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4.7 DRAINAGE This section evaluates the adequacy of existing and planned stormwater drainage infrastructure to serve the College Park project. The following evaluation uses information from previously prepared documents that address stormwater drainage at the Mountain House community,including the Mountain House New Community Master Plan(San Joaquin County Community Development Department [SJCCDD] 2000),Final Environmental Impact Reportfor Mountain House Master Plan and Specific Plan I(SJCCDD 1994),Mountain House Stormwater Master Drainage Plan(Mountain House Community Services District [MHCSD] 2003),and College Park at Mountain House New Community Specific Plan III(EDAW 2004).This evaluation is also based on technical drainage information and analysis prepared for the College Park project by the project engineer(MacKay& Somps), and technical information and analysis contained in the Specific Plan III Amendment to the Stormwater Master Plan Update(MacKay& Somps 2004)which provides details on the drainage improvements proposed as part of the proposed project. 4.7.1 PHYSICAL SETTING The project site is located in the San Joaquin Valley,which is bounded by the Sierra Nevada to the east and the Diablo Range to the west.The Diablo Range creates a rain shadow that causes the average annual precipitation to decrease quickly east of the crest of the mountains.Most of the annual precipitation falls as rain during the winter rainy season from November through April. The mean annual precipitation at the project site is between 10 and 12 inches per year(SJCCDD 1994). The project site incorporates a gentle northeastward-sloping alluvial surface at the base of the eastern flank of the Diablo Range.The elevation of the project site ranges from approximately 160 feet above mean sea level(msl) along the western boundary to approximately 60 feet above msl along the northern boundary.The eastern flank of the Diablo Range is drained by streams flowing to the northeast that discharge into the Sacramento—San Joaquin River Delta system. MOUNTAIN HOUSE CREEK The project site is located in the Mountain House Creek watershed.Mountain House Creek drains an area of approximately 6.5 square miles,the headwaters are in the Altamont Hills southwest of the project site.Mountain House Creek flows from the south to the north and is located to the west and northwest of the project site(see Figure 3-2 in Chapter 3,"Project Description"). Runoff generated on and flowing through the College Park project site drains to Mountain House Creek Although the creek does not flow through the project site,it is an integral component of the larger adopted drainage plan for Mountain House(Mountain House Storm Water Master Plan Update,Pacific Advanced Civil Engineering[PACE]2003),and would serve the proposed project in the future. Mountain House Creek is a watercourse that passes through culverts under Byron Road and flows into Old River through another culvert at the upstream end of"Dredger Cut,"a dredged channel that passes through the river levee. The culvert is higher than Mountain House Creek,therefore,runoff is forced to pond in the area behind the river levee and is subsequently pumped into Old River by existing pumps.Trimark began expanding Mountain House Creek in 2003 under the approved Mountain House Creek Improvement Project(MHCIP),which was subject to separate environmental review under the California Environmental Quality Act(CEQA).The primary focus of the MHCIP is to widen and deepen the main creek channel and connect it directly to Old River to accommodate a design for stormwater flows resulting from full development of the Mountain House community under the Mountain House Master Plan(MHMP)(including development of the College Park project site).After completion of the MHCIP,the water from Mountain House Creek will be pumped directly to Old River rather than allowed to pond at the south levee of the river before being pumped into the river.Improvements to Mountain House Creek were recently completed,except for plantings along the creek(MacKay&Somps 2005). College Park at Mountain House Specific Plan III Draft EIR EDAW San Joaquin County 4.7-1 Drainage r.