The sinkholes that opened up in the Anselmo Mine Yard parking lot last week likely aren't sinkholes after all.
Storm water runoff probably created the holes, said Tom Malloy, Butte-Silver Bow reclamation manager, Tuesday.
A storm water drop inlet at the edge of the parking lot that receives the Excelsior Avenue runoff had become plugged, causing the water from recent rainstorms to overwhelm the drain.
“This (drop inlet) is one I clean up after every storm,” Malloy said. “The water comes off Excelsior a high volume, moving really fast. It roars. It will move mountains of rock.”
Malloy said when he first visited the holes Friday he could see the white PVC pipe that carries storm water down historic Missoula Gulch, along which Excelsior Avenue runs. He said that led him to believe the supposed sinkholes were actually storm water-related instead.
“I looked up the mineshafts on our GIS database and there was nothing on record of there being one at this location,” he said. “There are underground mine workings, but those are several hundred feet down. This appears to be a sinkhole, but it’s actually a storm water washout.”
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A public works crew will dig up part of the parking lot and make repairs to the pipes, or replace them if need be, possibly with larger pipes to account for high-volume storm water runoff flows.
“It might be a side effect of the washout from a month ago,” Malloy said, referring to the heavy rainfall Aug. 21. “We’re still getting damage reports from a month ago.”
Malloy said his department repairs mine shafts that open on the Butte Hill upward of 15 times annually, and investigates another 20. Sinkholes are often exposed when someone notices unexplained holes in a yard or dirt moving inexplicably.