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Guide to the numbers:
681 feet msl: Lake Travis is considered full for water supply purposes at 681 feet msl. (At this level Lake Travis holds 1.13 million acre-feet, which is the amount of water it takes to fill more than 1.1 million acres to a depth of one foot of water; this is the equivalent of 369 billion gallons.)
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681 to 685 feet msl: When the lake is forecast to rise above 681 feet msl, LCRA begins flood-control operations. It can release 3,000 to 5,000 cfs by starting or increasing generation at Mansfield Dam hydroelectric power station, as long as the releases keep the river below specified levels at three control points on the Colorado River downstream of the dam. Mansfield Dam has the capacity to generate more than 106 megawatts of electricity. Usually no floodgates are opened within this range.
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685 to 691 feet msl: If LCRA projects Lake Travis to rise between 685 feet msl and 691 feet msl, LCRA may begin operating floodgates — but it depends on the time of the year. There are seasonal rules that guide how much water LCRA can release:
- May and June, September and October: During these months, if Lake Travis is forecast to rise above 685 feet msl, LCRA may release up to 30,000 cfs using a combination of hydroelectric generation and floodgates as long as this amount of water, when combined with local inflows below the dam, does not exceed specified levels at three control points on the Colorado River downstream. At this level, LCRA theoretically could open up to five floodgates given that each of Mansfield Dam's 24 floodgates can release more than 5,000 cfs; however, the potential for flooding downstream may limit this number.
- January-April, July, August, November and December (the rest of the year): During these months, if Lake Travis is forecast to rise above 685 feet msl, but not exceed 691 feet msl, LCRA may release up to 5,000 cfs until the lake falls below 683 msl, depending on downstream conditions.
| Key control points on the Colorado River downstream of Mansfield Dam |
|
Location
|
Control stage
(feet)
|
Est. equivalent flow
(cfs)
|
| Austin (Montopolis bridge at U.S. 183) |
20.5 |
30,000 |
| |
124.8 |
150,000 |
| Bastrop |
25.1 |
45,000 |
| |
126.7 |
150,000 |
| Columbus |
35.5 |
50,000 |
1Control stage when elevation is forecast to exceed 710 feet msl.
CFS: cubic feet per second
|
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691 to 710 feet msl: When the lake is forecast to reach this level, LCRA may release up to 30,000 cfs using a combination of hydroelectric generation and floodgates as long as this amount of water, when combined with local inflows below the dam, does not exceed specified levels at three control points on the Colorado River downstream. Within this range, LCRA theoretically could open up to five floodgates; however, the potential for flooding downstream may limit this number.
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710 to 714 feet msl: If LCRA projects Lake Travis to rise above 710 feet msl, it may release up to 50,000 cfs using a combination of hydroelectric generation and floodgates as long as this amount of water, when combined with local inflows below the dam, does not exceed specified levels at three control points on the Colorado River downstream. Within this range, LCRA theoretically could open eight floodgates; however, the potential for flooding downstream may limit this number. Corps guidelines allow higher stages for the river downstream when the lake reaches this level.
(Another lake level of note, 710.4 feet msl:Lake Travis' all-time high elevation was set on Dec. 25, 1991. At that time, LCRA opened five floodgates.)
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714 to 722 feet msl: If LCRA projects Lake Travis to rise above 714 feet msl, it may release up to 90,000 cfs using a combination of hydroelectric generation, floodgates and flow over the spillway — regardless of downstream conditions. Theoretically, LCRA could open as many as 15 floodgates; however the amount of water flowing over the spillway may limit this number.
If the lake rises above 714 feet msl, water flows over the spillway in the upper center of the dam.
(Another lake level of note, 716 feet msl: Currently this is the level of the 100-year floodplain on Lake Travis. This is set by the Federal Emergency Management Agency. However, the agency has issued preliminary maps that would raise the floodplain on Lake Travis to 722 feet msl. See information from the Texas Colorado River Floodplain Coalition. )
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722 to 750 feet msl: If the elevation of Lake Travis is forecast to rise within this range, the amount of water that be releases is determined by the U.S. Bureau of Reclamation by considering the dam's structural integrity. LCRA could release water regardless of downstream conditions.
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750 feet msl: This is the level of the top of the dam (not including a parapet wall).
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Glossary
MSL– mean sea level
CFS – cubic feet per second
100-year floodplain – The standard used by the National Flood Insurance Program to manage floodplains and determine the need for flood insurance. Local communities require new homes and businesses to be built at or above the 100-year floodplain. The term "100-year flood" can be misleading. It is not the flood that will occur once every 100 years. Rather, it is the flood elevation that has a 1 percent chance of being equaled or exceeded each year. Thus, the 100-year flood could occur more than once in a relatively short period of time.
Federal regulations guide operations of Mansfield Dam during floods: The Code of Federal Regulations, Title 33: Navigation and Navigable Waters, Part 208: Flood Control Regulations, § 208.19 Marshall Ford Dam and Reservoir (Mansfield Dam and Lake Travis), Colorado River, Tex.
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