Logjams aren’t really jammed at all, say geoscientists
One of the lesser-known consequences of forest fires is logjams—river channels clogged with wood. The mechanism is straightforward. Forest fires create vast areas of dead wood. During the winter, heavy snow leads to avalanches that push thousands of the burned logs to the bottom of river valleys, where they enter the water.
When the logs stretch across the river, from bank to bank, the river can no longer flow. It is this formation that has long captured the public imagination: the word “logjam” has come to mean a situation in which movement, physical or otherwise, is impossible.
As part of the natural system of forest renewal, logjams confer great benefits to wilderness regions. But they can also cause significant problems when they interfere with boats, bridges, or other artificial structures. So a better understanding of the way they behave is desperately needed.
Enter Nakul Deshpande and Benjamin Crosby at Idaho State University in Pocatello, who say that nobody has studied the way logjams evolve over time. “No in-situ field measurements have assessed the degree of arrest in a naturally-formed logjam,” they say.
Today, these guys change that by studying the evolution of a logjam for the first time. Their guinea pig is a logjam on Big Creek, a tributary of the Salmon River in central Idaho. Their surprising conclusion: logjams that appear locked solid aren’t jammed at all. Instead, the logs move downriver, albeit slowly, driven by the ebb and flow of water, as the river level rises and falls.
First some background. Logjams are relatively common in the Salmon River watershed, which drains 14,000 square miles of coniferous forest land in central Idaho. The US Forestry Service estimates there are currently around 20 logjams in the various tributaries that feed the river.
Deshpande and Crosby chose one on Big Creek, a steep, mountainous tributary of the Middle Fork Salmon River. In 2014, after several seasons of forest fires and a heavy winter, a series of snow avalanches delivered a large number of logs into Big Creek.
Over the next year, other logs from further up the river washed down and joined the jam. Today, the logjam contains more than 1,000 logs. It spans the width of the river and stretches for over 70 meters (230 feet). To the casual viewer, it is locked solid.
Deshpande and Crosby set up cameras to photograph the jam during May and June 2016. They used the images to track the positions and orientations of 132 logs and to create time-lapse videos of the phenomenon. They also recorded the flow rate and depth of the river.
Their results make for interesting reading. Their first observation is that the logs form a kind of herringbone pattern. This is partly because many of the logs are longer than the channel is wide. So one end of a log can float and move with the flow of water while the other is stuck on the bank.
Of course, this also implies movement. Indeed, Deshpande and Crosby’s time-lapse videos clearly confirm this. “Despite the namesake, we find that the logjam is not jammed,” they say.
Instead, the logs rise and fall with the water level, which changes with the rate of snowmelt. “As water rises and log-drag against the bed and banks decreases, they collectively translate downstream,” say the researchers. “As streamflow recedes and the logs reconnect with the bed and banks, the logs settle opportunistically amongst their neighbors.”
This leads to a cycle of pinning, free movement, and further pinning that depends on the river flow.
That’s interesting work that throws some light on a surprisingly misunderstood phenomenon. It also reveals how the process of pinning, clogging, and creep is similar to the behavior of other disordered materials—a science known as granular rheology.
That could help marine engineers better understand how to manage logjams and mitigate the damaging effects they can have on artificial structures. “This can lead to more informed wood retention structures for restoration, bridge pier design, river corridor management and flood hazard mitigation,” say Deshpande and Crosby.
But whether this discovery will ever change the common usage of the word “logjam” is another question entirely.
Ref: arxiv.org/abs/1911.01518 : Logjams are not jammed: measurements of log motions in Big Creek, Idaho
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