Historic Texas Flooding Fuels Fears Of Catastrophic Dam Failures

AUSTIN, TEXAS — Amid a looming threat of additional potential flooding in the region after recent heavy rains, area residents are casting a wary eye on the Central Texas skies wondering when the next deluge will come.

To be sure, downtown Austin is well protected from major flooding events given the Highlands Lakes system — a chain of six dams along the Colorado River designed as safeguard. The network of dams has been successful in protecting life and property since being built between 1938 and 1951 as protective buffer in the Colorado Basin.

But even then, anxiety builds in the wake of historic flooding this week across Central Texas as residents remain fearful of future deluge. And when one considers a recent National Oceanic and Atmospheric Administration report showing not only that major rainfall events are more frequent, but drop some 33 percent more water than previously believed, anxiety is heightened further still.

House in Marble Falls near Cottonwood Shores floats away in flood waters after recent Central Texas rains.

That leaves an untold number of residents in the immediate area —particularly those close to watershed areas — asking: What would happen if Mansfield Dam or Tom Miller Dam failed? Using data from the Lower Colorado River Authority, LawnStarter Inc., an app-driven landscaping service, devised illustrations dramatically illustrating just such nightmarish scenarios.

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Those LawnStarter-devised maps have a chilling effect when seen, rendering envisioned flood-caused catastrophe less of an abstraction. For starters, here’s what what river flow and lake levels normally look like, as illustrated by LawnStarter:

And here’s what the water paths would like like in the event Tom Miller Dam were to burst, as depicted by the red overlay:

Located near the Red Bud Trail in West Austin, the Tom Miller structure is the upstream dam that forms Lady Bird Lake.


Were it to fail, LawnStarter analysts found, the breach would cause moderate flooding of downstream areas — chiefly some of Auditorium Shores and along Cesar Chavez Street, according to researchers. Past Interstate 35, areas normally occupied by small creeks would swell and expand to the natural banks for the Colorado.. A large amount of Roy G. Guerrero park would be inundated.

Related stories:

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Incoming Heavy Rain Prompts Flood Warnings For Travis, Williamson

Property damage from a dam breach would be about $539 million, affecting 4,357 residents across some 1,995 housing units with 21 surrounding bridges in peril, according to LawnStarter predictors.

But in the same report, researchers at LawnStarter seemingly seek to assuage residents in Tom Miller Dam’s midst: “Damage is largely mitigated due to the relatively low volume of water contained by the Tom Miller Dam,” they wrote. “The dam is responsible for holding 21,000 acre-feet back, which is only 1.8 percent of the volume held back by Mansfield Dam.”


But in terms of Mansfield Dam, the devastation its failure would cause would be exponentially greater, according to LawnStarter researchers. Here’s what the escaped waters’ pathways would look like:

The resulting property loss, human toll and collateral damage a Mansfield Dam break could wreak rivals the worst scenarios of a Hollywood disaster movie script. To wit:

Widespread destruction would be assured given astonishingly massive volumes of unleashed water. Water escaping from the Mansfield Dam would dispel across an expansive area:

Mansfield Dam is located by FM 620 near Lakeway, west of Austin. “It is the only dam meant to hold back floodwaters in the chain, making it incredibly important for Austin and downstream cities,” LawnStarter analysts wrote. “If the dam were to fail it would unleash 1.1 million cubic feet of water per second. This high flow rate would also continue for a substantial amount of time due to the size of Lake Travis (1.1 million acre-feet).”

Analysts noted the projections take into account subsequent failure of Tom Miller Dam given its inability to withstand the surge of floodwaters in the envisioned scenario. “Needless to say, Austin would be heavily affected by a flood of this magnitude, with the potential loss of life in the thousands and the large-scale destruction of Central Austin.”

The sheer destruction by the initial wave of water will be catastrophic, researchers said. Depending on the size of the breach, a wall of water would be blasted out at speeds in excess of 1,000 miles per hour, and could still be traveling at 50 miles per hour by the time it reaches Austin, according to the research.

Thankfully, those are imagined scenarios even while rooted in actual available data. Which is not to say breaches haven’t happened in the past. Austin was twice flooded twice in the early part of the last century as a result of dam failures. “Early dam projects had trouble trying to control the mighty Colorado and actually left the city impoverished for years afterwards,” LawnStarter researchers wrote “It took federal funds and the founding of the Lower Colorado River Authority (LCRA) to build the current system of dams that protect Austin.”

Despite their flood-induced swollen state the dams are holding tight. To alleviate their burden, Lower Colorado River Authority officials have diligently been releasing water piecemeal by opening floodgates piecemeal toward mitigating over-capacity. With their considerable expertise, massive collective doses of hope and no small measure of prayer, those imagined LawnStarter scenarios will never come to pass.


Ironically, LCRA was in the midst of replacing the original floodgates on Tom Miller Dam before this week’s flooding as part of a project “…to help ensure the dam operates reliably and safely for generations to come,” officials wrote on their website. The $9.9 million project continues LCRA’s investment in the dam and marks the dam’s second extensive renovation since it was completed in 1940, LCRA officials explained.

During the 18-month project, plans called for the nine floodgates to be removed one at a time and replaced with newly constructed, custom-made floodgates as part of work that began in August 2018. “Tom Miller Dam will remain capable of responding to floods and water supply needs throughout the project,” LCRA officials wrote. For residents with questions about the project, officials direct them to: AskLCRA@lcra.org.

Located on the Colorado River within the city limits of Austin, Tom Miller Dam impounds Lake Austin, with its downstream side is the beginning of Lady Bird Lake. Aided by fund from the Public Works Adminstration, the city built the dam for flood control purposes and for generating hydroelectric power. It’s named after Robert Thomas Miller, a former Austin mayor.

The dam began operating in 1940, and is located where two previous dams once stood. Both of those previous dams were destroyed during major floods.


Formerly known as Marshall Ford Dam, the Mansfield Dam is located across a canyon at Marshall Ford on the Colorado River, 13 miles northwest of Austin. Its groundbreaking ceremony took place on Feb. 19, 1937, with U.S. Secretary of the Interior Harold L. Ickes in attendance. The project was a collaborative effort among the LCRA and the U.S. Bureau of Reclamation, with partial funding coming from the Public Works Administration. Construction of the dam was completed in 1941.

The name of the dam was changed to honor U.S. Rep. J.J. Mansfield. Lake Travis is the reservoir behind this dam, which is owned and operated by the LCRA. It’s 278 feet high, 7,089 feet long, and 213 feet thick at the base.

View LawnStarter Inc. study here.

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>>> Top photo: View of the rain-swollen Colorado River from the MoPac Expressway (Loop 1), just west of Zilker Park, early afternoon on Friday, Oct. 19, 2018, and video of same by Tony Cantú/Patch staff. Maps courtesy of LawnStarter Inc., used with permission.

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