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these facilities shall be cleared of debris and sediment. With implementation of these requirements,sediment buildup in Mountain House storm-drain facilities will be removed as part of annual maintenance. Therefore, a less-than-significant impact would occur. Project development would increase the amount of impervious surfaces at the College Park project site and increase the quantity of runoff. Currently,runoff flows through the site as sheet flow.Under the proposed project, runoff would be directed into storm drains and culverts and this,together with the increase in runoff associated with the project,would concentrate and increase the velocity of water exiting the project site.This could result in downstream erosion.However,MHMP Implementation 15.3(a)requires that erosion be minimized by using appropriate streambed protection energy dissipaters in downstream channels and culverts; MHMP Policy 15.4(b)requires that open-channels be designed to minimize erosion;MHMP Policy 15.6(d)requires the Mountain House Creek be designed to minimize erosion; and MHMP Policy 15.4(g)requires that erosion be minimized by designing and — constructing open channels to convey storm-water runoff at or below the allowable maximum velocity.The Mountain House Master Drainage Plan Update requires that,in locations where the velocity of the runoff can be expected to cause erosion,erosion protection measures(e.g.,concrete,riprap, soil-reinforcement products)would be used to prevent erosion(MHCSD 2003). With implementation of these existing requirements,a less-than- significant impact would occur. Mitigation Measure 4.7-1: Drainage-Ancreased Erosion or Sedimentation. No mitigation is required. IMPACT Drainage—Result in Flooding or Exceed the Capacity of the Storm-drain System. The proposed 4.7-2 project would increase the rate and amount of surface runoff and both increase the demand for capacity in the existing downstream MHMP storm-drain system and require the development of new storm-drain facilities to serve the project. However, this would not result in flooding or exceedance of the capacity of the MHMP storm-drain system because: the existing downstream MHMP storm-drain system has already been sized to accommodate runoff from the proposed project;and the proposed project would provide adequate capacity in the new storm-drain facilities required to serve it.A less-than-significant impact would occur. The project would require expansion of WQ81 and if not completed before project development,a significant impact would occur. According to the Mountain House Stormwater Master Drainage Plan Update(MHCSD 2003)and the proposed Specific Plan III Amendment to the Stormwater Master Plan Update(SJCCDD 2004b), extension of the trunk storm drains southward to the College Park project site,expansion of WQB1,completion of the upstream components of the MHCIP, and development of an on-site storm-drain system,would all be required to provide storm drain service to the College Park project.Interim on-site detention could allow for some development at the , College Park project site before completion of the aforementioned on-site and off-site storm-drain facilities. The watersheds established for the site's proposed storm-drain system closely resemble those identified in earlier drainage studies conducted for the MHMP area and discussed above.The proposed Specific Plan III Amendment to the Stormwater Master Plan Update(MacKay& Somps 2004) describes the proposed drainage facilities that would be needed to safely control and convey stormwater runoff through and off the project site in accordance with San Joaquin County policies and procedures.The drainage plan designates three separate drainage sub- watersheds in the project site(Watersheds A,B,and D).Each sub-watershed would be served by a gravity conveyance system of storm drains, detention facilities,and water-quality basins. The primary storm-drain improvements are shown in Figure 3-12 and described in Chapter 3,"Project Description."The proposed facilities,watersheds,and runoff quantities are identified in Figure 4.7-1. As indicated,the existing trunk storm drains in Central Parkway,DeAnza Boulevard,and Mountain House Parkway would be extended southward to and into the project site. EDAW College Park at Mountain House Specific Plan III Draft EIR Drainage 4.7-12 San Joaquin County