The Grant County PUD says preliminary results of an 11-week investigation has determined that a mathematical error during the pre-construction design of the Wanapum Dam is the "primary contributing factor" in a crack in the dam's spillway.
The fracture forming on spillway monolith No. 4 may have been exacerbated by a weak construction joint, PUD says.
Engineers found the 65-foot-long crack in the Columbia River dam in March and the PUD drained the reservoir behind it as a precaution.
A team of engineers discovered the mathematical error while examining the original design calculations of the dam spillway.
PUD says the investigation indicates that the fracture may have originated a number of years ago and spread gradually over time, ultimately allowing enough water into the fracture to push the upper portion of the structure visibly out of place.
PUD says repairs to the spillway are likely to occur throughout the summer.