Inspecting a stormwater tunnel during river drawdown

December 31, 2020

In October, for the first time since 2008, the U.S. Army Corps of Engineers drained the lower St. Anthony pool of the Mississippi River, immediately below the upper St. Anthony Falls lock and dam in Minneapolis. The drawdown briefly lowered the pool depth by 12 feet, allowing the Corps to inspect portions of its locks, dams, and related infrastructure.

There’s essentially a 30-foot-high waterfall under the city.

The drawdown gave Barr, the Bassett Creek Watershed Management Commission, and the city of Minneapolis a rare opportunity to inspect nearly 2 miles of a tunnel whose outlet into the Mississippi is normally submerged under several feet of water. The tunnel, which still carries the last reach of Bassett Creek as well as highway stormwater runoff, cuts under a baseball stadium and a business district before emptying into the Mississippi. According to Jim Herbert, the Barr project principal, its most interesting feature is a drop structure connecting its upper and lower sections. “There’s essentially a 30-foot-high waterfall under the city,” he says.

Barr, the watershed commission, and the city began coordinating with the Corps three years ago to plan for a three-day tunnel inspection. On October 6, a crew equipped with communication radios, lights, and safety gear entered the tunnel to assess its integrity. The inspection found no deficiencies requiring immediate attention.

To learn more about Barr’s tunnel inspection services, contact us.

About the author

Jim Herbert, vice president, senior civil engineer, has nearly 35 years of experience, primarily in water resources management. He leads projects involving urban stormwater management, tunnel and dam rehabilitation, and waste management. He has also overseen bluff and channel stabilization, led the development of watershed and lake management plans, and provided environmental compliance services for solar projects and transmission-line construction projects.