55C - ALTON AVE OVERCROSSING @ SR 55

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Draft Findings of Fact and Mitigation Monitoring Reporting Program (applies to CEQA only) Rationale: Implementation of these standard industry practices will reduce impacts related to seismic activity, liquefaction, expansive soils, and erosion to levels less than significant by ensuring that all construction incorporate Best Management Practices for erosion control and current Caltrans seismic design criteria. Standard industry practices also address liquefaction and expansive soils issues. No other mitigation is required. Reference: The discussion on setting, impacts, and mitigation related to Geology, Soils, and Seismicity are included in Section 4.1 of the FEIR/EA prepared for the Alton Avenue Overcrossing at SR-55. 7.2 HYDROLOGY, WATER QUALITY, STORMW ATER RUNOFF Potentially Significant Effects Permanent Impacts . The receiving waters are presently subject to direct runoff from the existing highway, land immediately adjacent to the highway, and adjacent residential, commercial, and industrial urbanization. The proposed project is approximately 1,700 meters (5,577 feet) long, with an average increase in pavement width of approximately 28.0 meters (92 feet), and an increase in impervious area of 4.78 hectares (11.8 acres). The Preferred Alternative would increase the rate and volume of runoff slightly. The rate and amount of any dissolvable and suspended materials would increase slightly. The increased rate of runoff and volume of runoff would be due to the increase in impervious surfaces. The overall watershed would not change as a result ofthe project. · Construction of the HOV direct-access drop ramps, with the associated widening of SR-55 to accommodate the median ramps, would require the modification of the OCFCD Lane Channel. Modifications would include replacing the channel with an enclosed box culvert that would cross under the freeway to the east side of SR-55 where it would head south, running parallel to the freeway under Pullman Street. The box culvert west of SR-55 would be 10 feet high by 13 feet wide and east of SR-55 the box culvert would be shorter and wider, 7 feet high and 12 feet wide, in order to accommodate the existing profile of Pullman Street (double box culvert). These modifications would affect approximately 1,310 square meters (2.36 acres). Implementation of the Preferred Alternative would not significantly increase storm water runoff volumes carried by Lane Channel and velocities of storm runoff after the project would not vary significantly from the pre-project conditions. All new storm drain improvements would be protected from any erosive velocities. Temporary Impacts (Construction) · Lane Channel would be relocated to cross under SR-55 just south of Dyer Road and continue parallel to the freeway, under Pullman Street. · During construction, the activities related to the Lane Channel relocation may have an impact on the receiving waters. The potential impacts to receiving waters would be Alton Avenue Overcrossing at SR-55 K-IO April July 2005 55C-28