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On-site Watershed Improvements The following describes the proposed on-site improvements for Watersheds A, B,and D. On-site Watershed _ "Clean"runoff from off-site Watersheds B,D,and E would be conveyed north through Watershed A to Grant Line Road through a proposed storm-drain system. "Urban"runoff generated in Watershed A would be conveyed via a separate storm-drain system to a proposed water-quality pond to be located at the southwest comer of the Grant Line Road and Central Parkway intersection. This pond would treat the"urban"runoff before mixing with "clean"runoff from the off-site watersheds.The combined runoff would then be conveyed through the Central Parkway trunk storm drain through the Specific Plan I and II areas and discharged to Mountain House Creek. The conveyance system through on-site Watershed A carrying"clean"runoff water from off-site drainage watersheds would be sized to convey the 100-year,24-hour runoff.For a portion of the distance,a volume equivalent to the 10-year, 24-hour runoff would flow via overland release(MacKay& Somps 2005).The separate pipe system to carry"urban"water runoff to the water-quality-control pond would be sized to carry the 10-year, 24-hour runoff. On-site Watersheds B and D + In on-site Watersheds B and D,`clean"runoff from off-site Watershed SB9A and"urban"runoff from on-site Watersheds B and D would be collected and conveyed in the same pipes.Trunk storm drains in these watersheds would be designed to transport the 10-year runoff flows,with the 100-year storm-runoff flows conveyed in the street right-of-way. In other words, storms that produce runoff flow rates greater than the drainage-pipe capacity may cause water flow above the level of the stormwater inlets, and the runoff would be carried by the street section.The MHCSD storm drainage guidelines permit this as long as the depth of flow does not exceed 6 inches ` above the top of curb or edge of pavement(MacKay& Somps 2004). The stormwater runoff from these watersheds would flow through the DeAnza and Mountain House Parkway trunk drains to the WQBI located north of Byron Road.WQBI would require enlargement,depending on the ` criteria adopted for stormwater-quality volume requirements to be adopted by MHCSD.WQB1 may require enlargement from 56 to 64 acre-feet(MacKay& Somps 2004).The MHCSD has not identified any issues surrounding this possible needed expansion(MacKay& Somps,pers. comm.,2004). _ MHCSD requires industrial and commercial properties to implement BMPs for stormwater runoff quality before discharge to the stoma-drain system. As a result,the proposed commercial and industrial areas in Watersheds B and D would drain into a proposed water-quality basin to be located just south of Mascot Boulevard and just west of Mountain House Parkway before the runoff from these areas entering the DeAnza Boulevard and Mountain House Parkway trunk storm drains. Drainage Improvements Phasing Specific details for each phase of drainage improvements to serve the project site and their locations are provided below. Phase I ► DeAnza Boulevard(Neighborhood B).Existing twin 66-inch trunk storm drains located in DeAnza Boulevard currently extend 500 feet south of Mascot Boulevard and would be extended south to Grant Line Road. Grant Line Road(Neighborhood B).A new storm-drain lateral system would be extended west to serve properties in Neighborhood B that drain northerly towards Grant Line Road. College Park at Mountain House Speciftc Plan III Draft EIR EDAW San Joaquin County 4.7-13 Drainage