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F, In order to ensure that peak flow rates in Mansville Ditch will not exceed their existing level, a series of small detention basins with gravity flow operation are planned along the length of the channel between the runways. Two larger detention ponds are planned at the north end of the main Frunway in the outer runway protection zone. These drainage features would be sized to avoid standing water on ground surfaces. A new pump station is planned to pump drainage water from the detention ponds into Mansville Ditch. The detention basins and ponds would provide short-term storage for stormwater drainage and would result in peak flow rates that are slightly less than under existing conditions. Peak flow rates in Weber Slough would decrease slightly from existing conditions because of the decrease in contributing area. According to the drainage study, Mansville Ditch will be subject to detailed evaluation during the final design process to determine whether its capacity is adequate to accommodate 1 anticipated discharge. If the capacity of the ditch is not adequate,the drainage study states that either the ditch will be enlarged to accommodate the anticipated discharge or all or part of the discharge from the retention basins will be diverted to Little John Creek to the north or to Weber Slough to the south. As discussed above and as indicated in the drainage plan, Weber Slough, from the location south of the Farmington Fresh facility to the pumping station and on to French Camp Slough, does not have sufficient capacity to carry existing and anticipated flood flows. The drainage study indicates that this section of Weber Slough will require reconstruction to increase its capacity to carry anticipated flows. In addition, the drainage study states that pumps in the lift station will be evaluated to ensure that they can accommodate anticipated flood flows. Implementation of the proposed drainage improvements are expected to avoid drainage impacts. However,because the actual land uses that are developed could differ from those assumed (subject to the Special Purpose Plan) and because of the inadequate capacity in Mansville Ditch and Weber Slough to accommodate flood flows, the potential impact of project development is { considered significant. Implementation of the following mitigation measure would reduce this impact to a less-than p g g significant-level. E Mitigation Measure DW-1: Develop and Implement a Comprehensive Drainage Plan x The Department of Aviation shall develop and implement a comprehensive drainage plan as part of the Special Purpose Plan to ensure that changes in runoff on the project site as a result of development do not increase peak flow rates exiting the airport property. Improvements could include expansion or modification of retention systems or modification of drainage channels. In addition, the plan shall specify specific improvements or changes to the airport drainage channels r so that flood flows are adequately accommodated in Weber Slough, Mansville Ditch, and related drainage channels. All improvements or modifications to drainage systems shall be designed and constructed according to San Joaquin County Department of Public Works standards. Any modifications to facilities located within the Stockton city limits shall comply with the City's Storm �- Water Management Plan, and the County shall pay all related fees as required. The comprehensive r Stockton Metropolitan Airport Project Chapter 5. Drainage and Water Quality Draft Environmental Impact Report 5-7 March 70, 1995 i