AUSTIN (KXAN) — Mansfield Dam forms Lake Travis, the largest water supply reservoir in the Highland Lakes and last line of defense against devastating floods funneling into Austin.
Built and placed between 1937 and 1942, the dam’s 24 floodgates are showing signs of corrosion and deterioration. While the dam is still safe and fully functional, the Lower Colorado River Authority says it is time to pull each of the 50,000 pound floodgates, rehabilitate and replace them.
Mansfield Dam by the numbers
278 feet high
7,089 feet long
3 miles of tunnels inside
1.8 million cubic yards of concrete
Two train tracks lie in a cool, damp tunnel 150 feet below the surface of Lake Travis. Walking the length of the tracks takes you by each one of the 25-ton, 30 foot tall metal slabs that hold back Lake Travis’ floodwaters.
In order to remove each floodgate, hoists lift the cartridge-shaped piece of metal out of the concrete, then place it on a cart on the parallel tracks. Since there is no locomotion inside of this four-foot wide tunnel, the brawn of five to six LCRA workers is required to manually push the slab down the tracks. Once they haul the cart and floodgate to its first daylight in more than 70 years, a massive crane lifts the assembly off of the dam.
After removing the floodgate from the dam, crews refurbish the more than 5,500 individual pieces, each designed with hand drawn blueprints in the 1930s. Floodgate motors are sent offsite to be refurbished by a contractor, while LCRA crews clean and replace other components. Each gate is fully tested before being hauled back into place.
Crews have five floodgates done with 19 still remaining. While they are able to take one or two gates offline at a time, complete flood readiness is required 24 hours a day at the dam, ensuring the repairs do not affect emergency operations during heavy rain.
During construction of the dam in 1938, historic rainfall flooded the Colorado River near Austin, with flows reaching 260,000 cubic feet per second — or 7 billion gallons an hour. In response to the flood and damage at the construction site, the height of Mansfield Dam was raised 78 feet, to 278 feet, where it remains today. This makes Mansfield the tallest dam in Texas. The spillway has never been topped, but floodwaters during the Christmas flood of 1991 came within 3.5 feet.