07-09-18 Drainage and Flooding Committee Meeting

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Land. t.Jse The majority of the 300 acre airport site is open area covered in native grasses. The impervious area on the site is made up of runways, taxiways, parking lots, storage lots, and some commercial area. Approximately 20 percent of the site is impervious. The Glen Meadows neighborhood to the west is a single family residential development with an average lot size of approximately 0.2 acres. The neighborhood to the north along N Avenue H is made up of larger residential lots of approximately 2 acres. The lots along N Avenue H are made up of mostly open space covered in grass. Existing Drainage Structures The drainage infrastructure on the airport is made up of Reinforced Concrete Pipe (RCP) culverts and ditches in that outfalls into an underground storm sewer in Spencer Highway. The existing onsite RCP culverts range in diameter from 15-inch to 36-inch. A roadside ditch along Farrington Drive receives runoff from approximately 70 acres. This roadside ditch, on average, is about 2.5 feet deep and top width varies to maximum 30 feet. Two culvert structures (See Exhibit 2) restrict flow to the system. The drainage infrastructure in the neighborhood to the north of the airport site consists of grass -lined drainage ditches and RCP culverts to transfer runoff beneath driveways. The ditch along the north property boundary receives runoff from approximately 76 acres. Three ditch systems converge to a 1.6 ft deep, 35 ft wide ditch which drains south along the east edge of the airport property toward Spencer Highway (See Exhibit 3). The drainage infrastructure in the neighborhood to the west of the airport site consists of an underground storm sewer system containing RCP ranging in size from 18-inch to 60-inch and streets that are curbed and guttered. The storm sewer flows west and has three separate outfalls to the Big Island Slough channel. Storm sewer information for the Glen Meadows subdivision was obtained from the City of La Porte online GIS system. The Glen Meadows Park along the Big Island Slough channel at the north end of the subdivision functions as a detention basin for part of the drainage system with a footprint of approximately 7.3 acres. Exhibit 1 presents the Harris County watershed boundaries. Exhibit 2 presents a detailed look at the existing drainage infrastructure for the Farrington Drive ditch. Exhibit 3 presents the existing drainage infrastructure for the ditch along the northern boundary of the airport. Existing Oulfffls The drainage system along North Avenue H to the north of the airport property outfalls into a roadside ditch system along Sens Road. La Porte Municipal Airport — Offsite Drainage Analysis HT&J, LLC, January 2018 Page 4