07-09-18 Drainage and Flooding Committee Meeting

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IML FIGURE 7 - XPSWMM MODEL LAYOUT For the 5-year and 10-year storm events, the tailwater was modeled with the water surface at the top of the outfall pipe (10 x 6 feet box outfall) into the Big Island Slough. For storm events larger than the 10-year event, the tailwater conditions were modeled using a stage time series. The time series was constructed from the HEC-HMS hydrograph and the HEC-RAS flow vs. stage relationships in the Big Island Slough channel (13106 00 00). Flow vs. stage data for different storm events was obtained from FEMA effective HEC-HMS and HEC-RAS models. The neighborhoods to the west of the airport property (both north and south of Spencer Highway) have their own storm sewer systems that outfall to the Big Island Slough separate from the Spencer Highway system. Ditches that overbanked or culverts that were surcharged were identified and on -site ponding was delineated. Exhibits 5, 6, and 7 show the inundation conditions in the existing system for the 5-year, 10-year, and 100-year storm events. Supplemental Table C presents the maximum water surface elevation for each culvert and natural channel for the 5-year, 10-year, and 100-year storm events. Existing System Analysis Results The existing drainage conditions do not currently comply with the FAA criterion of no encroachment of runways/taxiways during the 5-year storm event and no ponding on the center 50% of runways/taxiways during the 10-year storm event (See Exhibits 5 and 6). Page 120