If the Lewisville Dam were to fail, it would endanger more than 400,000 lives and $22 billion in capital investments, said Stacy Gray, project manager for the Dam Safety Modification Study.
It is highly unlikely the dam will fail. To further reduce the probability of failure, the U.S. Army Corps of Engineers has proposed modifications to the dam in its Environmental Assessment because of the critical damage that would result if it did fail.
The Corps held a public meeting to answer resident questions and discuss comments concerning the dam modification proposal Tuesday evening at the MCL Grand Theater.
The Lewisville Dam is an embankment that is about 33,000 feet long and 125 feet tall, Gray said. It was constructed between 1948 and 1955. The Corps has created 30 measures, or ways of addressing the potential risks, and condensed them into 13 alternatives, or measures put together to form plans.
“Everything is working exactly like it worked when we built it,” Gray said. “As low as reasonably practical, how low can we get that probability [of failure] where it still makes sense to [modify].”
Proposed modifications to the dam focus on risk-driving potential failure modes, according to the draft. Listed are two PFMs identified as risk-driving, followed by other considered PFMs that were presented at the meeting:
- Internal erosion of soil foundation that leads to seepage that progresses to piping and loss of embankment materials
- Instability, uplift and sliding
- Internal erosion of embankment along main water conduit
- Failure of municipal water lines along the embankment toe, resulting in erosion of toe
- Local instability of embankment leading to loss of crest
“Bottom line, we’re worried about seepage. We’re worried about stability,” Gray said. “We want to make sure those things don’t progress into something that can turn out bad.”
Embankments seep, she said, but the concern is if that seepage changes. The Corps monitors the color of the seepage, where it comes from, the amount in volumes that escapes and the water pressure of the seepage.
The draft proposes how to modify the dam for the risk-driving PFMs. The Corps would construct at two different locations inverted filter berms, which would make sure seepage remains clear of soil, with associated collection trenches for seepage flow at each downstream location.
The Corps would reduce risks concerning spillway instability by building post-tensioned anchors with an upstream geomembrane cutoff to support the spillway structure.
It would overlay the apron on the downstream side of the spillway and construct two barrier walls downstream of the spillway to prevent apron panels from moving. The latter would also reduce channel scour and erosion during spillway flow occurrences, the draft says.
The remaining three PFMs are not risk-driving but their inclusion takes advantage of construction efficiencies and reduces overall risk of failure, the draft reports.
To reduce the risks of erosion at the outlet conduit, the Corp would construct a new conduit to lessen stress caused by high volume flows, the draft continues.The Corps would increase slope stability to cut down risks related to slides on the upstream embankment.
“Lastly, the USACE is requiring the City of Lewisville to relocate waterlines that currently encroach on the embankment to reduce the risk of embankment erosion from a waterline rupture,” the draft states.
The Corps proposes to begin construction in early 2018. With different phases, the Corps estimates construction will continue through mid-2024.
Following construction of modifications, consequences remain high. The Corps will maintain a heightened state of awareness and communication with the emergency management community, a presentation slide notes.
Public review is open through Oct. 15. Written comments may be submitted to Marcia Hackett, USACE, Fort Worth District, P.O. Box 17300, Fort Worth, TX 76102-0300, or via email to email@example.com.