Thatcher Project Highlights Ingenuity, Community Partnerships

Officials with the Vicksburg Engineer District’s Thatcher Lock & Dam, the northernmost navigation structure on the Ouachita River in Arkansas, first noticed something wrong at the structure in late October 2021. What’s transpired since then has been a testament to the ingenuity of the Vicksburg District’s engineering and maintenance team and a good example of community partnership.

Thatcher Lock & Dam features a 200-foot-long hinged crest gate that stretches across the Ouachita River. The gate is about 5 feet high and, in normal conditions, has about a 1-foot head over the top. The gate is designed to maintain an elevation of 77 feet on the Ouachita. In high-water conditions, the level above and below the structure can equalize, at which point Corps officials can lower the hinged gate, allowing vessels to pass without going through the lock chamber.

In late October, though, with the river low enough that the hinged crest gate was in the raised position, lock operators noticed a problem. Even with the gate up, the structure wasn’t able to hold pool at 77 feet. Prior to that, lock operators had already noticed water flowing under the gate, but in normal conditions, the amount was insignificant relative to the total flow of the river.

However, with very little rain in the region last summer and fall, the leak in the gate became a material issue for holding pool.

“When you get down to very-low-flow conditions and you have this water flowing essentially between the hinge and the concrete, it became enough flow, with the limited inflow we had, that we were losing pool,” said Justin Giles, chief of the water management section in the Vicksburg District’s Hydraulics Branch. “At that point, we realized we had an issue at the structure. It wasn’t a minimal thing that could be worked around. It was something we needed to go in and inspect.”

On October 22, 2021, the Vicksburg District issued a public notice stating that the lake above Thatcher Lock & Dam had fallen below normal pool level and was continuing to fall. Lowering the pool and closing the lock would have minimal impact on commercial navigation, Giles said, although the district knew there would be impacts on recreational navigation.

The bigger potential impacts rested with the surrounding communities that draw their fresh water from the Ouachita, Giles said.

“They have intakes in the river that pull water out to provide water to those communities, particularly Camden, Ark., and the surrounding communities,” Giles said. “We had to work very closely with them to ensure that us lowering the pool to inspect and fix the issue was not going to cause them to lose water to the thousands of citizens in their community. There was a lot of close coordination with them to do that.”

Coordinating with community leaders, the district identified a water level of 72 feet as sufficient to maintain the water supply to community intakes. Since the pool would naturally bottom out at 71 feet (the height of the sill at Thatcher), the team at the lock used sandbags and other materials to make a small 1-foot levee just upstream from the lock. That way, they were able to maintain a pool of 72 feet while also “inspecting in the dry.”

Once the water level dropped enough to dewater the dam, Corps officials discovered that the problem was with the anchorages where the crest gate’s hinges were bolted to the sill. Corrosion had deteriorated the anchorages to the point that water was leaking through and had even stressed the gate itself. The force of the water was causing the gate to actually lift off the sill.

“It was actually so severe that the stress on these corroded anchorages was causing a lot of them to break and cause the gate to bend,” Giles said. “At the center of the gate, there was a 7-inch bend because the anchorages in the center had been corroded and broken.”

At that point, Giles and his team knew the gate had to be removed and replaced. Fortunately, Thatcher Lock & Dam is equipped with a “poiree needle system” at the upstream side, similar to temporary stop logs. Corps officials installed the poiree needle system to raise the pool while also removing the crest gate for an extended period. They bolstered the original poiree needle system by bolting a series of steel I-beams to the sill as well.

Giles called it a “long-term temporary” solution.

The Corps maintenance team then moved forward with cutting the hinged crest gate into three sections and hoisting them out of place. Giles said the largest section of the gate weighed 65,000 pounds. The maintenance team removed the old crest gate and fortified the poiree needle system in early February.

Vicksburg District leaders are working on securing funding for a replacement hinged gate, while engineers are redesigning the anchorages to prevent corrosion in the future. The timeline for replacing the gate is speculative at this point, with Giles estimating a one- to three-year window.

The poiree needle system continues to hold pool above Thatcher, Giles said.

The project thus far has been an effort in discovery and creativity, with a focus on good communication and collaboration with the surrounding communities.

“At first, there was a lot of anxiety in those communities because they thought they might lose water,” Giles said. “Once we started working with them and having conference calls each week, it was clear that we had a good plan, and they understood that plan. They were a lot more at ease once we were doing those conference calls each week.”

Caption for photo: A maintenance crew with the Vicksburg Engineer District removes a portion of the hinged crest gate from Thatcher Lock and Dam on the Ouachita River. The Corps removed the gate and installed the lock and dam’s poiree needle system in early February after discovering that the gate’s anchorages were leaking, causing the Corps to lose pool on Thatcher Lake. (Vicksburg Engineer District photo)