New Moses Lake North Dam construction starts soon

SUNDAY, DEC. 29, 2013

Construction on the new Moses Lake North Dam is expected to begin in early January, according to engineers working on the project.

The project to replace the Moses Lake Irrigation and Rehabilitation District-operated dam is currently in the permitting process, said Chris Comstock of Spokane engineering firm STRATA. The project could end up costing anywhere from $1.9 million to $2.9 million.

While construction is projected to take place until late May, Comstock said the dam itself will be finished before then.

Engineers set a March 31 deadline to have the facility hydraulically functional. Setting a late March deadline ensures the dam will be up and running in time to safely pass water from the spring runoff to avoid rising lake elevations, he said.

The dam and the road across it have been closed since Labor Day weekend, when MLIRD crews discovered a sinkhole near the structure.

Shortly after the sinkhole was discovered, MLIRD crews installed an emergency cut-off wall (cofferdam) to take the hydraulic pressure off the dam.

Since then, the district has been working with engineers to look at rehabilitation and replacement options, as well as with legislators and other agencies to secure funding for those options, according to a previous Columbia Basin Herald article.

Comstock said the dam, which was built in 1928, performed very well until the mid 1980s, when inspectors identified what’s known as a piping problem. That meant that water was washing soil below the dam and piping it downstream, he said.

Since about 1985, annual inspections of the dam have taken place. STRATA became involved in 2011, after the district asked the firm to look at some of the ongoing concerns at the dam.

At the time, Comstock said, the dam was not in danger of imminent failure.

After inspections of the dam this past September, engineers looked at various options to fix the dam.

“We did look at options to rehabilitate the facility and they became very cost-prohibitive by the time you retrofit the facility and upgrade the design,” he said.

Comstock said the district also wanted to look at options beyond simply rehabilitating the facility to avoid any future maintenance issues.

“They don’t want to have to replace the structure in 10 years, they don’t want to put a band-aid on a structure that can potentially fail and have to spend even more money 15 years down the road,” he said. “They want to fix it once, fix it right and fix it forever essentially.”

The dam replacement project STRATA presented to the district has a design life of about 50 years on paper, which usually translates to a life of about 100 to 150 years in real life, Comstock said.

The footprint of the new facility is not going to deviate much from the structure’s current footprint, he said.

“It will look very similar to what it does today, but it will have a bridge over it with an open channel rather than culverts,” he said.

The open channel will allow the dam to pass a lot more water more efficiently, Comstock explained.

To remedy the current piping problem, the new design will use a steel sheet pile system. Comstock said the emergency cofferdam MLIRD installed in September is an example of a sheet pile system.

“These piles are continuous and interlocking so they don’t allow water to pass through horizontally,” he said. “That prevents water from flowing underneath the dam.”

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