It's landslide season. Just in time for the mud rampages, a group of geophysicists have released a series of videos showing how to create the worst mudslides imaginable. The key? Water pressure.
Image by Damian Dovarganes/Associated Press
A team of researchers used an outdoor lab called the USGS Debris-Flow Flume, which is a 95-meter-long slide. Scientists send huge amounts of rocks, debris, and water shooting down the flume in order to understand the physics of, well, flowing debris. Lasers and other instruments mounted on the flume measure everything from the flow's speed and mass, to how compact the substance is as it flows.
The researchers had a simple question: Why do mudslides often speed up and gain mass when they are zooming over a wet landscape? It turns out that the answer is related to why cars hydroplane on rain-slick roads. When mudslides rush over wet ground, the grains of dirt and sand in the flow become very compact, creating millions of tiny pockets of water under extremely high pressure. Like that heavy car careening over a wet road, the mudslide is basically skating over the top of the wet ground on a thin layer of highly-pressurized water. That's why the mudslide gains speed over wet ground, but slows down over dry ground.
But unlike the hydroplaning car, the mudslide also gains mass. The same forces that make the mudslide zoom over the ground so quickly also allow it to "scour" its muddy bed, picking up the ground that it's skating over. So as the mudslide's speed increases, so too does its size. That helps to explain why mudslides that pour over a wet mountainside may seem relatively small when they start, but wind up causing enormous damage by the time they reach level ground.
Here's a video taken a couple of years ago when the researchers were doing the tests that led to the discoveries chronicled in their Nature publication yesterday. I love the way the lead researcher's introduction sounds completely dry, and then once the mudslide starts, you hear the whole geophysics group whooping and cheering as the watery mixture comes sliding down the flume, gathering speed and mass. (You'll see the mudslide repeatedly from several different camera angles.)
You can watch dozens more mudslide videos via the USGS Debris-Flow Flume website.
And you can read the full scientific paper about this research at Nature Geoscience.