The fluid mechanics of natural disasters

Although we’ve managed to tame large portions of the Earth’s surface over the last couple of millennia, there are still some forces of Nature that elude us. We eat humble pie when faced with avalanches, tsunamis, earthquakes, landslides, hurricanes and severe storms. At best, we manage to get out of the way and then pick up the pieces once the damage is done. At worst, hundreds, even thousands of people perish and billions of dollars’ worth of property is destroyed. We’re constantly reminded of our vulnerability; just this past January mudslides and flash floods in Brazil caused at least 700 deaths and left 14,000 others homeless. The best strategy is to avoid putting ourselves in their path in the first place. It is thus more important than ever to understand where, when and how these catastrophic natural events occur. Professor Christophe Ancey, head of ENAC’s Environmental Hydraulics Laboratory, studies natural flows – snow, debris, and mud – in an effort to contribute to this understanding.

In Switzerland and elsewhere in Europe, authorities manage territory – deciding where things can be built and where they can’t – based on the degree of danger each area presents. They typically estimate the intensity and frequency of catastrophic natural events by extrapolating from historical databases. “This works well if you have a lot of data,” notes Ancey. “But it doesn’t work well in areas that are less populated.” Switzerland has the added difficulty of mountainous terrain, which poses its own set of problems. “Hazards encountered on the plain – flooding, earthquakes and severe storms – are relatively well understood. But in the mountains these phenomena are more complicated.” For several years, Ancey’s lab has been developing tools that will help create a physical basis for understanding the fluid dynamics of snow, debris and mud flows in mountainous terrain, and predicting the environmental conditions that will set off these kinds of catastrophic events.

In natural debris flow and avalanches, a phenomenon known as “self-channeling” often occurs. Large chunks of debris pile up at the edges, channeling the rest of the flow into a narrow tongue that then travels long distances. Ancey explains that this is caused by a segregation of particles in the flow; as the large particles rise to the top and the smaller ones sink, energy dissipation is reduced and the runout distance increases by as much as a factor of ten. The smaller particles act as a kind of conveyor belt, keeping everything in motion. Ancey runs scaled down experiments in his lab, simulating flow with a viscous blue mass of goo, taking measurements to see the rate at which the particles are segregating in the various layers within the flow. He’s also collaborating with the United States Geological Survey (USGS ) in an experiment in Oregon in which debris is launched down a 100-meter flume.

We often assume that these phenomena only occur on steep slopes, but in fact they can also occur on gently sloping terrain, stopping and starting repeatedly over a long period of time. Ancey shows me a video clip from a mudslide that occurred in a small town in southern Italy (http:// wn.com/Italian_Landslide_Feb_2010). He interested in studying the factors involved in initiating debris flow such as this. What starts the ground moving? What are the relative roles played by soil type, water load and slope? And once it’s moving, what are the forces that keep it in motion? In the video, this element is dramatic; just when you think the ground has come to a stop, the entire hillside starts flowing again. With an improved understanding of these phenomena, explains Ancey, hazard maps could be established for regions – inhabited or not – that would prevent this kind of catastrophic loss of life and property. Because when these kinds of natural forces are at work, the best strategy is to just stay out of the way.