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San Joaquin County Community Environmental Setting,Impacts,and Mitigation Measures Development Department Surface Hydrology,Stormwater,and Water Quality Impact WQ-5: Effects of Tsunami or Mudflow (Less than Significant) Dry Pit The project area is located far from the ocean. Therefore,a tsunami would not pose any risk. In addition,the project area is relatively level,with little risk of mudflow. In the mining pits,excavated walls would present a potential for mudflow. However,as described in Impact GEO-8,the factors of safety for pit slopes during mining operations and in the non-saturated zone following reclamation operations would be greater than 1.0,indicating that slope failure is not probable. In addition,a mitigation measure has been added to the project (Mitigation Measure GEO-8)requiring that a bench of native(undisturbed) ` material be left at the groundwater elevation,which would support unsaturated reclaimed material above it. Therefore,implementation of Mitigation Measure GEO-8 would ensure potential for a mudslide in the pit area during mining or reclamation operations is reduced to a less-than-significant level. +� Wet Pit The potential effects and risk of a tsunami or mudflow during Wet Pit operations would be similar to those described for the Dry Pit operations. The potential for .. a mudslide in the pit area during mining or reclamation operations is minimal with implementation of Mitigation Measure GEO-8,which would reduce this impact to a less-than-significant level. Impact WQ-6: Effects of Seiche (Less than Significant with Mitigation) Dry Pit During the mining operations, the South Pit,which will be used as a settling pond,may contain ponded water at depths that could exceed 50 feet. As described in section 317,"Geology,"the probabilistic peak horizontal ground acceleration values for the project area range from 0.3 to OAg(where 1.0 g is the a force of gravity),which indicates that the ground-shaking hazard in the project area is moderate. Because a seiche is caused by ground shaking2, a seiche may be possible over the life of the Project, although its duration likely would be short. The seiche potentially could affect the stability of the pit walls if it eroded sediments into the pit and altered the pit wall slopes. Implementation of Mitigation Measure WQ-6 would reduce this impact to a less-than- JI significant level. Wet Pit r The potential effects and risk of a seiche during Wet Pit operations would be similar to those described for the Dry Pit operations. Implementation of Z A seiche is a prolonged oscillating wave in a body of water caused by seismic disturbances such as earthquakes. DeSilva Gates Quarry Project Draft Environmental Impact Report 3.G-11 J85 05105.05