Rock fragment characteristics, patterns and processes on natural and artificial mesa slopes

[Truncated abstract] Rock fragments on hillslopes interact with fine soil and vegetation by affecting infiltration, runoff, erosion and evaporation, and therefore have an important function in arid ecohydrological systems. Previous studies have described rock fragments and their spatial patterns using simple measures such as mean or median size. This study presents research conducted on three natural mesa slopes and a post-mining waste rock dump, in the Great Sandy Desert, Western Australia, to characterize the statistical properties of rock fragments and interrelationships between particle shape and size. Digital images of surface rocks were collected along transects placed on each hillslope. A total of 112,142 rock fragments from 263 locations were recorded. From these images perimeter, area, Feret's diameter and circularity of rock fragments were determined. On natural mesas, mean Feret's diameter, and similarly area and perimeter decreased while mean circularity increased downslope. The results indicated that larger and more angular rock fragments occurred on the top of these hills. From a suite of probability distributions tested, lognormal distribution was found to describe the Feret's diameter best. Furthermore, both the location and scale parameters of the lognormal distribution decreased approximately linearly with distance down each transect. None of the probability distribution functions tested sufficiently characterised the distributions of circularity. Transport process such as overland flow has been the predominant explanation for the observed particle sorting on rock armoured slopes. It has been suggested as a mechanism that selectively washes fine material away, leaving coarser particles on steeper part of hillslopes.