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August 22, 2006 at 8:22pm

Flooding Susitna River takes down 2nd transmission tower; river travelers once again cautioned

For the second time in a week a flooding Susitna River has disabled a transmission line tower after undercutting its foundation at Dynamite Slough.

On Monday morning Chugach Electric Association discovered the river had undercut the foundation under one leg of a transmission line tower on an island between the main channel of the Susitna and a normally quiet stretch of water called Dynamite Slough. After weeks of steady -- and sometimes heavy -- rains upstream in the Susitna Valley, the slough is now running full of water with very strong current.

The 70-foot aluminum tower knocked askew on Monday supports a 230,000-volt transmission line that carries power from Chugach's Beluga Power Plant. It is one of three transmission lines that move power from Alaska's largest power plant for delivery throughout the Railbelt.

The tower is the second one in a week at the same location to be damaged by a raging Susitna River. A similar tower on a parallel transmission line -- known to the utility as Beluga Transmission Line No. 3 -- was eroded and fell in the river on the evening of Aug. 15. It took Chugach a few days to safely stabilize that tower, free the wires and re-energize the line.

The most recent problem that affected the tower on Beluga Transmission Line No. 2 apparently occurred some between early afternoon Sunday and late morning Monday. Chugach flew patrols of the line at those times and days. Chugach has been closely monitoring the flooding since before the first tower went down, and daily for the past week and a half. After Line No. 3 was returned to service a few days ago, Chugach de-energized Line No. 2 knowing the water was threatening the tower's foundation. Consequently, customers did not experience any power outages or blinking lights when the tower tipped over. On Monday, the tower was leaning at about a 45-degree angle over the river, held aloft by its electrical conductors.

Although the line is de-energized, the three wires that normally carry electricity are still attached to the structure and are suspended approximately 10-15 feet above the Susitna River at their low point. Persons traveling the river are cautioned to watch for the low wires. As it did a week ago, Chugach notified both the Alaska State Troopers and the U. S. Coast Guard of the hazard.

Chugach is assessing the situation to determine the best way to safely separate the electrical lines from the tower. With one line out of service, Chugach has reduced the output from the Beluga Power Plant and is making or buying power from other generation resources in the region.

Even before the first tower went down, Chugach had been making plans to move towers further back from the river. It has been something the utility has had to do many times since Lines 1, 2 and 3 were built in 1967, 1974 and 1981, respectively. Not counting the two towers that have been eroded in the past week, Chugach estimates it has moved or replaced 15 towers in the past 15 years at river crossings between the Beluga Power Plant and Point MacKenzie.

Chugach is the largest electric utility in Alaska, providing power for Alaskans throughout the Railbelt through retail, wholesale and economy energy sales.

Tower udercut: A 70-foot aluminum tower holding 230,000-volt transmission line leans over the Susitna River after being undercut by flood waters. 

The towers that fell stood on an island between the main channel of the Susitna and a normally quiet stretch of water called Dynamite Slough.

According to the National Weather Service 4-to-9 inches of rain fell in locations that drain into the Susitna drainage over a 4-day period, which was just prior to the second of two Chugach transmission towers falling into the river.
For more information, contact Phil Steyer, 762-4766 (desk) 242-1417 (cell)