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U.S. Army Corps of Engineers Omaha District Hydraulic Engineer Paul Boyd, left, speaks to community members at the Springfield Community Center Tuesday in Springfield at the Missouri Sedimentation Action Coalition annual meeting. (Marcus Traxler/Republic)
U.S. Army Corps of Engineers Omaha District Hydraulic Engineer Paul Boyd, left, speaks to community members at the Springfield Community Center Tuesday in Springfield at the Missouri Sedimentation Action Coalition annual meeting. (Marcus Traxler/Republic)

Corps weighing sedimentation options

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news Mitchell, 57301
Mitchell South Dakota 120 South Lawler 57301

SPRINGFIELD -- The U.S. Army Corps of Engineers is running through all options when it comes to the sedimentation collection on a portion of the Missouri River.

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The corps on Tuesday explained during a Missouri Sedimentation Action Council annual meeting that there are several possibilities to clean the Lewis and Clark Lake reservoir, including potentially dredging the bottom of the lake. That was presented during a study update at the meeting, which was held at the Springfield Community Center.

"We don't have all of the answers," said Paul Boyd, a hydraulic engineer with the Omaha District of the corps, and the project manager of the Lewis and Clark Sediment Management Study. "But we've been able to understand the river much better, and we're answering some of the smaller questions that we have."

The corps hopes to have the study finished by July 31 and will likely present results at a public meeting. The focus is on the Lewis and Clark Lake, which is the reservoir behind the Gavins Point Dam, just west of Yankton on the South Dakota-Nebraska border.

Boyd presented on the study and said the corps isn't going to have all of the answers, but it's a chance to better understand what the realistic long-term options include, should the time come that resources are available to fix sedimentation problems.

Since the Gavins Point Dam was constructed in 1955, Lewis and Clark Lake has lost about 2,500 acre-feet of water storage each year due to sedimentation, and the lake has lost 29 percent of its original storage capacity, up 4 percent since a previous study in 2011. In addition, the delta formation at the mouth of the silt-heavy Niobrara River out of Nebraska has continued to expand.

Boyd said the corps is working on nine different sedimentation scenarios, that would include various flushing rates including 30,000 cubic feet per second, which is quite typical during the summer and 60,000 cubic feet per second, which Boyd said is very high. The study also examines frequencies and various dredging options, like mechanical dredging with barges or staged dredges along the river. Each considers options that would be a one-time scenario or something that is done annually for the considerable future.

"This is a first shot at it but what kind of order of magnitude numbers are we looking at?" Boyd said. "Let's get that number and let's understand what's possible and look at what the benefits are of doing it and what are the penalties for doing it."

The corps is also getting basic cost estimates of what a dredging project would include but the study will not take into account other economic, social or political factors. The New Orleans district of the corps, which has significant experience regarding dredging and cost estimates, is helping to understand what a dredging project could potentially cost.

"We have the technology and the wherewithal to dredge it," Boyd said. "But we all know that it's going to be really expensive.

The corps finished its first study in May 2013, which concluded sand would remain trapped within the lake, and transporting the sediment would be limited because of the size of the reservoir and the fact that the Gavins Point Dam spillway gates are more than 30 feet above the lake bottom. Models that were run by the corps showed that lower spillway gates would result in significantly more sediment delivery below the dam. Upon hearing public input on the first study, Boyd said it gave the corps different scenarios to try to model, and the challenge is weighing the impact of the lake to the channels below the dam.

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