DOUGLAS — A pair of engineering firms contracted by the state warn that cracks in the 112-year-old concrete LaPrele dam, along with deterioration in its geologic foundation, could result in catastrophic failure.
Such an event would threaten people and infrastructure downstream, including the town of Douglas, and likely destroy the Ayres Natural Bridge Park directly below the dam, as well as two Interstate 25 bridges, the Wyoming Water Development office said during a tour last week.
“This is more than an irrigation district matter,” Water Development Office Director Brandon Gebhart said. “It’s a hazard.”
Rehabilitating the dam might be impossible, according to Gebhart. Instead, the office and its governing citizen commission is considering building a new dam directly below the existing structure. The commission will consider seeking funds to develop a complete engineering study for a project that could exceed $50 million.
Such an effort may qualify, in part, under several provisions in the $1 trillion infrastructure bill passed by the U.S. Senate this month, buttressing a Wyoming policy ambition to impound more water for use before it leaves the headwaters state, according to state officials. Meantime, the state is examining multiple other funding sources due to the potential for loss of life and property if the dam were to fail.
The matter is urgent, officials say.
“It’s not a slow failure with this dam design,” Gebhart said. “It would be very abrupt.”
The water office organized a public tour of the dam Aug. 12 to better inform the public of the risks and potential solutions under consideration.
A rumble and a warning
When a magnitude 3.7 earthquake rumbled the towns of Glenrock and Casper on the night of Aug. 1, personnel at the nearby Ayres Natural Bridge Park said it felt like a sonic boom. They sprang into action, caretaker Dee McDonald said.
Dee and her husband Doug McDonald have served as caretakers for six years.
The natural arch was still intact. But their minds were on a more pressing danger still looming: water. So they evacuated the handful of campers in the park that night.
“Anybody with any sense would do that,” McDonald said.
The park is situated in a small geologic bowl filled with old boxelder and tall cottonwood trees, along with manicured grass and picnic areas surrounded by high sandstone cliffs carved out over millennia by LaPrele Creek. The creek runs directly under the natural bridge. The rare high-plains oasis was a retreat for European settlers traveling the Oregon Trail. Today it remains a popular leisure destination for locals and travelers alike.
Visitors from around the world packed into the park on Aug. 21, 2017 to experience the totality of the solar eclipse that swept across central Wyoming.
Less than two miles upstream, however, is a not-so-natural wonder: the LaPrele dam. Completed in 1909, the concrete structure today presents a danger to an area far beyond the Ayres Natural Bridge Park, state officials and engineering teams say.
Aging, patchwork infrastructure
The Ambursen-style-designed LaPrele dam consists of a series of concrete buttresses supporting an angled, flat slab on the reservoir side. It is 130 feet high and 325 feet wide, and serves late-season water to about 100 irrigators via 94 miles of irrigation infrastructure, according to the state. The dam is anchored into a fractured Madison limestone formation on both sides.
Construction was funded via the federal Carey Act of 1894, a measure pushed by Wyoming U.S. Sens. Francis E. Warren and Joseph M. Carey to help arid western states develop more water for irrigation.
In the 1970s, the LaPrele dam was determined to have reached the end of its useful life. But dozens of irrigators downstream still depended on late-season releases from the reservoir to help them eke out a living on the plains along the North Platte. Neither the state nor federal government were eager to pay for refurbishing the dam, so the Panhandle Eastern Pipeline Co. agreed to fund the repair effort in return for a share of water for a coal-gasification project.
Crews grouted cracks and added new layers of concrete to the dam. Panhandle Eastern Pipeline Co., along with the lead engineering company that oversaw the project, received the “Outstanding Civil Engineering Achievement of 1979” award from the American Society of Civil Engineers.
Panhandle Eastern’s coal gasification project never came to fruition. Grateful for the investment and newfound confidence in the dam, however, members of the LaPrele Irrigation District went about their business of relying on the patched up dam for late-season water.
Then, in 2016, a boulder in the west limestone wall behind the dam fell. The event was listed as a mere notation in records reviewed a couple years later by a team commissioned to assess water and water infrastructure along the North Platte.
Discovered by accident
When the team of engineers examined the rockfall behind the LaPrele dam, it alarmed them.
Peter Rausch of the engineering firm RESPEC said that during the initial inspection it appeared that if another large boulder in the same strata were to fall it might roll into one or more of the dam’s concrete fins. Then, looking at the potential targets, they noticed what looked like a large, unrepaired crack in one of the boulder-zone concrete structures.
RESPEC called another firm, HDR, which specializes in dams, to assess the structure itself. Apart from the potential of damage via rockfall, developing cracks in the dam pose a risk of catastrophic failure, according to the company.
In November 2019, the Wyoming Water Development Office ordered the LaPrele Reservoir be maintained at a lower level to avoid stress on the dam. Consequently, the LaPrele Irrigation District receives about 55% of its normal appropriation of water, according to the state.
That’s concerning enough for eastern plains irrigators facing a warming, drier Wyoming where late-season irrigation is becoming increasingly vital in the face of a global climate crisis. But the risk of a dam failure, and the catastrophes that might result, add to anxieties — even if it might move a new dam construction project higher up the list of infrastructure priorities.
“This is a classic example of aging infrastructure,” Gebhart said. “We were lucky to find this — it was happenstance that we found the deficiencies in this.
“I think there’s probably a lot of infrastructure in the state, not just irrigation but also municipal, that is maybe at or near the end of its useful life,” Gebhart said. “Some of it may not even be known. It’s an example of a significant problem I see in the state.”