The old Bassett Creek tunnel below downtown Minneapolis doesn't carry nearly as much water as it used to.
But as heavy rain storms become more frequent in a changing climate, officials are exploring whether it could play a bigger role in the future.
Construction on the old Bassett Creek tunnel began in the 1880s, when city planners wanted to build where the creek wound through downtown Minneapolis to the Mississippi River. A newer tunnel has since been built to carry Bassett Creek underground to the river, so the old tunnel now accommodates storm water runoff from a much smaller area.
Still, it remains critical to diverting storm water from all that development — buildings and parking lots and freeways — that went up above ground in the North Loop. So, on a recent morning about 10 engineers climbed down a manhole to the inside of the tunnel for an inspection.
It's not easy work, said Nathan Campeau of Barr Engineering.
"We have to work with the weather, we have to work with the rising and falling river levels from the Mississippi," he said.
In addition, the teams are coordinating a lot of different activities: gathering evidence of the tunnel's integrity, sampling sediment and measuring its depth. The tunnel is like a large concrete box roughly 10 feet wide by 10 feet tall. Some sections include side-by-side boxes. Other sections open up to 20 feet wide and nearly 14 feet tall, where old brick bridges formed the groundwork for the tunnel.
"These are huge infrastructure projects," said Katrina Kessler, director of the surface water and sewer division for the Minneapolis Department of Public Works. "Some are miles long and five stories underground. The Bassett Creek tunnel is more shallow, so you don't have to have the constraints of lowering everything down by crane."
During this wet spring, the bottom of the tunnel fills with backwaters from the Mississippi River. At the tunnel's entrance the water is about knee-high. The closer to the river, the deeper the water gets.
And on the tunnel's bottom, the engineers find all sorts of stuff: mud, sand, big pieces of concrete, trash. On this trip, a lone crutch glimmers in the light coming in from the manhole.
"Over time, when you have a lot of sediment building up in the tunnel, you lose capacity," Kessler said.
If anything, the city needs more capacity. Not only have storms become more intense in a changing climate, the existing infrastructure is crumbling, meaning the tunnels and pipes just can't handle the pressure.
More than 10 years ago, water burst from one of the tunnels because of too much pressure and not enough capacity, causing a mix of storm water and wastewater to overflow into the Mississippi River. The city plans to begin construction on a new storm water tunnel in 2020, but officials also want to know if the old Bassett Creek Tunnel could address some of the need.
The city is working with the Mississippi Watershed Management Organization to determine how much sediment has built up in the tunnel and what's in it.
"These tunnels were designed to move storm water, so certainly we want to free up the capacity of the tunnel," said Stephanie Johnson, projects and outreach director for the Mississippi Watershed Management Organization.
Engineers from Barr collect samples of the sediment in different areas of the tunnel and will analyze what pollutants are present.
"Heavy metals would be not uncommon to find in an urban area, coming off of brake pads, hydraulic fluid from cars," Johnson said.
Johnson said they don't yet know what it would take to move all the sediment and debris out of the tunnel. An analysis in 2012 found there were up to 3,500 cubic yards of it.