The decade-old Lake Brazos low-water dam is in urgent need of some $4.5 million in embankment repair, the engineer who designed the dam told the Waco City Council at a retreat Wednesday.

“If you lose it you don’t have a town lake,” said Victor Vasquez of Freese and Nichols. “Of all the things you can do for the dam, this is the most urgent.”

But that work on the eroded embankment just east of the dam is only the beginning of work the firm recommended for the dam. For $7 million, the city could build a boom and gate system that could control the logjams that form after heavy rain and cause higher upstream water levels, Vasquez said.

Together, the work would cost $11.5 million for improvements to a dam that originally cost $19 million.

Council and staff members said they would continue to weigh the brush control options, but they agreed to move forward with the embankment repair next year, along with bypass valves adjacent to the embankment.

“We obviously need to fix the embankment and the three bypass valves,” Deputy City Manager Wiley Stem III said. “I think the boom and gate need to be evaluated compared to what we’re paying now for maintenance.”

Vasquez said the dam itself, completed in fall 2007, is in sound condition. The dam’s “labyrinth weir” design allows water to flow rapidly over a zigzag edge. It replaced 1970-era gate dam that repeatedly failed when brush and logs jammed the gate and hydraulic system.

But that woody debris has posed a different kind of problem for the new dam. After heavy rains, it lodges in the teeth of the labyrinth weir and backs up hundreds of feet. The city has had to hire companies to clean it out periodically, including a major cleanup project this summer that cost $750,000 and removed and estimated 3,000 cubic yards of debris.

Vasquez said the Brazos River probably has the most floating woody debris of any major river in Texas, and it would be a challenge for any dam.

“What you saw in the last three years is a lot more (brush) than normal because of the extreme drought you had,” he said.

Freese and Nichols staff proposed spending about $1.5 million on a floating boom that could allow city workers to skim and divert floating debris.

They also proposed a $5.5 million “bascule gate” just upstream of a section of the dam that could both regulate the river’s flow and intercept debris. Vasquez said the gate would be “much more robust” than the failure-prone gate dam of past decades.

Vasquez and city officials said they would seek funding assistance for the project from the U.S. Army Corps of Engineers.

The brush issue is largely unrelated to the embankment problem.

The embankment is on the east side of the river, where contractor F.M. Young built a bypass system in an effort to capture hydroelectric power. At the outfall of the bypass is a metal wall that is now compromised, with the soil behind it eroded from flooding.

Vasquez said Freese and Nichols’ original design in 2004 called for reconfiguring the bypass area with a stilling basin that would minimize the impact of flooding. But the cost came in too high, and the city rebid the work without those features. State inspectors noted erosion damage after a flood swamped the area during construction in 2007 and recommended that the city continue to monitor the embankment.

At the meeting Wednesday, Mayor Kyle Deaver asked how much it would cost to replace the embankment if it failed during a flood. Vasquez and city staff said it could cost millions more than repairing it now, and it would require the lake to be lowered for more than a year.

In an interview afterward, Deaver said the bank stabilization project is not optional.

“The possibility of Lake Brazos going down for more than a year would have tremendous economic impact,” he said.

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