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MHMP(1,206 cfs under the College Park project vs. 1,249 cfs under the MHMP).This would include a slightly (0.49%)greater increase in runoff in Watershed A than under the MHMP, which the project engineer has _ determined is statistically insignificant(MacKay&Somps 2004),and a lesser increase in runoff than under the MHMP in Watersheds B and D. Based on this information,the proposed project would result in an incremental increase in stormwater runoff that is less than or equal to that for which the existing downstream MHMP storm- drain system has been designed to accommodate,and the proposed project would not exceed the capacity of this existing downstream system.A less-than-significant impact would occur. The Central Parkway,DeAnza Boulevard and Mountain House Parkway trunk storm drains have been developed to their ultimate capacity, and Mountain House Creek will soon be developed to its ultimate capacity under the MHCIP.WQBI has not been developed to its ultimate capacity, however, and would require expansion to serve the College Park project.The required expansion would likely be from 56 to 64 acre-feet(MacKay& Somps 2004).As indicated previously,WQBI is proposed to be expanded as part of the College Park project to accommodate project runoff from on-site Watersheds B and D(472 and 127 cis,respectively,during a 100-year storm event).However,Figure 3-12 in Chapter 3,"Project Description,"does not identify when WQB 1 would be developed. If project development were to occur in Neighborhoods B or D before either expansion of WQB 1 or provision of on-site retention in-lieu of expansion of WQBI,a significant impact would occur. As indicated in Figure 3-12 in Chapter 3,"Project Description,"the extension of the existing Central Parkway, DeAnza Boulevard and Mountain House Parkway trunk storm drains, and the development of the water-quality basin as part of the Central Parkway storm-drain system,would occur during Phase I of the proposed project. In addition,MHMP Policy 15.3(a)requires that adequate storm transport systems be provided to ensure that all off- site drainage from watersheds is safely conveyed to terminal drains;MHMP Implementation 15.3(c)requires that each segment of the storm-drain system be designed as needed to provide protection for each phase of development; the San Joaquin County Development Title,MHCSD Design Manual,and Mountain House ` Stormwater Master Drainage Plan Update set forth requirements for storm-drain facilities that must be complied with at the Tentative Map and development stages; and all drainage facilities must be permitted by MHCSD. Therefore, adequate storm-drain infrastructure would be in place to convey project runoff to the existing downstream storm-drain system by backbone storm-drain infrastructure to serve the proposed project when required.With implementation of the proposed phasing plan and compliance with the above identified MHMP policies, a less-than-significant impact would occur. _ Mitigation Measure 4.7-2: Drainage—Result in Flooding or Exceed the Capacity of the Storm-drain System.The College Park developers shall expand WQBI as required to accommodate runoff from the College Park project, or shall provide the required on-site retention in-lieu of the required expansion of WQBI,before development of those portions of Neighborhoods B and D to be served by the DeAnza Boulevard and Mountain House Parkway storm-dram systems. Implementation of the above mitigation measure would reduce the potential for the proposed project to result in flooding or exceed the capacity of the existing downstream MHMP storm-drain system to a less-than-significant level. IMPACT Drainage—Require Dewatering that Could Lead to Flooding. Because groundwater levels at the College 4.7-3 Park project site are a minimum of 40 feet bgs, it is not anticipated that dewatering would be required during project construction, and no potential would exist for dewatering-related flooding or exceedance of the capacity of the storm-drain system.A less-than-significant impact would occur. As indicated previously, the minimum groundwater level at the College Park project site is 40 feet bgs. Because it Y is not anticipated that project excavations or other project construction activities would reach this depth and thus not encounter groundwater,no potential would exist for flooding or exceedance of the capacity of the existing or - planned storm-drain system associated with dewatering.At WQBI, which is located north of Byron Road, groundwater may be 5 feet bgs(assuming that groundwater levels at WQBI are similar to those in adjacent EDAW College Park at Mountain House Specific Plan III Draft EIR Drainage 4.7-16 San Joaquin County