07-09-18 Drainage and Flooding Committee Meeting

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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 (B 106 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 4, 5, and 6 show the inundation conditions in the existing system for the 5- year, 10-year, and 100-year storm events. There are two areas of interest that border the La Porte Municipal Airport property, one to the north along N Avenue H, and the other to the west of the airport, west of Farrington Drive. Both areas were evaluated to determine the following: • sheet flow direction • maximum channel conveyance capacity of the bordering airport ditches • overflow rates for the 2-, 5-, 10-, 25-, 50- and 100-year storm events • water surface elevations for the 2-, 5-, 10-, 25-, 50- and 100-year storm events Exhibit 4 presents the ponding during the 5-year storm event for the existing conditions. Exhibit 5 presents the ponding during the 10-year storm event for the existing conditions. Exhibit 6 presents the ponding during the 100-year storm event for the existing conditions. Farrington. Drive (West of Airport) Hydraulic Analysis The airport channel that runs along Farrington Drive slopes from an elevation of 21.5 ft to an elevation of 16.8 ft. The centerline of Farrington Drive serves as the high bank to the west. Figure 4 and Figure 5 below display the profile and typical cross section of the channel. La Porte Municipal Airport — Offsite Drainage Analysis HT&J, LLC, January 2018 Page 9