07-09-18 Drainage and Flooding Committee Meeting

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IML Floodplains The La Porte Municipal Airport is split between FEMA FIRM panels 48201C0940M and 48201C0945M, both effective 1/6/2017. Only the southwest corner of the airport site is located within the 500-year FEMA floodplain, as shown in Exhibit 3. The rest of the site is located in the "Area of Minimal Flood Hazard"; outside the 100-year regulatory floodplain. Exhibit 3 presents the FEMA Floodplain at the airport site. The 100-year Base Flood Elevation (BFE) ranges from elevation 19.70 near Spencer Highway to elevation 21.0 at the north property boundary line. Topography and Drainage Topography of the site was developed using the detailed survey provided by Gessner Engineering. Digital Terrain Model (DTM) was developed from the detailed survey and drainage areas were determined for each culvert and drainage structure using GIS tools. Overall, the site generally slopes north to south, with isolated high and low points. The ground elevations on site range from approximately 24 feet to 20 feet. For the modeling, the total site is divided into 120 sub basins for detailed analysis. Land Use 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. Existing Drainage Structures The drainage infrastructure is made up of Reinforced Concrete Pipe (RCP) culverts and natural channels in a connected network that outfalls into an underground storm sewer in Spencer Highway. The existing on -site RCP culverts range in diameter from 15-inch to 36-inch. The entire site primarily drains into a roadside ditch along Farrington Drive to the west, a shallow swale along the north and east property line that are ultimately connected to storm sewer system at Spencer Highway. The Roadside ditch, on average, is about 2.5 feet deep and top width varies to maximum 30 feet. Two culvert structures 118 and 166 (See Exhibit 3.3) force this roadside ditch to act as a linear detention pond. In a regional watershed divide, the area north east of Runway 12/30 is delineated to the San Jacinto and Galveston Bay watershed and is supposed to flow northwards. However, the previously installed internal drainage of interconnected swales along the north and the east property line allows runoff to flow southwards instead. See Exhibit 3 for the Harris County watershed boundaries and Exhibit 4.0 for the drainage infrastructure overview. Page 115