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Flawed study led to sediment miscalculation on Elwha project

May 8th, 2013 - 6:20am

(Port Angeles) -- The Port Angeles City Council last night heard the latest on the project to remove two dams on the Elwha River.

They also heard a lot about sediment. Sediment has overrun water treatment facilities on the river.

City officials are concerned unless there's a fix soon, the city's water supply could be threatened.

Chief hydrologist on the project, Andy Ritchie, says there were 8-million more cubic yards of sediment than first thought trapped behind the dams. That fine sediment was stored in the form of mud on the lake bed that doesn't wash down the river like gravel and sand.

Ritchie says engineers were using a flawed survey, done in 1917, that resulted in a massive error in estimating the amount of sediment under Lake Mills.

Several aerial views of the Elwha delta show a tremendous buildup of washed-down sediment. It is forming a large beach where there used to be none. But Ritchie thinks tidal action of the Strait of Juan de Fuca will eventually dissipate it to some extent.

Even with the sediment problems, Ritchie still feels the project is moving along well.

Ritchie says one thing that would help clear the river bed faster than usual would be heavy rains, resulting in rushing waters to help move more sediment faster than is now happening.

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