AUSTIN – LCRA’s handling of the latest round of heavy rains and floods reflects the guidelines it must follow in maintaining flood releases from Lake Travis and Mansfield Dam.
Lake Travis peaked early Sunday afternoon (Aug. 19) at 686.64 feet above mean sea level (msl), almost 6 feet into its flood pool. LCRA is discharging the floodwaters through releases from the hydroelectric power station at Mansfield Dam, a process that should return Travis to its full elevation of 681 feet msl by the Labor Day weekend if there are no additional rains.
That procedure may have surprised some residents who live deep in the Lake Travis flood pool and who want LCRA to open floodgates at Mansfield, as it did during the last flood in late June, to accelerate the releases.
But LCRA did not open floodgates because it is following operating guidelines developed by the U.S. Army Corps of Engineers for Lake Travis and Mansfield Dam.
Mansfield Dam, Lake Travis designed to provide flood protection To put things in perspective: LCRA built Mansfield Dam and formed Lake Travis in the late 1930s and early ’40s, among other reasons, to protect residents and communities downstream of the dam from the worst effects of floods. Mansfield and Travis are the only elements in the Highland Lakes chain specifically designed to capture and hold floodwaters until they can be safely released downstream.
When Lake Travis exceeds its full elevation of 681 feet msl, it is in its “flood pool,” meaning LCRA may inundate property in that area, as necessary, to hold floodwaters until they can be released downstream.
“Flood protection for downstream communities is the primary reason why Lake Travis and Mansfield Dam are designed they way they were,” explained Mark Jordan, manager of LCRA River Management Services. “If Travis and Mansfield had been designed primarily for other purposes, such as recreation, they would be operated quite differently.”
LCRA follows guidelines in making decisions on flood releases
When floodwaters push Travis above its full elevation, the Corps of Engineers guidelines spell out how LCRA must release floodwaters from Lake Travis without endangering downstream communities, using time of year and downstream conditions as governing factors.
For eight months of the year, including August, LCRA can open floodgates at Mansfield Dam only if the elevation at Lake Travis is forecast to go 691 feet msl or higher and if the releases would not push downstream river elevations above certain flood stages.
“The goal is not to release floodwaters on top of floodwaters if it’s raining heavily downstream,” Jordan explained.
During the other four months – May, June, September and October – LCRA could begin opening floodgates at Mansfield Dam once the lake’s elevation is forecast to reach 685 feet msl or higher and downstream conditions are met. The lower trigger elevation reflects a history of much wetter weather during those four months.
These factors guided LCRA’s decision not to open floodgates at Mansfield Dam for the latest flood event. The highest lake elevation forecast for Travis was 690 feet msl, 1 foot below the 691 minimum for August. Had the flood occurred in September, LCRA might have been able to open a floodgate – but only if the releases would not create or add to flooding downstream.
Flood protection is intended for downstream residents
When floodwaters flow into Lake Travis, news coverage often focuses on the few dozen or so residences deep in the lake’s flood pool – often overlooking the point that the affected residents have chosen to live in an area that was designed to hold floodwaters to protect Austin and other communities downstream of Mansfield Dam.
“The more than 1 million residents in these downstream communities are the intended beneficiaries of flood-management operations at Mansfield Dam,” Jordan noted. “Many of these communities have existed along the Colorado River for more than a century and a half. On the other hand, most of the residences in the Lake Travis flood pool have been built in the years since Mansfield Dam’s completion in the 1940s. We continue to communicate the flood risks to those property owners.
“In any flood situation, LCRA must balance the interests of those who have chosen to live in a flood-control reservoir against the interests and safety of those for whom Mansfield Dam and Lake Travis were built to protect,” Jordan said.
Key elevations for Lake Travis during floods
LCRA operates Lake Travis and Mansfield Dam during floods under federal regulations developed by LCRA and the U.S. Army Corps of Engineers. These regulations determine how much water LCRA may release, depending on how much water is in the Lake Travis flood pool and downstream conditions. Below is a summary. Click on the colored ranges for more detail. |
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Guide to the numbers:
681 feet msl: Lake Travis is considered full 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 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 |
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Location
|
Control stage
(feet)
|
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 msl. |
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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 note710.4 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 Floodplain Coalition.
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722 to 750 feet msl: If the elevation of Lake Travis is forecast to rise within this range water is released to protect 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.
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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" is 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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