by: Deborah Horne Updated:Near Carnation, Wash. —
The huge Tolt pipeline that provides 30-40 percent of the water for much of the Puget Sound region could crack under the weight of the shifting earth if something isn't done. "The earth is moving some 90 feet below the surface," said Senior Water Systems Manager Eugene Mantchev, "to the tune of about three to four inches a year."
Seattle Public Utilities (SPU) engineers noticed the movement a dozen years ago when a forestry company began legally clearing the land above the Tolt pipeline. Fewer trees meant more water runoff, saturating the ground and making it unstable. Then five years ago they spotted cracks in the asphalt around the pipeline. New instruments were installed and geologists identified the area as "an ancient slide," said Mantchev. "This information was really not available when Tolt One was constructed, or even when Tolt Pipeline Two was added in 1998."
Now SPU is planning to spend just under a half-million dollars to helicopter in 20-30 mature trees and their root balls to create a man-made log jam. It is meant to shore up the bank along the Tolt River to help stop the erosion below the slide. Mantchev said it will take months, perhaps even years, to know if the plan gets the earth to stop moving. "But we hope to get there," he said, "because the other alternatives are very expensive. We're trying to accomplish a reliable supply from the Tolt at the lowest cost."
SPU officials said even if the Tolt failed, it would not stop the flow of water to their customers. Most of the water comes from the Cedar River pipeline. But the Tolt is essential as the system's backup.
Work on the Tolt pipeline could start this summer.