Dynamics of talus formation

Abstract

Many areas at the foot of steep rocks are subject to periodic falls as shown by the presence of abundant stone accumulations, called talus. Taluses are relatively common formations that may occur at rather different scales: from a few meters to thousands of meters. Numerous field surveys have provided quantitative information on the characteristics of a talus deposit. However, much less has been done on the dynamics of a talus evolution. In this work, a further effort is made to understand the dynamics of talus formation based on experimental studies at the small scale. Firstly, the complexity of the processes forming a talus deposit is simplified by studying the interaction of only two kinds of grain sizes. Grains of one size class are cast from a certain height down a plate covered by a grain of another size class. The plate is inclined with a certain angle. The final distribution of grains as a function of the distance from the fall point is then measured and analyzed. In addition, a high-speed camera is used to monitor the instant of impact. When small grains fall on large grains, a nearly exponential decrease of grain distribution as a function of the distance from the fall point is observed. On the other hand, large grains falling on a granular bed formed by smaller grains lose much more energy in the impact, but then may roll down slope down the whole plate length. Because rolling friction appears so important in the dynamics, a dedicate study was also devoted to study the process of rolling on a granular medium, and found that large spheres may actually reach a longer run-out, the density being the same. Hence, these experiments in conjunction can clarify in a more quantitative manner the distribution of rock size along a talus slope. In a further study a small-scale talus was created, allowing for many different grain sizes to interact among each other. We found that talus evolution is not always reducible to elementary processes. Collective processes may occur as well such as creep, grain migration through the granular bed, and avalanching. Finally, a particular class of relatively uncommon talus slopes was considered: the ones formed by flat stones. After measuring the properties of these talus in one field example from Southern Norway, a small-scale replica was made. It was found that these taluses are dominated by a more uniform rock size distribution along slope, which is a consequence of the predominance of sliding versus rolling.