High rainfall triggered action at Cooper Lake

The deluge of rain that hit Southcentral Alaska in mid-September called for some immediate action at the Cooper Lake hydroelectric power plant on Alaska’s Kenai Peninsula, according to a report submitted to the Federal Energy Regulatory Commission, or FERC, by Chugach Electric Association, the power plant’s operator.

Chugach Electric told FERC that, with 13.8 inches of rainfall in the second half of September, compared with a 20-year average of 3.1 inches in that time period, the water level behind the dam at Cooper Lake had climbed towards the normal high operating level of 1,194 feet above sea level. The spillway height at the dam is 1,215 feet, but the maximum licensed water level is 1,201 feet.

Apparently, Chugach Electric normally plans for a relatively high water level in the lake in the fall, in anticipation of high winter power and natural gas demand, and recognizing that rainfall input to the reservoir drops in the winter. And the utility had scheduled inspection and maintenance of the power plant prior to the onset of winter.

With the water level in the lake rising and a maintenance shutdown set to suspend the flow of water through the generation system, a water flow that is the only means of lowering the reservoir level, Chugach Electric decided to expedite the maintenance program: Crews worked 10-hour days, seven days a week, to limit the maintenance outage to 11 days, compared with the normal 19 days, the utility said.

The reservoir level crested at 1,194.5 feet on Oct. 19, before starting to drop back, Chugach Electric told the commission. No damage was caused to the facility or to private property, and no one was injured, Chugach Electric said.

—Alan Bailey