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Economic and dam related articles

Corps Finds Error in
Bonneville Dam Spill Measurements

by CBB Staff
Columbia Basin Bulletin - July 30, 2004

The U.S. Army Corps of Engineers has identified a discrepancy in the amount of water spilled at Bonneville Lock and Dam causing less water to be released than reported.

Engineers say that due to incorrectly calibrated gate openings, when trying to meet targeted volumes, up to 30 percent less water has been spilled from the dam's spillway than has been reported to regional fish and water management officials.

Adjustments are being made to ensure the correct volume of water is released from the dam.

The discrepancy may date back three decades when the Corps replaced the spillway gates at the dam in the early 1970s. The height of gates was increased, leading to the calibration error. However, due to historical spill patterns the affect was negligible until recent years.

Questions about flow volumes in the river system have circulated for several years. The Corps has investigated these issues and resolved several, but some inconsistencies have remained. This led engineers at Bonneville Dam to look at the calibration of the gate openings.

The error was identified this week after engineers measured the gate openings and calculated the expected resulting flow. The Corps is in the process of validating the data. That process will take about two weeks.

"Now that we have identified the discrepancy we can make an adjustment to ensure the right amount of water is released from the dam," said Cindy Henriksen, chief of the Corps' Reservoir Control Center in Portland. "Adjustments are fairly simple to make."

Henriksen said water releases from a dam spillway aren't as precise as some might think. "The narrower the opening in the gate, the more error is experienced," she said.

Henriksen said the size of the discrepancy was exacerbated in recent years as a result of new spill patterns used for fish passage at Bonneville Dam. Prior to 2002, the Corps spilled the majority of the water from the left- and right-most gates on the spillway though wider gate openings. However, following the installation of submerged flow deflectors designed to reduce saturated gas levels -- which allows for better conditions for fish -- the Corps changed its water releases to go through all 18 gate openings. That means smaller openings in each gate and larger margins for error.

Henriksen said the Corps this week thought it was releasing 75,000 cubic feet per second of water over the spillway, when it was actually releasing closer to 64,000 cfs. Nighttime spills for juvenile fish are not affected by this problem, she noted.

"The amount of water passing the dam remains the same. We are not holding water back," she said. "The difference is how the water passes the dam. The requirement is to send 75,000 cfs over the spillway during the daytime, and that is what we are trying to do.

"That's why we are making the adjustments now, and not waiting until the data is validated."

In addition to recalibrating the gates at Bonneville Dam immediately, the Corps will be looking at all its dams in the Columbia and Snake river system.

CBB Staff
Corps Finds Error in Bonneville Dam Spill Measurements
Columbia Basin Bulletin, July 30, 2004

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