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 GEOLOGY AND SOILS Other Geologic Hazards Ground Subsidence Land subsidence due to groundwater withdrawal is common in groundwater basins. Subsidence of approximately 0.5 inches per year near Santa Ana was observed between 1993 and 1999, coinciding with a period of net water withdrawal in the region. However, there has been no indication that the land surface changes caused, or are likely to cause, any structural damage in the area. The potential for problematic land subsidence is reduced by maintaining groundwater levels and basin storage within its historical operating range. In the event that land subsidence becomes a problem in a localized area, the Orange County Water District would work with local officials to investigate and remediate the problem (OCWD 2009). Expansive Sails Expansive soils contain certain types of clay minerals that shrink or swell as the moisture content changes; the shrinking or swelling can shift, crack, or break structures built on such soils. Arid or semiarid areas with seasonal changes of soil moisture experience a much higher frequency of problems from expansive soils than areas with higher rainfall and more constant soil moisture (COGS 2011). The City of Santa Ana is in a semiarid region with marked seasonal changes in precipitation: most rain falls in winter, and there is a long dry season in summer and autumn. Therefore, Santa Ana's climate is such that a relatively high incidence of soil expansion is expected where soils contain the requisite clay minerals. Collapsible Sails Collapsible soils shrink upon being wetted, being subject to a load, or under both conditions. 5.4.2 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: G -1 Expose people or structures to potential substantial adverse effects, including the risk of loss, injury, or death involving: i) Rupture of a known earthquake fault, as delineated on the most recent Alquist- Priolo Earthquake Fault Zoning Map issued by the State Geologist for the area or based on other substantial evidence of a known fault. (Refer to Division of Mines and Geology Special Publication 42) u) Strong seismic ground shaking. iii) Seismic - related ground failure, including liquefaction. iv) Landslides. G -2 Result in substantial soil erosion or the loss of topsoil. October 2014 Page 5.4 -9