pier walls (totaling an area of 0.05 ac). In the existing condition, a hydraulic jump occurs upstream of the bridge (i.e., flows transition from supercritical to subcritical, which represents a high energy loss with erosive potential). The proposed Project would improve the river hydraulics upstream of the bridge by lowering the water surface elevation and reducing the length of the subcritical flows by approximately 300 ft. Therefore, implementation of the proposed Project would have a beneficial effect on the river hydraulics upstream of the Project area. Additionally, the proposed Project would maintain the overall drainage patterns in the Project area. Therefore, implementation of the proposed Project would R:\WKE1702\ISMND\Final ISMND\Fairview Street Bridge Final ISMND.docx (05/26/20) 3-61