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.
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.”