Tag Archives: Saint Anthony Falls Laboratory

Bridges and Structures

Continuous Scour Monitoring Improves Bridge Safety

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A leading cause of bridge failure is bridge scour, which occurs when rapidly moving water erodes riverbed soil around abutments or piers.

Monitoring bridge scour with traditional inspection methods can be dangerous and difficult, so MnDOT has been working with researchers from the University of Minnesota’s St. Anthony Falls Laboratory to develop a continuous monitoring system to test certain bridges more safely and efficiently.

MnDOT currently monitors 45 scour-critical bridges — and local Minnesota agencies monitor 360 more — using visual inspections or water data websites during flooding events. Once a predetermined threshold is exceeded, portable scour monitoring equipment is deployed to measure scour depth.  If scour has undermined the foundations of a bridge, inspectors close it for repair.

But portable scour monitoring systems can be difficult and dangerous to deploy from the bridge deck or boat in fast-moving water. It can also be difficult to get inspectors to sites quickly enough in areas subject to flash flooding.

A better alternative for such situations are fixed scour monitoring devices that continuously monitor scour and send data wirelessly to bridge personnel, alerting them when scour reaches a dangerous level.

The Highway 43 Bridge in Winona was affixed with continuous bridge scour monitoring equipment.
The Highway 43 Bridge in Winona was affixed with continuous bridge scour monitoring equipment.
New technology

MnDOT has not historically made use of fixed scour monitoring equipment, but as advances in technology have made these devices more affordable and reliable, the agency  became interested in exploring the use of fixed monitoring equipment at locations where the use of portable equipment is problematic. ( A major concern for fixed scour monitoring is damage from debris and ice.)

Researchers have installed fixed remote monitoring stations on four such bridges.

Stations on the first two bridges (Highway 14 over the Minnesota River in Mankato and Highway 43 at the Mississippi River in Winona, pictured at top) ran successfully for three years, with outages due to primarily to power and communication issues.

Researchers learned valuable lessons from these bridges and have now installed monitoring equipment on two more: The Old Hastings Bridge (Highway 61 over the Mississippi River), on which float-outs were installed; and the Dresbach Bridge (Interstate-90 over the Mississippi River), which had a tilt meter and underwater sonar device installed.

“The less familiar personnel are with technical equipment, the less they tend to use it,” said Andrea Hendrickson, State Hydraulics Engineer, MnDOT Office of Bridges and Structures. “This research project gave us the familiarity and technical information we need to be comfortable using fixed scour monitoring equipment on bridges that warrant it.”

Related Resources

*Editor’s note: This article was adapted from an article in the latest issue of our newsletter, Accelerator. Read it online, or sign up to receive it by mail. 

Environment, Materials and Construction

Flume research simulates Red River flooding to test road protections

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Flooding in the Red River Valley is an almost annual occurrence, and the cost to roads, property and lives is huge.

Highway 1 gets torn up year after year, only to be rebuilt in time for next year’s flood, joke residents in the little town of Oslo, which becomes an island whenever the roads close.

While not much can be done to prevent swollen farm fields from overflowing, what if a road embankment itself could be bolstered to prevent physical damage to the underlying structure of the road?

“We can’t just raise the road because it would create backwater upstream,” explained JT Anderson, Assistant District 2 Engineer. “Our best bet is to let the water over-top the road and try to protect the road when it does.”

Researchers have built a flume inside the University of Minnesota’s St. Anthony Falls Laboratory to test six methods of embankment protection specific to the needs of towns like Oslo.

“It is not uncommon for one over-topping site to have a half-mile long stretch of road being damaged,” said university research engineer Craig Taylor. “One road being protected should cover the cost of the study and the cost of deploying the erosion control product for that road.”

Nationally, research of this kind has mostly been restricted to high-intensity flooding.

“Those really high-depth, short duration events, you can only protect an embankment with concrete and boulders,” Taylor said. “With longer duration, low-depth floods, we may be able to protect roads with soft armoring, like reinforced vegetation.”

The damage in northern Minnesota has been the worst on east-west roads, where the river flow runs perpendicular to the center of the road, causing the road to act like a dam and the water to jump at the edges.

“It eventually eats through that road embankment and makes the road collapse,” Anderson explained.

Researchers will examine how a cross-section of a road holds up under various erosion control methods at different levels and speeds of water-flow.

The damage from flooding was less in 2010 after engineers added rocks and vegetation to the side of Highway 9, near Ada.
The damage from flooding was less in 2010 after engineers added rocks to the side of Highway 9, near Ada, Minn.

One test will be to slow the flow of water by covering the road shoulder with a rubberized membrane and temporary water-filled tubes.

Permanent schemes — such as turf reinforcement mats and rocks — will also be tested.

“These methods have been deployed in the field, but you never really know under which conditions they survived or failed,” Taylor said.

In the Red River Valley, MnDOT engineers have tried a combination of vegetation and boulders, as well as concrete blocks covered with topsoil, to protect highways. Flattening a slope is another option.

“I expect that a single erosion protection technique will not cover every situation our road embankments may be exposed to at any given location,” Anderson said.  “Rather, I expect we would look at using several different techniques in concert to develop an effective erosion protection system for the expected velocities.”