State: New dam preferred option for LaPrele Creek

The 111-year old concrete LaPrele Dam, completed in 1909, has come to the end of its lifespan, Wyoming Water Development (WWDC) Director Brandon Gebhart said Dec. 16.

The historic dam is not sitting pretty these days and contains documented structural damage. LaPrele is the only concrete dam in Converse County, and, should it fail, it could possibly have catastrophic results in this county, according to the emergency action plan posted on the Converse County website.

The LaPrele Irrigation District owns the dam, located approximately 27 miles outside of Douglas. 

The irrigation district applied to the Wyoming Water Development Commission to start a level one master plan study in 2018, Gebhart said, which is when potential rockfall was discovered and cited as being a danger to the dam.

The commission contracted with engineering firm RESPEC, of Rapid City, South Dakota to study the dam and pinpoint not only the problems LaPrele was experiencing, but solutions for the LaPrele Irrigation District, he said.

“We looked at the structure and the transmission of water. We took a cursory look at the dam. The consultants identified potential rock fall above the dam. As a result, we entered into a level two study, where we looked at the potential of falling rock and what the impact would be on the dam. As a result, we took a more-detailed look and found deficiencies in the dam which warranted further analysis. We put the rock fall on the back burner and began to focus on the deficiencies.

“Through the deeper analysis, we found out about significant issues with the dam and extended the level two study to fund additional analysis on LaPrele,” Gebhart said.

The commission made the decision earlier this year that LaPrele Dam has reached the end of its lifespan and other alternatives needed to be found.

The water development commissioners discussed the dam’s rehabilitation in a board meeting Dec. 10. Gebhart said among the many options under consideration, rebuilding a dam further downstream was the preferred choice of action, rather than investing possibly the same amount of money – anywhere from $50 million to $80 million – to fix the existing dam.

“All of the alternatives being evaluated for either rehabilitating the existing LaPrele Dam or replacing the dam would be on LaPrele Creek.  All of the alternatives evaluated to date would be immediately downstream and in very close proximity to the existing dam,” he said.

WWDC’s LaPrele Dam Project Manager Chace Tavelli said five alternatives have been identified; four different types of dam structures which would be built just downstream of the existing dam; and the fifth alternative would be to in-fill the existing dam.

“Another alternative would be to decommission the dam and breach the existing structure to the extent necessary that it cannot store water.  This option exists but is not desirable, as it would mean the water users would no longer have stored water to irrigate their crops,” Tavelli explained.

Water district commissioners made a decision to request $4.3 million to continue with the LaPrele Dam project. The money will be put toward a new dam’s conceptual design, some of the required permitting which will be necessary for it to be built, as well as looking for funding from outside sources, Gebhart said.

“It can be tricky at this point to pin down a number, but the total project could come in the range of $50-$80 million. Multiple options also fall into that range, but at this point, we’re looking at rebuilding the dam as the preferred choice. That doesn’t mean it is a final alternative yet, without losing sight that other alternatives could be brought to the table for consideration,” the director stated.

Last year, the commission (via the legislature) allocated $650,000 toward the LaPrele Dam project.

“We’ll ask for $4.3 million in the water bill which will go to legislation. The commission makes recommendations every year, one for planning and one for construction. The recommendations go to the legislature and that’s how funding is appropriated. If we’re successful with the funding request, then we will continue to identify and secure additional funding for the project, continue designs and fieldwork, and initiate permitting analysis,” Gebhart said.

 LaPrele Irrigation District President Colt Rodeman said in March during the shareholders’ meeting that there’s vertical and horizontal cracking in the dam’s buttresses and he expected it would cost upwards of $100 million to repair the structural damage.

The LaPrele Dam is the oldest and tallest open-front dam in the nation. The structure is located above Ayres Natural Bridge on LaPrele Reservoir about 21 miles out of Douglas via I-25 and about three miles down Natural Bridge Road.

State engineers have mandated the dam’s water level stay at or below 5,475 feet, which equates to being about 60 percent filled. The lower level of water minimizes stress on the dam.

Rodeman said in March that concerns regarding the dam’s overall integrity and the safety and security of area residents have been expressed to Converse County Emergency Management Agency officials and Converse County commissioners.

Should the dam fail, people at the Ayres Natural Bridge  park just a ways downstream of the structure would have only 10 minutes to evacuate before the area is flooded, Rodeman said. He also told shareholders approximately 75 people would be at risk between the Ayres Natural Bridge State Park and Douglas. The dam is under 24/7 monitoring and emergency protocols are in place.

Wyoming is facing aging infrastructure issues all over the state, though, it’s not limited to the LaPrele Dam, Gebhart stated.

“This dam is an extreme example of some of the issues we are facing throughout the state with aging infrastructure. (LaPrele) is a high hazard dam. There are other structures throughout the state which are nearing or ending their useful lifespan as well. Identifying and addressing the aging infrastructures throughout the state is a large issue. It’s not just germane to LaPrele, but all over the state. It’s not just irrigation facilities. Communities are facing aging infrastructures as well,” Gebhart said.

The state engineer’s office and LaPrele Irrigation district worked together to prepare an updated emergency action plan, which helps identify LaPrele Dam’s risks.

LaPrele Irrigation District Secretary Anna McClure said there are no changes to the evacuation procedures in place which Rodeman last spoke about during the shareholders’ meeting in March.

LaPrele Dam irrigates 14,612 acres of land and holds approximately 21,000 acre feet of water when not under restrictions.

Rodeman said that he will see the LaPrele Dam project through, no matter how long it takes.

“We’ve got a real long ways to go. We’re all excited about the progress we’re making. But, it’s going to cost us more money to repair the dam as it would to build a new one. The state alone can’t fund us, it’s going to take federal money. We’re exploring every option we can explore. Hopefully by this time next year we’ll have things lined up and we’ll go from there,” Rodeman said.