Sediment dynamics below retreating cliffs

The retreat of cliffs may constitute the dominant erosional response to base-level fall in arid settings underlain by horizontally-bedded sedimentary rock. These vertical cliffs typically loom above a relatively straight bedrock slope ('plinth') that is mantled with a thin layer of sediment and perched near the angle of repose. In detail, a plinth consists of a system of quasi-parallel ridges and channels. We ask how the sediment supplied from a retreating cliff influences the erosion of the plinth hillslopes and channels, and how this affects the rate of cliff retreat. Motivated by field observations and high-resolution topographic data from two sites in western Colorado, we develop a two-dimensional (2D), rules-based numerical model to simulate the erosion of channels draining a plinth and diffusive erosion of the intervening interfluves. In this model, retreat of a cliffband occurs when the height of the vertical cliff exceeds a threshold due to incision by channels on the plinth below. Debris derived from cliff retreat is distributed over the model plinth according to the local topography and distance from the source. This debris then weathers in place, and importantly can act to reduce local bedrock erosion rates, protecting both the plinth and ultimately the cliff from erosion. In this paper, we focus on two sets of numerical model experiments. In one suite, we regulate the rate of rockfall to limit the cliff retreat rate; in most cases, this results in complete loss of the plinth by erosion. In a second suite, we do not impose a limit on the cliff retreat rate, but instead vary the weathering rate of the rockfall debris. These runs result in temporally steady cliff-plinth forms and retreat rates; both depend on the weathering rate of the debris. Copyright (C) 2011 John Wiley & Sons, Ltd.