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Sector Five Chapter 2 - Today The Natural Environment Geology Sector Five, along with Sectors Four, Six and Seven, is located in the Blackland Prairie to the east of the Balcones Escarpment. The underlying material in the area is erodible chalk and clay-shale. The base is generally covered by thick, organic rich clay soils. Most of the terrain is nearly flat. Topography Land elevations in Sector Five vary from 683 feet above sea level behind the Central Texas Medical Center to 548 feet above sea level at the Cape Road bridge crossing over the San Marcos River. Sector Five topography is shown on the Natural Features map on the following page. Edwards Aquifer Zone The Edwards Aquifer is the water-bearing underground network of porous and honeycombed limestone formation of the Edwards Plateau. It is the source of water for San Marcos, as well as over 1.5 million other Texans, and is the source of the San Marcos River. Sector Five is located in the saline zone. The saline zoned is an area of saline ground water that abuts the artesian zone. Watersheds A watershed is an area that drains into a river or creek. Sector Five lies in the San Marcos River, Cottonwood Creek, and York Creek watersheds. Floodplains There are floodways and floodplains along both the San Marcos River and Cottonwood Creek. Approximately 20% of the land in Sector Five is located in these floodways or floodplains. There are 148 acres of Sector Five in the 100-year floodplain, described as an area which has a 1 % chance of being inundated by floodwaters in any given year. The 100-year floodplain has been adopted by the Federal Emergency Management Agency as the basis for floodplain management nationwide. There are 106 acres of Sector Five in the 500- year floodplain, an area which has a .2% chance of being inundated by floodwaters in any given year. There are 107 acres of the sector located in floodways and river channels. These areas must be kept free of encroachments so that a 100-year flood can be carried without substantial increases in flood height. In San Marcos, development is prohibited in the floodway, a floodplain permit is required for any development in the 100-year floodplain, and floodplain permits are not required for development in the SOO-year floodplain. Sector Five floodways and floodplains are shown on the Natural Features map on the following page. Adopted: April 22, 2002 2-3