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4 ► Any new or expanded storm-drain facilities required to safely collect,convey,and in some cases treat,the .. runoff to be generated by new development under the College Park project must be in place before construction.It has not been determined how extensive of water treatment would be required,if any. The San Joaquin General Plan requires treatment of stormwater runoff from industrial uses prior to release into the stormwater conveyance system,in this case,stormwater from the residential and commercial uses. The type of treatment facility needed to serve the industrial uses associated with the proposed project shall be determined by the College Park developers' engineer and provided in construction documents to the County and MHCSD for approval. Under the proposed College Park project,impervious surfaces at the project site would increase from approximately 15%of the total land area to approximately 47%(MacKay& Somps 2004).The increase in .. impervious surfaces would increase the quantity of runoff generated on the project site.Table 4.7-2 below identifies the modeled stormwater runoff quantities for the three on-site watersheds under the following scenarios: existing conditions,development of the College Park site under the existing MHMP, and development of the College Park site as currently proposed. Watershed A would result in a net decrease in peak flow as shown when analyzed for existing and proposed ` conditions. This is because of the timing effect of the subareas in the watershed. A large portion of Watershed A is undeveloped, located in the upstream portion of the watershed,and the runoff would increase in peak flow and volume of runoff; however,flow would peak sooner than when the watershed is undeveloped. The juxtaposition of the flows from the undeveloped area and the developed area would have a wider gap in times of peak. The resultant peak flow decreases; however,volume of runoff would increase. Table 4.7-2 College Park Existing/Future Stormwater Runoff Quantities ' On-site Watersheds Existing 100-Year Developed Under MHMP 100- Proposed Project 100- Runoff(cfs)b Year Runoff(cfs) Year Runoff(cfs) Watershed A 610` 602` 607` (Central Parkway System) Watershed B 282' 5004 472° (DeAnza Boulevard System) Watershed D 38e 147` 1270 (Mountain House Parkway System) Total 930' 1,249' 1,206' Note:Above values include stormwater drainage originating from off-site watersheds(i.e.,Watersheds D,E,B,and SB9A). ' The runoff quantities in this table are in cis and represent the runoff entering or eating the project site. b These values were calculated by PACE in the Mountain House Drainage Master Drainage Plan Update,May 27,2003. } ` Approximately 491 cfs of this is runoff from off-site(pass-through flows). e Approximately 158 cfs of this is runoff from off-site(pass-through flows). ' All of these flows originate on-site. ' Approximately 649 cis of this is runoff from off-site(pass-through flows). Source:MacKay&Somps 2004. As shown in Table 4.7-2,the proposed project would increase the total stormwater runoff from the project site during a 100-year storm event from 930 to 1,206 cfs,a 29.7%increase in runoff.A slight decrease in existing runoff would occur in the on-site portion of Watershed A,while relatively large increases in runoff would occur .� in on-site Watersheds B and D.However,whether the increase in runoff under the proposed project would exceed the capacity of the existing downstream infrastructure is determined by comparing the runoff quantities of the College Park project to runoff from development of the site under the existing MHMP. As indicated in Table 4.7-2,the proposed project would result in a smaller increase in total stormwater runoff from the project site during a 100-year storm event than would buildout of the project site under the existing 4 College Park at Mountain House Specific Plan III Draft EIR EDAW San Joaquin County 4.7-15 Drainage