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

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NT MENTS INITIAL STUDY/MITIGATED NEGATIVE DECLARATION FAIRVIEW BPIOGE REVLACEM REST TO STREET BEET) PROJECT L S A MAY 2020 (9Tx STREET To ANTx STREET) L, OLEGT SANTA ANAL CALIFOPHIA groundwater supplies or interfere substantially with groundwater recharge such that the Project may impede sustainable groundwater management of the basin, and impacts would be less than significant. c. Would the project substantially alter the existing drainage pattern of the site or area, including through the alteration of the course of a stream or river or through the addition of impervious surfaces, in a manner which would: i. Result in substantial erosion or siltation on- or off -site 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; however, no construction would occur in the river during storm events. 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 reduce impacts to water quality during construction, including those impacts associated with soil erosion and siltation. Compliance with the CGP and implementation of the construction BMPs would ensure that construction impacts related to on- or off -site erosion or siltation would be reduced to less than significant with mitigation incorporated. The proposed Project would increase impervious surface area on the Project site by approximately 0.2 ac compared to existing conditions, and could potentially increase on -site storm water runoff during a storm event. In the proposed condition, the impervious surface areas would not be prone to erosion or siltation. Erosion and siltation would be minimized in the landscaped areas, where soil would be stabilized by vegetation. Therefore, the proposed Project would not increase on -site erosion or siltation. An increase in impervious surface area can potentially increase storm water runoff generated from a project and increase erosion and sedimentation in receiving waters. However, as discussed previously, the proposed Project would slightly increase the impervious surface area on the Project site compared to existing conditions (an increase of 0.2 ac), which would slightly increase the volume of storm water runoff generated from the Project site. However, post - construction BMPs would be implemented to treat new impervious surface runoff. In addition, the Project includes BMPs that would reduce on -site erosion during storm events. Downstream erosion would not occur, as all pervious areas would be stabilized with landscaping and BMPs and the downstream conveyance channels that receive runoff from the Project area are engineered and hardened and not subject to erosion, siltation, or hydromodification (i.e., channel modification or channelization from alteration of flow). The increase in impervious area would increase the volume of storm water runoff from the Project area into the Santa Ana River. However, the Santa Ana River is a stabilized concrete R:\WKE1702\ISMND\Fina1 ISMND\Fairview Street Bridge Final ISMND7c0582S% 3 3-59