- Associated Press - Sunday, April 20, 2014

HELENA, Mont. (AP) - When a leak sprang in one of Toston Dam’s 23-year-old rubber bladders in 2012, it took fewer than three hours for the entire 327 acre reservoir to drain and for farmers to lose irrigation water for five days. Current construction on the dam aims to stop a similar event from happening again, and so far it is coming in under budget.

Toston Dam, also called Broadwater Dam, has the distinction of being the only state-owned hydroelectric dam. The 6 to 10 megawatts of power produced is small compared to other dams - Canyon Ferry Dam produces over 277,000 megawatts - but it does earn the state between $4 million and $5 million in revenue annually.

Revenue varies year to year based on river flow. In years where the Missouri flows consistently between 6,000 and 6,500 cubic feet per second, the plant can operate at its peak. When flows bump above or fall below that level, the power generation dips.

DNRC uses the revenue from Toston Dam for maintenance of Montana’s 20 other state-owned dams that do not produce electricity.

Toston Dam is also vital surrounding agriculture land. The dam provides enough water to irrigate 194 square miles of ground.

When the dam was built in 1940, farmers controlled the water level by inserting boards called needle beams to raise the water level. In 1989, the state completed construction of the powerhouse and turned to a new technology to control water levels. They replaced the needle beams with seven rubber bladders that can be inflated and deflated to control water flow over the spillway.

“It was a risky idea back in ‘89, but it ended up being fabulous,” said Mike Sims, DNRC engineer. “They perform extremely well in our cold climate.”

The early bladders did have an engineering flaw, however. A seam that runs the length of bladder can fail after 20 years of use. On the morning of Sept. 22, 2012, the seam failed in Bay 6, and a leak developed. As a result of the leak, all the bladders except one began to deflate, with two fully deflating. DNRC shut the powerhouse down and scrambled to stop the leak.

The water flowing over the spillway climbed from 1,500 cfs, hitting a peak of 9,000 cfs, but staying well below flood stage of 25,000 cfs. No property damage or injuries resulted.

To fix the leak, DNRC turned to old technology, inserting needle beams and wooden flashboards, but the fix proved labor intensive, and five days passed before irrigation water could flow again.

“Fortunately the leak occurred in September, so most of the irrigating was done for the year,” said plant manager Brian Carroll. “We took the failure as an opportunity to make improvements on the dam.”

DNRC commissioned the building of two steel bulkheads that can quickly go into place to stop a leak. The state estimates the time irrigation water would be lost due to a leak would be three days at most.

“Farmers can make it three days without water, but they can’t go a week,” Carroll said.

After inspecting the remaining bladders, engineers determined they were also approaching the end of their lives. The agency decided to replace all the bladders.

“There was no question that we’d replace them with what we had before,” Sims said. “Ice doesn’t stick to them like steel gates that go up and down. The benefits far exceed the negatives.”

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