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MOUNTA1N HOUSE PROPosEn ALTAMONT I-IFIGnrs TENTATIVE MAP 4. ENVIRONMENTAL CHEcrcz.isT 4.8 HYDROLOGY AND WATER QUALITY Less Than Potential SI'IIiEIR Significant 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 SPIREIR Required i) Expose people or structures to a ■ F-1 ❑ ❑ El ❑ 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 ■ ❑ E F1 ❑ ❑ tnudflow? SETTING Climate and Topography Average annual rainfall in the vicinity of the project site 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 existing topography varies from elevation 190 at the top of the small hill in the southeast corner to less than 155 feet in the Swale at the northern boundary. Surface Hydrology A drainage swale runs southwest to northeast across the site. An excavated drainage ditch has replaced the natural swale in some locations. Current drainage at the site consists of overland sheet flow toward the drainage swale and then along the swale to the northern property boundary. A sharp bend in the Byron Bethany Irrigation District's Canal 155 is located approximately 50 feet north of the project site's northern boundary. Water in the swale flows into the canal through a 10-inch-diameter pipe(Siegfried Engineering,2006a). The project site is located in Watershed A-3 as defined in the SPIIIEIR(EDAW, 2005b, Figure 7-1). This watershed is within the larger 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 in Old River. Runoff generated on and flowing through the project site would drain through a storm drain system to Mountain House Creek. Runoff flowing through the project site comes from 1.9 square miles of undeveloped area southwest of the Mountain House Community that currently flows into the drainage swale. This runoff is considered "clean"because it is not polluted by urban land uses. Seepage from both the Delta Mendota Canal and the California Aqueduct,located approximately 2,000 and 5,000 feet west of the project site,respectively,results in year-round water in the swale(Siegfried Engineering, 2006a). Master Drainage Plan 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 4-41