Flume damage at 'worst case' level

Public Works director Mark Clanton, looking up, emerges from under Shelbyville to give officials the bad news -- about 200 feet of the city's drainage system has been severely damaged and it could get worse if more heavy rains come. (T-G Photo by Brian Mosely) [Click to enlarge] [Order this photo]

Public Works director Mark Clanton had bad news for city officials Thursday after he emerged from underneath Shelbyville following the inspection of a collapsed stormwater flume.

"Our worst fears are realized," Clanton told Mayor Wallace Cartwright, city manager Ed Craig and others who gathered at the entrance to the flume Thursday morning.

A section of about 200 feet has been undermined by water flowing under the flume and has pushed up the metal pipe about 10 to 11 feet, Clanton said.

But that's not all. The top of the flume has also fallen in at one point and there is a three-inch hole "with water just pouring in ... it's running in like a waterfall from the top."

Water is still getting through the flume because there is currently a normal flow, channeling around both sides of the damaged area, which has been pushed up in an inverted "V," Clanton said. The area where the water is flowing is about three feet wide.

"The only question is what happens when we get a big rain event," he said. He said more water would keep undermining the flume.

Clanton also said that back pressure occurred inside the flume when the Duck River rose during this past weekend's flooding.

Major cost

A 200-foot damage stretch is shown. (Photo by Mark Clanton) [Click to enlarge]

Clanton said he had rough estimates of what the costs could be to fix the damage when the problem was first discovered this past weekend, saying to just to replace the pipe would cost in the neighborhood of $100,000 to $200,000. This cost would not include the excavation, he said.

But now that he's seen the extent of the damage underground, he's saying it "will be at least double that amount ... it's hard to really put a figure on it."

The trick is replacing the flume, Clanton said, but when excavation work is done, workers will have to find where the pipe is undermined.

"But if we keep getting rain before we get anything done, it's going to keep undermining ... the cost could be anywhere from $100,000 to astronomical," Clanton said.

"It's a major undertaking," Clanton said about repair work facing the city after he emerged from underground. He also said he believed that the underground damage stretches beneath North Main Street.

Clanton estimated from what he observed that there is 70 to 80 percent restriction of water flow in the flume.

"It's 12 foot tall by 18 foot wide and the entire bottom (of the flume) is bowed up," Clanton said.

The damage is also working its way back to one of the entrances to the flume system near the corner of Madison Street and North Brittain Street, Clanton said.

Also, the right hand side of the walls coming from Madison is "undermined" and is bowed in at least three feet, Clanton said.

Could get worse

Aside from looking underground, Clanton was keeping an eye to the skies Thursday, because the National Weather Service was calling for more rain for our area, and that would make the situation worse, Clanton said. He explained that more water flowing under the flume could buck up the pipe further.

The city is moving in an expeditious mode to fix the problem, Clanton said, to "get on this and get it fixed before it gets too far gone."

Clanton is hoping that workers will be ready to get started next week to repair the damage.

"If we get more rain, it could be too dangerous of a situation ... it could float the flume plumb out of the ground," Clanton said, adding there are still a lot of unknowns with the flume damage.

"It's major and it will go further," he told Craig and Cartwright. "If we have another flash flood, we're going to have a problem."

Clanton also told the mayor and city manager that "this is pretty much what our worse case scenario is."

He added that the city is now at a point where an engineer needs to enter the tunnel as soon as possible and inspect it, which he said was to happen today.

Clanton said the engineer is a specialist in this type of work and exact measurement will be taken of the damage and then a plan of action can be devised.

Road closed

Another concern is traffic on North Main Street. City officials closed off about a block of the main thoroughfare that contains a number of businesses because "we were worried about the unknown," Clanton explained.

But now that they've seen the extent of the damage, no traffic at all can be allowed on the road because any vibration on the flume or near it could magnify the undermining effect.

North Main will be closed for "30 days at a minimum," Clanton said. "It's going to be a long process, it really is."

Even a closure of up to 60 days is not out of the question, he said.

The northbound Lane of North Cannon Boulevard is also closed, as is the section of North Brittain Street that was previously closed.

A number of firefighters accompanied Clanton into the flume, which had to be cleared of a great deal of logs and debris before they could enter. Firefighters were tied together at 100-foot intervals in case something went wrong, according to fire chief John Habel.