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MOUNTAIN HOUSE NEIGHBORHOOD"F"PROJECT INITIAL STUDY J. HYDROLOGY AND WATER QUALITY Provided below is a general review of the overall hydrologic conditions on the site and a brief overview of the planned project improvements as they pertain to Neighborhood "F"and the Storm Water Master Plan for the entire planned community. Climate and Topography The Mountain House project site is located just south and east of Altamont Pass, on the western edge of the San Joaquin River delta. Elevations on the site range from approximately sea level to 180 feet relative to the National Geodetic Vertical Datum(NGVD). The average annual rainfall at the project site and surrounding watershed is approximately 11 to 13 inches. The climate of San Joaquin County is generally characterized as Mediterranean, with wet winters and dry summers. The region's"rainy" season extends between the winter months of November and April, with relatively dry conditions for the remainder of the year. Hydrologic Characteristics Most of the site drains through a combination of overland flow and drainage ditches to the natural drainage courses of Mountain House or Dry Creek. Both Mountain House and Dry Creeks flow north to the Old River Channel, which is linked to the San Joaquin River Delta. The drainage characteristics of the surrounding drainages and on-site creeks consist of ephemeral (seasonal)hillside streams. Due to the steep eastern slopes and rainfall patterns, runoff patterns in the hillside streams can be short duration and high intensity and typically last only for the duration of the storm event. Presently, runoff from the site consists of a combination of sheet(unconcentrated) flow, small rill (concentrated) flow, and channel flow. The Mountain House Creek watershed is the dominant watershed. This watershed drains through a neutral open channel including existing storm drain pipes that pass under the California Aqueduct and the Delta Mendota Canal and which eventually discharge into the Old River channel. Mountain House Creek flows to the north and northwest of the site, crossing under Byron Road(through three, 36-inch culverts)before entering the Old River Channel through a single 48-inch culvert at a location called the Dredger Cut. In many places, Mountain House Creek does not have capacity to contain large magnitude flood flows. The southern portion of the channel, south of Byron Road, generally has significant channel capacity when not constricted by farm crossings. The north segment of the creek, downstream and north of Byron Road, has been significantly impacted by agricultural operations. Minimum channel capacity in this area is around 70 cubic feet per second(cfs). The culvert at the Dredger Cut has a capacity of 150 cfs. All flows in excess of 150 cfs either break out from the creek channel or back up behind the Dredger Cut culvert and pond along the interior of the levee. Three pumps then discharge ponded storm water into Old River. The pump sizes and capacities are listed below in Table V-16: Table V-16 OLD RIVER PUMP INFORMATION Pump Horsepower Elevation 1 60 15 2 30 8 3 15 3 C.VOBMCVNMAL SMYTINALINnlALMMY-PARMOC(7/10/00) 91 V.J. HYDROLOGY AND WATER QUALITY