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A consulting firm told the Newport City Council Monday night that there is bad news and not-so-bad-news about the two dams on Big Creek that provides Newport with nearly all of its drinking water. The bad news is that the lower dam is in pretty bad shape. It was built in 1951 with some clay but also a lot of sand that has made the dam itself rather mushy. The consultant said in a Cascadia Subduction Zone earthquake it would likely fail. City Public Works Director Tim Gross says such a breach would flood the residential areas immediately to the west, letting loose torrents of water that would eventually head for Agate Beach State Park. Before that happened, the consultant said, the city could gradually drain the lower lake and abandon the dam, thereby relying on the upper dam to provide water to the community. They and Gross have stated in the past that the although the lower dam is holding back a considerable amount of water, it’s not as much as it appears. They say decades of siltation has filled up the lake bed behind the dam so the lake is very shallow and doesn’t hold nearly as much water as what’s behind the upper dam.
The upper dam was built in 1968 and was constructed a bit better. There’s more clay in it, say the consultants. But in the event of a severe earthquake, it would be weakened.
The consultants said now that they’ve got a geologic snap shot of both dams, the next step would be to do more detailed analyses to determine how to strengthen them. They said depending on what they find, they could insert columns of strengthening materials and supports to buttress them against a major earthquake. They didn’t venture a cost estimate due to all the unknowns.
But to make those ‘unknowns’ known will take more investigating. Gross said it might be upwards to $400,000 to produce enough data to devise a plan to make the dams stronger. Or, to abandon the lower dam, drain it, and just work on the upper one. Lots to learn. The council asked Gross to continue to work on the issue and report back with more detailed cost estimates for further analyses and how to pay for it.