Chugach moves third tower threatened by flooding Susitna
While many people were enjoying time off and good weather during the long Labor Day weekend, engineers and line crews from Chugach Electric Association were living up to the heritage of the holiday by moving a 70-foot transmission tower across a soggy island in the middle of the Susitna River - with minimal assistance from motorized equipment.
The tower was the third of three that earlier this summer had stood on dry land a safe distance back from the bank of a normally quiet side channel off the Susitna River called Dynamite Slough. That was before heavy rains throughout August greatly increased the flow of the river, changed its channels and flooded vast areas of lowlands near the point where three Chugach transmission lines cross the river in a remote area on the west side of Cook Inlet.
Two of the transmission lines (3 wires or electrical conductors to a "line") are energized at 230,000 volts and one at 138,000 volts. The lines carry power from Chugach's Beluga Power Plant across 40 plus miles of low, open land to the utility's Point MacKenzie Substation where power is routed to different points on the regional power grid. In early August three 70-foot aluminum towers had stood side-by-side near the western shore of an island that separated a main channel of the Susitna from Dynamite Slough. Each tower provided support and clearance for a separate transmission line.
On the evening of Aug. 15, the northernmost tower -- which supported one of the 230-kv lines -- fell into the water after its foundation was undercut by the raging river. On the morning of Aug. 21, a Chugach crew on aerial patrol discovered that the next tower downstream (which supported the second 230-kv line) had fallen into the flooding river sometime during the previous day after its foundation was eaten away by the strong current.
Both of those cases required Chugach crews to brave the elements to separate the downed towers from the de-energized power lines. Once that was done and the lines had returned to safe clearances above the river, Chugach was able to return the lines to service. Fortunately the incidents happened about a week apart, so Chugach was able to keep two lines in service at all times and ensure the Beluga generation continued to flow to the grid.
Chugach carefully monitored the condition of the river and the steady loss of the riverbank that protected the foundation of the third and final tower. The bank continued to disappear into the water that threatened the stability of the tower supporting the 138-kv line. Last week the utility devised a plan to save the structure by moving it to a new, temporary foundation 200 feet back on the island.
It was an engineering and operational challenge. No heavy lift helicopter was available to lift and fly the structure to its new location.
So how do you safely move a 70-foot tower, weighing more than two tons, on a remote island with no heavy machinery? The only piece of motorized equipment Chugach had on the island was a little tracked mini-excavator about the size of a Volkswagen Bug that had been taken to the island via a small landing craft.
Smart people came up with a smart solution. Chugach's line crews and engineers designed a strong, temporary foundation system using a pair of 12-foot culverts imbedded vertically in the soil of the island. Line crews centered a 16-foot H-beam within each one and then filled the culverts with a mixture of gravel and silt that hardened and formed a strong base.
Linemen then climbed the threatened structure and freed the outside two electrical conductors and loosened the hardware that attached the conductor in the center of the cross piece of the tower. With fore and aft guy wires holding the tower upright, crews were able to detach the structure from its foundation, jack it up and reset it on a pair of field fabricated wooden skids. Crews then systematically worked a system of rigging devices to slide the tower out of harms way. The tracked mini-excavator provided enough power to ease the tower forward on its skids in four-foot increments.
Slowly and carefully, the skilled crew moved the tower about 200 feet to the east of its former location, jacked it up and set it on its new, temporary foundation. By Saturday night, the bulk of the project was done and the tower had been saved. On Sunday, the tower was leveled, conductors reattached and the line was returned to service at about 4:00 p.m. Best of all, no one was injured in the process.
Chugach feels the transmission lines will be safe from the river until this winter when new, larger foundations and replacement towers will be constructed.
The governor has declared a disaster for the Mat-Su flooding, and Chugach has been working closely with state emergency management personnel to document the damage to the towers at the Susitna River and capture costs.
Chugach is the largest electric utility in Alaska, providing power for Alaskans throughout the Railbelt through retail, wholesale and economy energy sales.