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Mountain House Specffic Plan III ........................................................................................................................................................................... 15.2.2 Specific Plan III Description Neighboring offsite watersheds have been defined previously by the Master Plan and the MHCSD Storm Water Master Plan and updates. The definitions of these watersheds contained in these documents have been reviewed by MHCSD and are still valid for utilization as the basis for all off-site watershed and on- site conveyance systems. The runoff from neighboring off-site Basins designated B,D,and E is — considered"clean"water since the runoff is generated from undeveloped lands or is treated prior to entry into Mountain House(see Figure 15-1). Should development occur within these off-site watersheds, the MHCSD will work with the permitting agencies with jurisdiction to require BMP's prior to releasing runoff from such areas which may enter into Mountain House. The existing"clean"runoff from these off-site basins will be conveyed through Neighborhood A north to Grant Line Road. It is proposed to convey urban runoff from within the Specific Plan III area in a"parallel"pipe system and to provide a water quality control pond to capture and treat the low flows and"first flush"runoff volume. After the — urban runoff from the Specific Plan III area is treated in the water quality control pond, the onsite flows will be combined with"clean"runoff from the offsite basins and will be conveyed north through — Neighborhood C and thence to Mountain House Creek. • The conveyance system through Neighborhood A carrying the"clean"flow from off-site Basin B will be sized to convey the 100-year,24-hour runoff.The separate pipe system to carry"urbanized"runoff to the water quality control pond will be sized to carry the 10-year, 24-hour runoff. • The conveyance system through Neighborhoods B and D is designed to convey the 10-year, 24-hour design storm with runoff in excess of the pipe capacity flowing within the street right-of-way as — overland release. a. The storm water runoff from Neighborhoods B &D,as well as offsite water from south of Watershed _ B, flows through existing Water Quality Basin#1 (WQB 1),located north of Byron Road. WQB 1 may require enlargement, depending on the criteria adopted for storm water quality volume requirements to be adopted by the MHCSD. 15-2 Draft 03(14/05 ...........................................................................................................................................................................