Managing floods in Flash Flood Alley

The Hill Country and Central Texas have a greater risk of flash flooding than most regions of the United States. The region of Texas is called Flash Flood Alley because of the area’s steep terrain, shallow soil and unusually high rainfall rates. Heavy rains can quickly transform into walls of fast-moving water with great destructive potential.

During a flood, LCRA operates the Highland Lakes dams to reduce the intensity of flooding downstream by managing the flow of floodwaters through the lakes and river system.

The Highland Lakes system

When flooding on the lakes or their tributaries is imminent, LCRA works to manage the floodwaters by holding or moving water as needed through a series of dams along the Highland Lakes. Of the six Highland Lakes, only Lake Travis – formed by Mansfield Dam – is designed to hold back floodwaters that otherwise would flood Austin and downstream communities. See how the Highland Lakes system works.

Lake Travis is considered full at 681 feet above mean sea level (feet msl). At that level, the lake contains 1.1 million acre-feet of water in its conservation pool for water supply. Lake Travis is designed to hold an additional 776,062 acre-feet of water in what is referred to as the flood pool. The flood pool stretches from 681 feet msl up to the Mansfield Dam spillway at 714 feet msl.

Upstream of Lake Travis, Lake Buchanan has no flood pool. It has little additional capacity for floodwaters when it is considered full. The smaller, pass-through lakes – Inks, LBJ, Marble Falls and Austin – have no extra capacity for floodwaters.

Managing a flood

The hydrologists and engineers in the LCRA River Operations Control Center (ROCC) monitor the lower Colorado River and Highland Lakes constantly. Crews at the dams are on around-the-clock alert whenever floods threaten, and work closely with the experts in the ROCC to open floodgates and move water through hydroelectric generation as needed to manage floodwaters.

LCRA conducts flood operations at Buchanan, Inks, Wirtz and Starcke dams according to a 1990 agreement between LCRA and the Federal Emergency Management Agency (FEMA). Read more about the operation of Buchanan Dam.

LCRA conducts flood operations at Mansfield Dam according to the U.S. Army Corps of Engineers (USACE) Water Control Manual for Mansfield Dam and Lake Travis. The USACE updated its flood regulations for Mansfield Dam and Lake Travis in 2014 to reflect changes since the previous regulations took effect in 1979. The updated Water Control Manual continues to limit flood releases from Mansfield Dam based on key Lake Travis elevations and expected conditions along the Colorado River downstream of Mansfield Dam.

RESOURCES

Operating Mansfield Dam

The Water Control Manual includes the 2014 Letter of Understanding and Water Control Agreement between LCRA and U.S. Army Corps of Engineers.

Federal flood control regulations for Mansfield Dam and Lake Travis.