Helena utility crews still investigating why Montana Ave. storm drain overflowed

Posted at 8:16 PM, May 10, 2018
and last updated 2018-07-05 15:01:09-04

HELENA – Helena utility supervisors are still trying to determine exactly what caused water to gush out of a city storm drain after heavy rain on Wednesday.

Several viewers sent MTN video of the dramatic occurrence, which happened near the intersection of 11th and Montana Avenues, across from Safeway.

Kevin Hart, the city’s utility maintenance superintendent, said it was most likely a “surcharge event,” which happens when more water flows in from uphill than can go through the rest of the system.

“Water has a tendency to go the path of least resistance, so if you have pressure from up above and an open drain like that, then you see the water tending to scoot out,” he said.

The drain in question sits at the bottom of a hill. Storm water travels from there across the Safeway property and is eventually carried to discharge ponds near the former Kmart building.

Hart said crews inspected the area around the drain Thursday, but weren’t able to determine conclusively what happened. He said there may have been some blockage downstream. Debris can wash into the storm water inlets and build up in the system.

“We’re going to try and do some TV camera inspection work of the underground piping system, to see if we see anything awry,” said Hart.

But he said it’s also possible there was simply more water in the system at one time than the outflow pipe could handle. Wednesday’s storm brought a great deal of rainfall over a relatively short time.

“That’s typically where Helena’s storm drain system has experienced problems,” Hart said.

City leaders say Helena’s storm water system includes 71 miles of pipes, ranging in size from 90 inches to just 8. Crews clean the more than 1,000 storm inlets yearly, and use high-pressure jets to clean the pipes.

Hart said he hopes to get some of the inlets cleaned soon, to prevent more debris being carried into the system with a strong storm. But he said the crews will be limited once the rain begins.

“There isn’t a lot we can do during a storm event, except to hopefully prevent flooding of homes and businesses,” he said.

Hart said the city recently completed a storm water master plan that will help determine what improvements need to be made to the drainage system.