Scree Slope Development under Conditions of Surface Particle Movement

Abstract

This paper examines the characteristics of slope form and material sorting which develop when a scree is subject to regrading only by the movement of individual blocks immediately after falling on its surface. A basal concavity may develop in the profile because higher energy particles escape the straight slope to accumulate at the slope foot. As the height of the headwall decreases, so the basal concavity becomes shallower, since the energy available to move particles declines, until, with no headwall, the scree tends to adopt a rectilinear form at the maximum angle of stability. Sorting may result from individual particles moving over the sieve-like surface of the scree, because smaller particles tend to be trapped more easily in the depressions. Strong down-slope orientation of particles occurs because particles align themselves to offer least resistance to motion. Field data collected on Cader Idris may be compared with the predicted characteristics of the model. Almost all profiles show some basal concavity with the largest concavities beneath the highest headwalls. A significant downslope sorting was found in nine of the eleven traverses and, at almost all sample points, particles possessed a strong down-slope orientation. These field observations and those of other workers suggest that the model is viable for many rockfall screes. SCREES developing under conditions of rockfall are subject to the impact energy of particles hitting the scree surface. The energy of a falling particle is utilized in moving itself, and other particles with which it is in collision, down-slope. These surface movements will be responsible for regrading the slope and, providing no other processes of redistribution are in operation, developing the form and character of the deposit. Should the process be insufficient to maintain the slope below the maximum angle of stability, dry avalanching will assume the dominant role. A traditional assumption about scree slopes is that they are at the angle of repose (O. Fisher, i866; J. C. Marr, I900; W. H. Ward, I945), and that small disturbances may give rise to large mass-movements. This is sometimes, but by no means always, the case. J. T. Andrews (1961) found that screes in the English Lake District had a wide variation in slope angle, and concluded that many were well below their maximum stable angle. P. J. Howarth and J. G. Bones (1972) have noted that screes having high rates of basal removal, and therefore at their angle of repose, are steeper than adjacent screes which have no removal at the foot. R. J. Chandler (I973) has also confirmed this observation and suggests that angles of repose may also be attained under high rates of deposition. H. Jeffreys (1932) considered that, on impact with a scree surface, a particle acquires a rotation, and its velocity is reduced. Between any two successive bounces the important constraints on motion are a loss of energy owing to friction and imperfect restitution, and a gain owing to work done by gravity. If the loss is greater than the gain the particle tends to stop, thus steepening the slope, and if the reverse is true, it tends to accelerate off the slope. This implies an equilibrium between the supply of energy by impact and the loss of friction, the form of the slope being adjusted to that equilibrium.