Daily Sitka Sentinel
Green Lake levels are back where they should be, the city electric department reports.
But Blue Lake levels are still below where they would be on an average rainfall year, Utility Director Chris Brewton said.
The electric department publishes the “rule curve” which shows lake levels during an average rainfall year for both Blue Lake and Green Lake. The same graph shows current lake levels.
Generation System Manager Andy Eggen described the “rule curve” as “where we need to be to keep the lights on in May, and with average weather the lakes will refill to the start of the new water year (in November).”
The lake levels generally reach their maximum height in October and November, when water actually spills over the tops of the dams. Once the weather gets cold in the late fall, the lakes stop filling and levels start going down through mid May. In the spring, electrical demand goes down, the snow is melting, and the lakes begin to refill. The hydro generators are at their most efficient when the lake levels are at their highest.
After last year’s colder and drier weather, neither lake filled completely. The problem was made more acute by the high price of oil, which led to more residents using electricity for home heating.
To maintain the lake levels as high as possible, the electric department started running one of its the diesel generators to supplement the hydro power. Green Lake provides a maximum of 12 megawatts of power, and Blue Lake 6 megawatts.
Brewton said that running the diesel generators allowed the lake levels to rise slightly in December, when they ordinarily would start dropping. The department also ran one hydroelectric unit at Green Lake at maximum efficiency instead of two units at inefficient levels, he said.
“When you run one at five megawatts and one at five megawatts, it’s inefficient and you’re wasting water,” Brewton said. The department ran one generator at its maximum capacity, and supplemented its output with diesel generation from the Jarvis Street plant, he said.
He and other department officials calculated that the city could get back on the “rule curve” – and get lake levels back where they should be – through this strategy.
“This is a perfect example of why we did what we did, and why we’re in good shape now,” Brewton said. “That’s the reason we ran the diesel and it worked.”
For the longer term, the city is in the process of increasing the Blue Lake dam height by 83 feet, which will expand the lake and increase its power- generating potential.
In the meantime the city has imposed a surcharge on electrical bills to cover the cost of the makeup diesel generation.