Flooding and flow path selection on alluvial fans and deltas

Abstract

[1] The surfaces of alluvial fans and river deltas (collectively fans) are often dissected by a small number of channels radiating from the fan apex. On long timescales, channels migrate via avulsion, the process of channel bed deposition and abandonment that often results in catastrophic flooding and loss of life on densely populated fans. We present results of an experimental fan that creates realistic channel patterns by avulsion. The avulsion cycle occurs with a period that is predictable from conservation of mass. Selection of a new flow path is inherently stochastic; however, once a network of 4-5 channels is established, flow oscillates among these channels indefinitely. We demonstrate that a directed random walk model with memory quantitatively reproduces these dynamics and limiting behavior, and is consistent with natural fans. Citation:

Causes and effects of noise in landscape dynamics

“…The characteristic scales of avalanches can be readily derived from the feed rate of sand, the size of the container, and the friction between the grains of the material. Amazingly, the more complicated system of an experimental alluvial fan-in which a self-formed river channel cuts and fills a sediment wedge via sediment transport driven by a turbulent fluid-displays avalanching dynamics identical to those of the sand cone in the hourglass, both of which lead to noisy sediment transport (Figure 1) [Reitz et al, 2010]. This stick-slip cycle of sediment accumulation and failure is a generic consequence of transport close to the threshold of motion.…”

Experimental alluvial fans: Advances in understanding of fan dynamics and processes

“…The type of flow, subcritical or supercritical, also has an impact not only on the surface flow patterns but also on the backfilling of unoccupied channels (Hamilton et al, 2013), impacting the surface topography and potentially the future positioning (and reoccupation) of channels as they migrate across the fan surface (Reitz et al, 2010;Reitz and Jerolmack, 2012). The impact of a critical slope threshold on channel incision was noted in early experiments (Hooke, 1968;Schumm et al, 1987;Whipple et al, 1998), and then further highlighted by the slope control between sheetflow and channelized flow in autogenic cycles (van Dijk et al, , 2009(van Dijk et al, , 2012Hamilton et al, 2013), and the initiation of avulsion events (Bryant et al, 1995;Reitz et al, 2010). However, downstream boundary conditions can interfere with the usually dominant upstream processes on alluvial fans and alter the slope threshold values and system response dramatically (Clarke et al, 2010).…”

“…A series of experiments simulating conditions of steep alluvial fans to isolate the dynamics of the avulsion process during fan evolution were run at the University of Pennsylvania (Reitz et al, 2010). These found that avulsions followed a regular cyclic pattern, which consisted of the following stages: (i) flow abruptly ACCEPTED MANUSCRIPT collapses into a single channel, focusing rapid deposition at the channel mouth; (ii) progradation causes a decrease in slope until the channel can no longer transport the supplied load; (iii) within-channel deposition drives backfilling from the fan margin toward the apex; and (iv) produces a brief finding phase (i.e., a myriad of potential flow paths are filled with water) (Reitz et al, 2010, p. 1).…”

Shape and dynamics of seepage erosion in a horizontal granular bed

“…The flow of water erodes the land which in turn modifies the flow producing meanders and networks of rivers [1][2][3]. Of significant interest is the case of seepage erosion or sapping [4][5][6] in which a flow of water occurs inside a permeable and erodible layer of sediment above an impermeable layer of a different composition.…”