Debris flows are concentrated mixtures of water and loose rock and mineral material that flow downslope, usually in a preexisting channel, under the pull of gravity. Speeds range from a slow walk to a speeding automobile. They differ from ordinary sediment-transporting streamflow in that the motivating force for the downslope movement comes directly from the pull of gravity on the sediment–water mixture. In sediment-transporting streamflow, in contrast, the water flows because of the pull of gravity, and the sediment is moved by the flowing water. (See the earlier chapter on rivers for a lot more material on sediment-transporting stream flow.)
The classic debris flow has sediment concentrations by volume of over fifty percent—almost to the point at which the sediment particles lock together and prevent movement. It also is characterized by extremely poor sorting of the solid material, from clay-size particles to house-size boulders. Up until recently, it was generally believed that the proportion of mud in a debris flow had to be high. It’s know known, especially from laboratory experiments on debris flows, that such flows can happen even at concentrations of muddy material as low as five percent. But below that concentration of mud, debris flows can’t happen.
The classic debris flow is nonturbulent to only weakly turbulent: the mixture moves smoothly and without the vigorous turbulent mixing that’s so characteristic of sediment-transporting streamflow. Many debris flows, however—the ones that are relatively “thin” and somewhat more like ordinary streamflow—are clearly turbulent.
One of the big sticking points in accounting for the existence of debris flows has been the mechanism (or mechanisms) by which large clasts can remain suspended in the flow in the absence of turbulence. A number of effects seen to contribute.
- First of all, there’s the buoyancy effect: the large clasts feel themselves to be immersed in a fluid medium with density much greater than that of clear water, because of the high concentration of finer sediment. The large clasts are therefore only slightly negatively buoyant.
- In flows with relatively high concentrations of clay-mineral particles, what is called matrix strength, caused by the electrostatic cohesive forces between clay particles, probably plays a role.
- Finally, the upward dispersive effect of strong collisions among clasts as the mixture is sheared is thought to be an important effect.
Debris flows are of much more than scientific interest. That’s because deposition from debris flows is typically by rapid halt of the entire mass rather than incrementally along the path of the flow. Large, fast-moving debris flows can bury entire valleys, demolishing villages and all of their inhabitants, when the moving mass finally comes to rest. Stream valleys on the flanks of active explosive volcanoes in humid regions are especially susceptible, because volcanic ash weathers readily to fine-grained, clay-rich material, and heavy rains can mobilize such material into a massive debris flow, called a lahar (an Indonesian word).
Bloom, A.L., Geomorphology; A Systematic Analysis of Late Cenozoic Landforms, Third Edition. Prentice Hall, 482 p. (Chapter 9)
Easterbrook, D.J., 1999, Surface Processes and Landforms, Second Edition. Prentice hall, 546 p. (Chapter 4)