Infrared Laser Examines Mudslides

Daniel C. McCarthy

Mudslides cause millions of dollars of damage and kill hundreds of people every year. Unlike other natural phenomena such as hurricanes or even earthquakes, mudslides are more challenging to predict; consequently, they have received little focused research. One attempt during the 1970s in Japan resulted in several deaths when a mudslide crossed a valley and buried researchers on the opposite slope.

An infrared GaAs diode-pumped laser from LMI Selcom Inc. is helping to determine characteristic forces inside mudslides in this controlled experiment at Oregon's Willamette National Forest.

A new approach is under way in Oregon's Willamette National Forest where researchers led by US Geological Survey hydrologist Richard Iverson are inducing controlled mudslides and measuring their progress down a 150-foot vertical drop through a 310-foot cement flume. Iverson's team measures the depth and character of each flow using a GaAs diode laser (780 nm) from LMI Selcom Inc.

Science has documented and come to understand the flow dynamics of the solids in avalanches or the fluids in floods. However, mudslides are a mixture of these solid and fluid forces and present characteristics of both, Iverson explained. "What we've found is that flows move as surges, and the surge front is dominated by solid forces. Fluid forces, in the form of liquefied sediments, follow behind. Knowing that the character of these forces change in relation to location helps us predict how fast and how far a mudslide will move," he said.

In an earlier analysis of debris-flow behavior the researchers attempted to reconstruct sediment origins from deposited mud. Later, Iverson's team tried ultrasound to measure flows in action, but this method failed to provide time/space resolution effectively. Then came attempts with visible spectrum lasers, but these performed poorly in daylight. Finally, Iverson affixed LMI Selcom's infrared laser at an angle above the flume. Light from the laser reflects off the flow passing beneath and into a photodetector. In combination with pressure and flow sensors in the concrete bed, this optical triangulation enables precise measurements of the mudslide's flow depth from which Iverson can calculate the nature and strength of forces within the mudslide.

The types of lasers that Iverson uses also have been fastened to automobile suspensions to monitor vehicle response to road surfaces. "In this case, instead of the laser moving over a surface to derive a measurement, we're moving the surface under the laser," Iverson said.