After November’s torrential downpours, the Army Corps of Engineers discovered a 95-foot slide on Lake Grapevine’s earthen-rolled dam.
The corps’ Fort Worth District, which oversees the lake, insists there is no risk to the dam’s overall integrity and said Tuesday that the $1 million repair should start in April.
Even though there is no immediate risk, corps officials changed Lake Grapevine’s dam classification last year from low urgency to high urgency, the second-most-serious category. Lewisville Lake, which also had a slide last year, was also placed in the high-urgency category in 2014.
At both lakes, the ratings have nothing to do with the recent slides. Instead, the ratings take into account the risk downstream should a catastrophic failure occur.
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Lewisville Lake, which was built in 1955, releases water from the Elm Fork into the Trinity River. Lake Grapevine, which was impounded in 1952, also feeds into the Elm Fork by Denton Creek near Carrollton. The Elm and West forks converge in Dallas County and form the Trinity River, flowing through the heart of Dallas.
“In the previous assessments, we did not look at consequences,” said Stacy Gray, a corps project manager. “We just looked at probability. This realizes consequences as part of the risk.”
Last spring, the corps found a 161-foot-long slide on the upstream face of the embankment at Lewisville Lake.
‘The key is to fix it’
Clay Church, a spokesman for the corps’ Fort Worth district, said the Lake Grapevine slide is not as serious as the one at Lake Lewisville.
Structurally, it’s part of the dam but away from where the water stands.
Clay Church, corps spokesman.
“The slide took place at Grapevine Lake dam on what is normally referred to as the dry side on the east side of the spillway,” Church said. “It's not right on the spillway. Structurally, it’s part of the dam but away from where the water stands.”
The repairs will require the closure of Fairway Drive for about a month. Repairs are expected to begin during the first week of April, but the corps doesn’t have a specific date.
“It’s almost at the top of the roadbed,” Church said. “You basically have to dig it out.”
There is no risk to residents downstream of the dam from the slide, and the lake level will not be lowered.
“The key is to fix it when it happens so it doesn't progress,” Gray said.
The lake is now 10 feet above capacity.
Grapevine spokeswoman Mona Burk said city officials are not concerned about the dam’s safety.
We are confident in the corps’ ability to maintain the dam.
Grapevine spokeswoman Mona Burk
“We are confident in the corps’ ability to maintain the dam,” Burk said.
More assessments coming
In 2014, the corps classified the Lewisville dam as high urgency “because of the risk associated with stability of the spillway during very high flows in the spillway associated with rare flood events and seepage that could lead to internal erosion of the foundation and/or embankment.”
Despite that classification, the corps has downplayed the risk of a failure at Lewisville Lake.
A contractor is working on a $6.4 million repair to the Lewisville dam with work expected to be completed in late spring or early summer, depending on weather.
At Lake Grapevine, it will be at least two years before a hydrology and hydraulics study is completed that could recommend more improvements to the dam.
The corps’ Fort Worth District owns and operates 25 reservoirs statewide. The only other dam with a high-risk rating is the Stillhouse Hollow reservoir in Bell County.
But other North Texas corps lakes will be assessed under the new guidelines for 10 years. Other corps lakes include Benbrook, Joe Pool, Lavon, Ray Roberts and Whitney.
Gray stressed that the nothing has changed about the dams.
“Know the structure is exactly the same structure that's been in place for decades,” Gray said. “It's doing exactly what it's supposed to do. The corps has simply started factoring consequences into their assessment of risk."
Dam classification system
- Class I (Very High Urgency): Dams where progression toward failure is taking place under normal operations and the dam is almost certain to fail under normal operations in a time ranging from immediately to a few years without intervention; or, the combination of life or economic consequences with probability of failure is extremely high.
- Class II (High Urgency): Dams where failure could begin during normal operations or because of an event. The likelihood of failure from one of these occurrences, before remediation, is too high to assure public safety; or, the combination of life or economic consequences with the probability of failure is very high.
- Class III (Moderate Urgency): Dams that are significantly inadequate; or the combination of life, economic or environmental consequences with the probability of failure is moderate to high.
- Class IV (Low Urgency): Dams that are inadequate with low risk such that the combination of life, economic or environmental consequences with the probability of failure is low, and the dam may not meet all essential corps engineering guidelines.
- Class V (Normal): Dams considered adequately safe, meeting all essential agency guidelines, and the residual risk is considered tolerable.
Source: Army Corps of Engineers