Eroding hillsides create trouble for local motorists; Iowa DOT considering options

by Kelli Boylen

A downed tree occasionally blocking Highway 76 between Effigy Mounds and Marquette is nothing new, but it certainly has started occurring with greater frequency in recent years. Since spring of this year there have been at least four incidents of trees down blocking that highway and mudslides from the bluffs in that location which have affected motorists, often in the days following a heavy rain.
Major rainfall events (three or more inches of rain in a day) saturate the soil. The water weakens the ground so it’s easier for the weight of the trees to pull the roots out of the dirt and down off the bluffs.
Pete Hjelmstad with the Iowa Department of Transportation (DOT) explained that the heavier rains in the last several years have caused erosion, exposing more tree roots. “This weakens the base of the tree and increases the risk of a tree going down. Also, each year many of these trees get larger, making them heavier, also increasing the chance of one falling.” Hjelmstad is the Field Services Coordinator for the Iowa DOT District 2.
During heavy rains the soil washes away, and in the days and weeks following a heavy rain the exposed roots continue to weaken to the point the tree falls. Often, heavy rain events also involve wind. “Add that into the mix with the leaves catching the wind and it greatly increases the pressure on the base of the trees.” Hjelmstad said. Wet leaves also increase the stress on the trees, which is an added factor of why they fall with more frequency immediately following a rain.
These major rain events also greatly increase the chances of gullies developing, leaving the hillside at greater risk of future mudslides and tree falls. There are now at least 12 large gullies coming down the hillside in that bluff-lined stretch between Effigy Mounds and Marquette that didn’t exist just a few years ago. As the gullies become deeper and wider, they drain the water from the hillside very quickly with little to slow down the flow of water, resulting in mudslides occurring with more frequency.
According to Todd Shea, meteorologist with the National Weather Service, in the last eight years there have been at least nine incidents of major rainfall events in Clayton and Allamakee counties. Some of these heavy rainfalls were very localized, some impacted wider areas. Many caused mudslides, flooding and heavy erosion to hillsides.
When a tree blocks the roadway, motorists are forced to sit and wait until highway crews can saw up the trees to remove them, or detour their route. Not being able to take Highway 76 can turn a usual 12.5-mile trip from Harpers Ferry to Marquette into a journey of at least 30 miles on gravel, or nearly 40 miles if a driver wanted to stay on blacktopped roads.
The Iowa DOT is aware of the situation and is considering its options.
“As far as the trees falling on IA 76, we are monitoring it and will be doing some clearing along the bluff this fall once the leaves fall off the trees. Right now it’s really too hard to even see what needs to be taken out. We also expect the number of slides to decrease once the leaves fall off the trees because there won’t be as much weight hanging on the limbs. The leaves also hold water and catch the wind, making the tree itself less stable,” Hjelmstad said. “As far as a long term solution, we don’t have anything planned right now but we are researching our options.”
The Iowa DOT is currently working between New Albin and Lansing to deal with the instability of a bluff face along Highway 26, but this area  is a different situation than the project that would be needed on Highway 76.  The nature of the issues are different, with the Highway 26 project dealing with rocks, versus trees along Highway 76, and varying degrees of slope. Also, Hjelmstad added, “That project covers a few hundred feet, the area on IA 76 from Marquette north is several miles long.”
The blasting and removal of debris on Highway 26 is going well, but Hjelmstad said the project is now running just a bit behind schedule. They were hoping to have the highway reopen by mid-October, but it now appears that it will likely be early November.