2014-069 - Final Environmental Impact Report No. 2014-01

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HARBOR BLVD. MIXED USE TRANSIT CORRIDOR PLAN FINAL FIR CITY OF SANTA ANA 5. Environmental Analysis HYDROLOGY AND WATER QUALITY potential for flood inundation from Prado Dam. The Army Corps of Engineers is also planning to update the dam inundation maps by 2020 (USACE 2013). The latest available dam inundation map for Prado Dam was produced in 1985 by the Army Corps of Engineers (USACE 1985). Note that this map was prepared prior to all of the improvements to the dam and the Santa Ana River and therefore overestimates the flood potential and dam inundation area. The peak outflow of the hypothetical flood wave would be contained within Santa Ana Canyon and then spread out across the alluvial plain to encompass an area more than 10 miles wide by the time it reached the project site. The nearest cross - section to the project area shows that the first arrival time of the flood wave would be approximately 6.25 hours with a maximum depth of flow of 4 feet at 7.25 hours. With improvements in the Santa Ana River, which can now convey 30,000 cfs (3.2 times the previous capacity), this could result in a proportional decrease in the flow depth to 1.25 feet in the vicinity of the project area. 5.7.2 Thresholds of Significance According to Appendix G of the CEQA Guidelines, a project would normally have a significant effect on the environment if the project would: HYD -1 Violate any water quality standards or waste discharge requirements. HYD -2 Substantially deplete groundwater supplies or interfere substantially with groundwater recharge such that there would be a net deficit in aquifer volume or a lowering of the local groundwater table level (e.g., the production rate of pre - existing nearby wells would drop to a level which would not support existing land uses or planned uses for which permits have been granted. HYD -3 Substantially alter the existing drainage pattern of the site or area, including through the alteration of the course of a stream or river, in a manner which would result in a substantial erosion or siltation on or off site. HM-4 Substantially alter the existing drainage pattern of the site or area, including through the alteration of the course of a 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. HYD -5 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 HYD -6 Otherwise substantially degrade water quality. HYD -7 Place housing within a 100 -year flood hazard area as mapped on a federal Flood Hazard Boundary or Flood Insurance Rate Map or other flood hazard delineation map. HYD -8 Place within a 100 -year flood hazard area structures which would impede or redirect flood flows. HM-9 Expose people or structures to a significant risk of loss, injury or death involving flooding, including flooding as a result of the failure of a levee or dam. HYD -10 Be subject to inundation by seiche, tsunami, or mudflow. Page 5.7 -16 PlaceWorkr