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LSA ASSOCIATES, INC. INITIAL STUDY/MITIGATED NEGATIVE DECLARATION JULY 2010 FIRST STREET BRIDGE REPLACEMENT PROJECT CITY OF SANTA ANA overflowing its banks. Throughout Santa Ana River Basin the potential exists for massive downstream flooding resulting from the failure of the Prado Dam, which is located 24 miles north- east of the project site. If Prado Dam failed while at full capacity the project site would be located within the potential flood inundation area. However, the proposed project would not expose people or structures to a significant risk of loss, injury or death involving flooding to a greater extent that under existing project conditions. Therefore, less than significant impact is expected and no mitigation is required. J. Inundation by seiche, tsunami, or mudflow? No impact. The proposed project is located within a developed urban area and there are no water bodies such as lakes or reservoirs that could generate a seiche large enough to inundate the area. The proposed project is located inland, approximately 9.5 miles from the ocean (USGS mapping), and is not within a tsunami zone.' The project site and surrounding area are relatively flat and there are no hillsides or slope areas adjacent to the site that could generate a mudflow. Therefore, no impact related to seiche, tsunami, or mudflow is anticipated to occur in the project area. No mitigation is required. IX. Land Use and Planning Would the project: A. Physically divide an established community? Less than significant impact. The communities around the project site consist mostly of the manufactured housing parks, and apartment complex buildings that are already physically separated by the existing First Street and bridge over Santa Ana River. The proposed project will not change this configuration of the First Street and bridge. The proposed project will widen the First Street by adding one through travel lane in each direction and replace existing bridge structure. The project will also incorporate continuous pedestrian walk along the First Street and Santa Ana Bridge, thus improving the mobility among the manufactured housing park communities. Because the project will implement a sidewalk, it would provide beneficial effects for the surrounding communities. The bridge replacement will be conducted in two phases in order to reduce impacts to traffic. During northern bridge structure replacement, the traffic will continue to use the southern bridge structure. After completion of the northern bridge structure activities, the southern bridge structure will be replaced, and the traffic will utilize the northern bridge structure. Therefore, First Street will remain open during all construction activities. The construction activities will temporarily obstruct the movement of bicycles on the Santa Ana River Trail and therefore will require implementation of the detouring plan. The plan will reduce the temporary construction impacts allowing the cyclists to use nearby streets during the time of construction. Please refer to Section 2.0 for details regarding the detouring plan. Less than significant impacts are anticipated and no mitigation is required. ' http://nmviewogc.cr.usgs.gov/viewer.htm (USGS mapping), 2007-08-30. 07/25/10 «PA\DMJ070l\MND\4.0 Environmental Analysis.doc 411_Ift, 4-25