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MOUNTAIN HOUSE NEIGHBORHOOD"F"PROJECT INITIAL STUDY Potentially Significant Potentially Unless Less Than Significant Mitigation Significant No Impact Incorporated Impact Impact c) 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 substantial erosion or siltation on-or off- site? f) 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? g) Create or contribute runoff water which would exceed the capacity of existing or planned stormwater drainage systems or provide substantial additional sources of polluted runoff? h) Otherwise substantially degrade water quality? i) 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? j) Place within a 100-year flood hazard area structures which would impede or redirect flood flows? k) Expose people or structures to a significant risk of loss,injury or death involving flooding,including flooding of as a result of the failure of a levee or dam? j) Inundation by seiche,tsunami,or mudflow? Environmental Checklist Explanations a) Violate any water quality standards or waste discharge requirements? fn terms of the surface water discharges, the project applicant has proposed to construct a series of storm water quality basins as outlined in the Phase 1 Neighborhood F Storm Water Report prepared by Condor Earth Technologies(CET). A potential water quality problem could arise with flood inundation of the effluent storage ponds for the wastewater treatment plant. The current plan is to pump water from behind the Old River levee through a series of rented pumps. No information was reviewed that indicates the elevation of the impoundment levees for the treatment plant effluent storage pond. One concern is that storm water could overtop the effluent ponds and possibly cause an uncontrolled release. Specific critical assumptions are used in the design procedure which should be revisited. This procedure has direct bearing on the temporary storage concept and the calculation of the maximum 100-year water surface elevation in the storage areas. The primary concern is that water stored behind the Old River levee under 100-year storm conditions may exceed water surface elevations predicted in the CET study. The plan for pumping the accumulated storm water is not well defined at this stage. The CET study indicates that 100-year water surface elevations are 11.65 feet in the area around the effluent ponds. The study also cvoeMMINMAL=YTMAL1NMMMUDY•PARn.DoC(7rza00) 95 V.J. HYDROLOGY AND WATER QUALITY