Twain Harte Lake Dam deemed safe after cracks appear

Local News

TUOLUMNE COUNTY, Calif. (KTXL) — Homeowners in a portion of Tuolumne County can breathe a little easier Thursday night after officials determined a nearby dam was safe despite cracks in it. 

Officials said an inspection of the dam found it was “structurally sound and clear for continued use.” 

A report of a large “explosion noise” prompted first responders to investigate the dam, the Tuolumne County Sheriff’s Office wrote in a Facebook post around noon. 

According to the sheriff’s office, the loud noise was most likely from the rock cracking. The Twain Harte Lake Association says the dam is situated between two granite domes. 

Something similar happened back in 2014. During that time, the left side of the dam was damaged and cracked after one of the granite domes naturally broke down in a series of events called exfoliation. 

“Natural rock stress release events are caused by years of weathering of an unprotected rock surface and fluctuation in temperatures. The resulting rock fractures, known as exfoliation joints, form onion like patterns where it appears that layers of the rock are peeling away – just like an onion,” the association’s site reads.

The damage cost around $900,000 to fix. The Twain Harte Lake Association said exfoliation would likely happen again, and it did in 2016, leading to a temporary closure of the Rock.

“The dam is now monitored by the (California Division of Safety of Dams) twice yearly and regular movements on the Rock are monitored by friends at the US Geological Survey,” the association says on its site.

The sheriff’s office said the Twain Harte Lake Association will continue to drain water from the lake and monitor it. 

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