Missing logs part of Hebgen problem

 By Andy Malby, editor

PPL hopes to fix broken intake without draining lake



Engineers at Hebgen Dam this week used Sonar cameras to have a look at the inner workings of the structure, which malfunctioned nearly two weeks ago, causing a four-fold increase in water to flow into the Madison River from Hebgen Lake.

A 150-square-foot hole in one of the four intake towers on the dam was discovered Wednesday, according to David Hoffman, external affairs representative for PPL Montana, the company that operates the nearly 100-year-old dam.

The hole exists because of the loss of several “stop logs” — giant logs, horizontally stacked in the intake, that control the flow of water through the dam.

“We did see that 17 stop logs are missing from gate no. 4,” he said. “That’s a 17-foot-tall hole that’s nine feet wide.”

Hoffman said engineers are still in the dark about why the stop logs are gone, but, “The good news is that now that we have a clearer picture down there it’s a lot easier to engineer a solution.”

Meanwhile, with water flowing through the hole at nearly 3,200 cubic feet per second, Hebgen Lake is dropping by about five inches per day, and as of Thursday was nearly 5.5 feet lower than when the problem began on Aug. 31. The lake is six inches below the “bottom recreation level” PPL is required to maintain, Hoffman said.

“The recreation level is something that under our license we strive to maintain for recreation on the lake — boating and so forth,” he said. “But at this level people ought to have their boats out anyway.”

Engineers spent ten days following the malfunction fashioning a steel bulkhead to block the flow of water from the malfunctioning intake, but that effort failed early this week, Hoffman said.

The plan now is to “slowly, carefully” lower new stop logs into place in gate no. 4, to try and slow the outflow. If successful, the outflow could then be diverted to other gates or to the spillway while engineers install a steel bulkhead and repair the damages to the hydraulic gate on the intake system.

It sounds complicated, but Hoffman said it actually is “a fairly routine practice.”

Once the bulkhead is sealed in place, the water can be pumped out of that intake so workers can get in and fix the problem that caused the stop logs to float away.

To ensure the problem doesn’t repeat on other gates, PPL probably will replace stop logs in all four gates, since they already will have the flow of water stopped, he said.

“We’ll do one at a time, possibly two at a time, once we get the engineering done on that,” he said.

He could not speculate about a timeline for getting the work done.

“Our number one priority is public safety and worker safety, and number two is not doing something to aggravate the situation,” he said. “The number one thing we don’t want to do is aggravate it somehow. But since we do have some pretty good images showing that the piers are intact then that risk is lessened.”

If the effort to replace the stop logs fails, though, the lake may have to be drained — not at all a preferred option.

“Again I’d probably have to say that I could not give a blind guarantee that that’s not going to happen,” he said. “We certainly hope it’s not and right now don’t think it will.”

If the lake does need to be drained, it could remain that way for an extended period, and could impact the blue-ribbon trout fishery downstream.

“It’s a big lake, and I’m not sure we’d have the ability to refill it, if it were drained, within one season,” he said. “It could also impact pulse flows down on the Madison.”

Pulse flowing is a process by which, during the summer months, PPL opens the dam to release “pulses” of water, to keep downstream river temperatures stable for fish, he explained.

Hebgen Dam, built in 1914, is about 20 miles northwest of West Yellowstone. It holds back the 15-mile long by 2.5-mile wide Hebgen Lake and is used to manage the Madison River fishery and keep Ennis Lake, about 45 miles downstream, full.

“We use Hebgen Lake to keep Ennis Lake full,” he said. “We need to keep the temperatures low enough to protect the trout.

“We don’t generate electricity at Hebgen Lake,” Hoffman added. “It’s really a regulatory lake to protect the resource, the fishery, and secondly to create recreation opportunities both above and below the dam.”