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LSA ASSOCIATES, INC. INITIAL STUDY/MITIGATED NEGATIVE DECLARATION JULY 2010 FIRST STREET BRIDGE REPLACEMENT PROJECT CITY OF SANTA ANA Sediment Control BMPs to prevent soil erosion during soil disturbance activities. Implementation of the Erosion and Sediment Control BMPs, as specified in Mitigation Measure WQ-1, would prevent adverse soil erosion and subsequent siltation impacts during construction. Existing runoff from the project site sheet flows into the curb and gutters and then into catch basins. The street widening and bridge replacement would not alter the drainage pattern on or off-site and runoff would continue to follow the same drainage pattern as the existing conditions. The Proposed project would increase the impervious surface area by approximately 0.30 ac compared to the existing roadway facility. Any increase in volume and velocity of runoff from the 0.30 ac increase in impervious surface area would not be substantial and would be conveyed to the existing downstream conveyance channels which are engineered, hardened and regularly maintained. Project runoff would not be directed to unpaved areas prone to erosion. The replacement of the bridge piers and nosing would occur within the Santa Ana River, which is concrete -lined within the project area and not subject to erosion. Therefore, the modifications within the Santa Ana River would not increase erosion. For these reasons, erosion and siltation impacts as a result of changes to existing drainage patterns would be less than significant and no mitigation is required. D. Substantially alter the existing drainage pattern of the site or area, including through the alteration of the course of stream or river, or substantially increase the rate or amount of surface runoff in a manner, which would result in flooding on or off-site? Less than significant impact. As discussed above in VIII.C., the proposed project would not alter the existing drainage pattern of the site. The project would increase the impervious surface area by approximately 0.30 ac compared to the existing roadway facility. However, the rate and volume of surface water runoff from the facility would not increase substantially and would not cause flooding in the area. Therefore, impacts related to capacity of the existing stormwater drainage system would be less than significant. E. Create or contribute runoff water which would exceed the capacity of existing or planned stormwater drainage systems or provide substantial additional sources of polluted run-off? Less than significant impact with mitigation. The proposed project would not create or contribute runoff water which would exceed the capacity of existing or planned stormwater drainage systems or provide substantial sources of polluted runoff. The Proposed project would increase the impervious surface area by approximately 0.30 ac compared to the existing roadway facility. This small increase in impervious area is not large enough to exceed the capacity of the existing storm drains. However, an increase in impervious area could result in an increase in pollutant loading to receiving waters. As specified in Mitigation Measure WQ-2, Source Control, Site Design, and Treatment BMPs would be incorporated into the project design to treat pollutants of concern in runoff from the project site prior to discharge to the City storm drain system. Therefore, with implementation of Mitigation Measure WQ-2, impacts related to addition of polluted run-off to the stormwater drainage system would be reduced to less than significant. F. Otherwise substantially degrade water quality? 07/25/10 «TA\DMJ0701\MND\4.0 Environmental Analysis.doc» 20A-74 4-23