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Chapter 2 Sector Six Today I The Natural Environment Geology , Sector Six, along with Sectors Four, Five, 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 generally flat. Topography Land elevations in Sector Six vary from 555.1 feet above sea level along the city limit between Animal Shelter Road and Old Martindale Road to 669.1 feet above sea level in the San Marcos Technology Park along 1lI-35. The Sector Six Topography map is presented 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 Six is located in the saline zone. The saline zone is an area of saline ground water that abuts the artesian zone. I t Watersheds A waterslied is an area that drains into a river or creek. Sector Six lies in tl)e San Marcos River, Blanco River, and Hemphill Creek watershed. Flood Plains There are floodways and flood plains along the San Marcos River, Blanco River, and Bypass Creek. Approximately 33% of the land in Sector Six is located in either the floodways or flood plains. There are 880 acres of Sector Six in the 100-year flood plain, described as an area which has a 1% chance of being inundated by floodwaters in any given year. The 100-year flood plain has been adopted by the Federal Emergency Management Agency as the basis for flood plain management nationwide. There are 144 acres of Sector Six in the 500-year flood plain, an area that has a .02% chance of being inundated by floodwaters in any given year. There are 224 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 increase in flood height. I In San Marcos, development is prohibited in the floodway, a flood plain permit is required for any development in the 100-year flood plain, and a flood plain permit is not required for development in the 500-year flood plain. The Sector Six Flood Plain map is found following the Sector Six Topography map page. 2-3