USA — The news this week has been full of warnings about debris flows, especially in and around the area that was burned last year in the Station Fire. But for someone like me, who came to California from the flatlands of the midwest, the danger may be difficult to grasp.
I understand the damage done by tornados – I’ve seen tornados. But debris flow? Just what are they talking about?
Geomorphology “is the scientific study of landforms and the processes that shape them.” One of those processes is “mass wasting“, when “soil or rock moves downslope under the force of gravity.” This movement can be slow or fast. Debris flow is fast: “A mixture of water-saturated rock debris that flows downslope under the force of gravity”. As the flow moves downslope, it picks up all sorts of debris – large boulders, trees, even parked cars. And it can move quickly – often greater than 35 miles per hour and as much as 52 miles per hour.
Debris flow is both predictable and unpredictable. It’s predictable in that we can measure the factors leading up to the onset of debris flow: topography, wildland fire, and rainfall. The steeper the slope, the more water-saturated the soil, the less vegetation, the greater the chance of debris flow.
It’s unpredictable is that we don’t know exactly when and where the downward force of gravity will overcome the shear strength (internal force of cohesion and friction). For a clearer explanation of how this works, check out thiswebsite.
Geomorphologists have names for different types of debris flows. They can be a mixture of water and small dirt particles – a mudflow. Or they can be a mixture of water and volcanic particles – a lahar. And they can be a mixture of soil and water sliding over permafrost – solifluction (a condition that occurs in the polar regions or on high mountains). What they all have in common is that they are a slurry – a suspension of solids in a liquid, they move fast and they can cause significant damage, including loss of life.
It some ways, tornados and debris flow are alike – accompanied by rainfall, generally predictable, specifically unpredictable and fast moving. But if I had a choice, I think I’d prefer a tornado to debris flow. At least, with most tornados, you can see them coming long enough to head down to the basement.