Riverbank collapse causes Prince Albert to move Rotary Trail back from river
Published Wednesday, September 23, 2020 6:32PM CST Last Updated Wednesday, September 23, 2020 6:54PM CST
SASKATOON -- One week ago, a resident from Prince Albert alerted the city to an area where a piece of earth had broken away from the riverbank along the Rotary Trail in the West Flat neighbourhood.
“We didn't expect anything like this. I don't think anybody did. But that's Mother Nature doing its thing,” said David Fischl, the chairperson of the Rotary Trail Committee.
He says the engineers who helped design the trail didn’t anticipate river sloping. The trail was designed to provide trail users with a view of the river while they walked, jogged or biked along the North Saskatchewan River.
“Right now they've got a fence here which is keeping people away from it which I think is a great idea but the trail is still usable,” Fischel said.
Director of public works Wes Hicks says this is the third location where the riverbank has sloped since 2017.
The city rerouted a section of the trail further away from the river this summer near 16th Street West because the riverbank had receded. The city maintained the same distance of the trail in the process of moving it further away from the riverbank.
Hicks says the high water levels this spring contributed to the changes in the land.
“This last year was the highest (river levels) in 35-years. We had the river rise up over top of the banks. And it scoured the shoreline so it cut into the banks of the river and as it receded away it left cliffs. Straight up and down vertical cliffs, some as much as five-metres in height,” said Hicks.
“As the summer progressed and the grounds starts to dry out and you get cracking and suddenly they give-way.”
University of Saskatchewan geography professor Alec Aitken says the changes to the riverbank are known as a rotational landslides.
High water levels and higher than average rates of river flow contributed to the erosion of the riverbank. Aitken says the south side of the river bank has a faster flow.
The soils at the base of the bank were disturbed and caused the rotational landslide. Excess moisture in the soil could have also compromised the strength of the soil in the riverbank and contributed to the sloping.
“As you remove material by the river the slope becomes steeper and steeper. And eventually the force of gravity overcomes the strength of the soils and sediments in the riverbank and causes them to fail and what you're seeing are classic examples of rotational landslides,” said Aitkens. He says the phenomenon is taking place along river systems across western Canada.
Aitken says monitoring of the riverbank by looking for tension cracks and measuring ground water pressure could help find areas where the river bank is weakening and prevent mishaps.
“And we can use that information to monitor to predict when the strength of the soils might be compromised and the riverbank will fail,” said Aitken,
Hicks says the city’s ground crews alert the city to problems and they have no plans on implementing a monitoring program in the future. He says the city is working to re-route the trail in the affected area.
The affected area is near 18th Street West and is fenced off to protect the public from getting to close to the drop-off.
The city took over the maintenance of the Rotary Trail once it was built.