75B - PH ADOPT RESO FOR PR NO 15-6827

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L S A FAIRVIEW BRIDGE REPLACEMENT AND STREET IMPROVEMENTS (9TH STREET TO 16TH STREET( PROJECT INITIAL STUDY/MITIGATED NEGATIVE DECLARATION SANTA ANAL CALIFORNIA MAY 2020 channel and is not susceptible to hydro mod ification.40 Therefore, increasing flow to this channel would not change sediment transport or increase downstream erosion and accretion. In addition, the proposed Project would not alter the course of a stream or river. As such, operational impacts related to on -site or off -site erosion or siltation would be less than significant. ii. Substantially increase the rate or amount of surface runoff in a manner which would result in flooding on- or ofilsite Less Than Significant with Mitigation Incorporated. During construction activities, soil would be exposed and disturbed, drainage patterns would be temporarily altered during grading and other construction activities, and there would be an increased potential for soil erosion and siltation compared to existing conditions. Additionally, during a storm event, soil erosion and siltation could occur at an accelerated rate. As discussed above in Response 3.10.2(a), Mitigation Measure HYDRO-1 requires compliance with the CGP and preparation of a SWPPP to identify construction BMPs to be implemented as part of the proposed Project to manage and convey storm water during construction. Proper management of storm water during construction would reduce impacts associated with flooding. Therefore, impacts related to on- or off -site flooding would be less than significant with mitigation incorporated. 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 have a beneficial effect on the flood control functions of the surface waters upstream of the Project area. Additionally, the proposed Project would maintain the overall drainage patterns in the Project area. Therefore, the proposed Project would not substantially increase the rate or amount of surface runoff in a manner that would result in flooding on or off site. As such, operational impacts related to on - site or off -site flooding would be less than significant. iii. Create or contribute runoff water which would exceed the capacity of existing or planned storm water drainage systems or provide substantial additional sources of polluted runoff Less Than Significant with Mitigation Incorporated. As discussed above in Response 3.10.2(a), there is a potential for chemicals, liquid products, petroleum products (such as paints, solvents, and fuels), and concrete -related waste to be spilled or leaked and transported via storm runoff 40 County of Orange. 2012b. op. cit. 3-60 75 4ND\FinalISMND\Fairview Street Bridge Final ISMND.docx(OS/26/20)