Hikers who cross stream channels in mountainous regions commonly encounter mysterious ridges of bouldery rubble that parallel the channel margins. These well-defined ridges, known as natural levees, can appear almost manmade. Similar features are observed on desert alluvial fans and even in images of Mars. The mechanism of formation of natural levees has remained a puzzle, however, owing to a lack of data obtained under well-controlled, replicable circumstances. Better understanding of the process has practical implications because natural levees channelize hazardous flows and thereby influence areas most likely to be inundated downstream.
Now, the levee-formation process has been explained by analysis of data obtained in large-scale experiments conducted by an international, multidisciplinary team of scientists working at the U.S. Geological Survey (USGS) Debris-flow Flume near Blue River, Oregon. The results were published in the March 22 issue of the Journal of Geophysical Research1. The lead author of the study, Christopher Johnson, is an applied mathematician originally at the University of Manchester (UK) and now based at the University of Bristol. Collaborators include other mathematicians and geoscientists at the USGS Cascades Volcano Observatory in Vancouver, Washington, and at the University of Manchester and University of Liverpool.
Geologists have long known that subtler versions of natural levees can be formed when rivers deposit sediment as they overtop their banks, and that prominent lateral levees can form when debris flows, pyroclastic flows, and other sudden surges of rocky debris shoulder aside part of their load. The new study reveals, in unsurpassed detail, the character of the shouldering process. As a surging flow advances, the largest grains rise to the surface by processes known as kinetic sieving or squeeze expulsion. Once on the surface, where the flow moves fastest, large grains travel to the head of the flow. After arriving at the head, some large grains remain trapped there, pushed forward by the ensuing debris, but others are shunted to the side, overridden, and eventually deposited in levees. The process is self-perpetuating because newly formed levees help preserve downstream flow momentum.
Richard Iverson, director of the USGS flume and an author of the new study, said that the levee research represents the latest in a continuing series of investigations that began at the flume when it constructed 20 year ago. "We were very fortunate to have the wherewithal to build the facility and hire good people when we did," said Iverson. "Our return on investment has been enormous."