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Water Quality Management Plan (WQMP) <br />Mainplace Mall Specific Plan <br />Santiago Creek is subject to hydromodification effects. The portion of the Project that drains to <br />Santiago Creek will be evaluated to determine if the proposed 2-year storm runoff peak flow rates <br />and volumes exceed existing conditions by more than 5%. <br />IIA Post Development Drainage Characteristics <br />Describe post development drainage characteristics. Refer to Section 2.2.4 in the <br />Technical Guidance Document (TGD). <br />MainPlace Mall is tributary to both Santiago Creek (County drain E08), which lies south of the <br />south leg of Mainplace Drive along the Project frontage and the Santa Ana River (County drain <br />E01), which lies west of the site, across Interstate 5. Santiago Creek is part of the Santa Ana River <br />watershed and discharges to the Santa Ana River about one mile west of the Mall. The Santa Ana <br />River drains south to the Pacific Ocean. Areas tributary to Santiago Creek are subject to <br />hydomodification analysis. Areas tributary to Santa Ana River directly are not subject to <br />hydromodification analysis. <br />Existing Local Infrastructure: The site generally drains to three separate storm drain systems: <br />Main Street Strom Drain System: The City owns and maintains an existing 36' to 42" diameter <br />storm drain pipe in Main Street, immediately east of Mainplace Mall. The pipe drains from north <br />to south and discharges into Santiago Creek. Mall drainage, other private development drainage, <br />and City street drainage are commingled in this drain. <br />Caltrans South Drainage System: When Caltrans widened Interstate 5 and State Route 22 for the <br />5/22 interchange project, it replaced a significant amount of local City drains with Caltrans drains <br />within Caltrans right-of-way. Included is a small local storm drain network that drains southerly <br />to Santiago Creek. This system commingles City drainage, Caltrans drainage from the northbound <br />Interstate 5lanes, and drainage from a portion of the mall. <br />Caltrans North Drainage System: Caltrans owns and maintains a substantial amount of drainage <br />infrastructure for the 5/ 22 interchange project, with significant pipes, channels, and box culverts. <br />The Caltrans north drainage system accepts local street drainage, private development drainage, <br />including from Mainplace Mall, and freeway mainline and interchange drainage. This system <br />discharges directly to Santa Ana River at the State Route 22 bridge over the River. <br />Existing Site Infrastructure and drainage pattern: The site is served by an existing storm drain <br />system discharging to six different locations. On -site drains range in size from 12" to 30" diameter. <br />Centennial Section II <br />Conceptual Specific Plan WQMP Page 13 <br />