Dallas residents have been haunted by the specter of a 65-foot-tall flood wave thundering down city streets ever since the deteriorating condition of the Lewisville Lake Dam became public knowledge last year. If the 60-year-old earthen dam gave way, Dallas would be under 50-feet of water.
This nightmare scenario sent North Texans into panic, Corps officials into damage control and local, state and federal politicians into motion. They raised enough money to complete repairs on the 160-foot long shallow slide, but completion of the project, which could cost several hundred million dollars, is still several years away.
The Corps is now seeking public comment on a proposed draft of their plan to fix the dam, which it calls Environmental Assessment Proposed Dam Safety Modifications:
The purpose of the Proposed Action is to minimize the potential for dam failure by addressing deficiencies at the Lewisville Dam. The Proposed Action accomplishes this by addressing the seepage deficiencies, spillway weir instability, and apron failure at the Lewisville Dam for safe and effective functioning at authorized capacity, while reducing the risk to the downstream public to tolerable levels.
Deficiencies at the Lewisville Dam became apparent during the May 2015 record rainstorm when floodwaters filled the Lewisville reservoir, causing the Corps operations teams to spend days and nights, with flashlights in hand, walking the dam, searching for signs of distress. They found it in the form of a “sand boil,” a small whirlpool sprouting from under ground.
A sand boil indicates excessive seepage eroding soil material from the dam’s foundation. It’s a problem that could lead to a condition called “piping” and, if not repaired, to the nightmare scenario. A layer of sandbags was used to equalize the pressure causing the whirlpool, preventing the expansion of water and soil particles in the dam’s foundation.
“It would be a much bigger magnitude to the Dallas area than Hurricane Katrina was to New Orleans,” one former Corps dam safety coordinator told The Dallas Morning News in December 2015. “It’s a nightmare scenario.”
In the draft of the environmental assessment, the Corps points out that the probability of the nightmare scenario occurring is remote but not remote enough to ignore.
The Corps plans to reinforce the areas where dam failure is likely. The plan proposes to build trenches, braces, water conduits and anchors to reduce the risk of dam failure. The Corps also wants to construct new conduits to reduce erosion and stress from high waters, increase the embankment to reduce the risk of slides and require the city of Lewisville to relocate water lines currently encroaching on the embankment.
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Construction may begin in early 2018 or 2019 and finish in 2023 or 2024. The draft of the environmental assessment of dam safety modifications is open for public comment until Oct. 15. Comments can be submitted via email to firstname.lastname@example.org
A public hearing will be also held from 6 p.m. to 8 p.m. on Tuesday, Sept. 27, in the Black Box Theater Room at the Lewisville Grand Theater.
“Life safety is our main priority within the Army Corps of Engineers," Col. Calvin C. Hudson II, commander of the Corps Fort Worth District, said in the press release. “Our Dam Safety Program seeks to ensure that Corps-managed and operated dams present minimized risks to life, property and the environment. Dams are designed and built to reduce flood risk, but they cannot eliminate all risk."