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LMOUNTAIN HOUSE PROPOSED MACHADO TENTATIVE MAP 4. ENVIRONMENTAL CHECKLIST ` 4.8 HYDROLOGY AND WATER QUALITY Less Than Potential SPIIIEIR Significant h. Significant Requires Impact Due New New Impact Additional to Mitigation Additional Additional Less Than Adequately Review:No Measures in Impact Not Mitigation Significant Addressed in Significant Project Addressed in Measures Issue or No Impact SPIIIEIR Impact Description SPIIIEIR Required it i) 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� SETTING Climate and Topography Average annual rainfall in the vicinity of the Machado property is 12 inches with most rainfall occurring from November to April. Average temperatures range from 38 degrees Fahrenheit in winter months to 93 degrees Fahrenheit in summer months(Western Regional Climate Center, 2002). The project site is relatively flat, sloping gently at approximately one percent to the northeast. Current drainage at the project site is generally in a northeasterly direction via overland sheet flow, small surface swales, ditches and pipelines that convey runoff and excess irrigation water into larger collectors. Surface Hydrology The project site is located in the 15-square mile Mountain House Creek watershed. The headwaters of the creek are in the Altamont Hills approximately 6.5 miles southwest of the Mountain House Community. The creek enters the Mountain House Community at the western boundary of Neighborhood E and flows in a northeast direction to its discharge to Old River. The creek is approximately 8,000 feet northwest of the center of the Machado property. Runoff generated on and flowing through the project site would drain through a storm drain system to Mountain House Creek. Master Drainage Plan r Drainage improvements for the Mountain House Community are described in the Mountain House Master Drainage Plan(PACE, 2002)that was updated in the Storm Water Master Plan L Update, Addendum I(PACE, 2004). Drainage infrastructure includes primary and secondary storm drain systems, improvements to Water Quality Basin(WQB)No. 1, and improvements to Mountain House Creek. New drainage infrastructure would be constructed to convey ` runoff from the Specific Plan III area, in which the project site is located. Discussion of storm drainage facilities for the proposed project is included in Section 4.16: Utilities and Service Systems. r 4-43 it