Lake level drops a foot to fix spillway

By Scott Aust, Journal staff | Friday, May 23, 2008

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The water level in Canyon Lake was reduced by about a foot over the past week as part of the effort to fix the sinkhole undercutting the spillway.

Filling the sinkhole is part of a short-term solution to shore up the structure for up to two years while the city continues to prepare for a complete reconstruction or replacement of the spillway.

Since the city declared an emergency in March, a standpipe was built to help reduce the lake level. Workers with Corr Construction began Thursday afternoon, plugging and sealing the sinkholes with gravel, sand and bentonite, a material that expands when exposed to water.

The standpipe allowed water to be pumped off the top of the lake instead of releasing water through an existing gate in the dam. The gate is buried in silt and would have contaminated the creek, likely leading to fines by the Environmental Protection Agency.

Robert Ellis, public works director, wasn't sure what impact Thursday's rain would have on the plugging effort, but he expects the work won't take long to complete after it starts.

"I don't know if they'll get finished up today. They very well could, but if they don't, they'll finish up tomorrow," Ellis said Thursday morning. "They are actually dropping stuff in the lake right now."

The sinkhole will be filled with a mixture of heavy, coarse gravel and bentonite to start, then a mixture of bentonite and smaller gravel, followed by a mix of bentonite and sand. Ellis said the project will be evaluated in a few days to determine if there has been settling and whether any more material is required.

"In a couple weeks, we're going to have divers go back out there and take a look underground, do some video for us, and see what's changed out there," Ellis said.

When asked about the "rooster tail" shooting water out of the face of the spillway, Ellis said the work may eliminate it, but it hasn't been the biggest concern. Although it is the most visible evidence of a problem, Ellis said, it isn't the problem that affects the spillway integrity the most.

"It looks like the most precarious part of it, but the boil is what we were really concerned with," Ellis said.

When the plugging operation is done, Ellis said, the lake level will be allowed to rise. It won't be left at a lower level while the long-term solution is prepared. He said the lower level also likely won't affect Memorial Day weekend activities, though he noted some paddle boats that were under water a few days ago are now sitting on the rocks.

"It should fill within a couple of days," he said. "Hopefully, things will be back to normal by Monday. With any luck, we'll be done by the end of the day."



The city is interviewing two engineering firms interested in designing the spillway replacement or reconstruction project. It may take more than a year to design the project and secure the necessary permits from state and federal agencies.

Efforts to fix the spillway were sparked by an engineering study earlier this year that recommended taking immediate action to plug the sinkhole and a major renovation or replacement should be done within two years.

Contact Scott Aust at 394-8415, or scott.aust@rapidcityjournal.com