Relationship between sediment supply and avulsion frequency in braided rivers

Abstract

The interplay between sediment supply (S s ), sedimentation rate (S r ), and the frequency of channel avulsion (A f ) exerts a primary control on alluvial architecture. In order to investigate the effect of sediment supply on avulsion frequency, four Froude-scale model experiments of an aggrading braided river were undertaken in which the magnitude of S s was progressively increased over an eightfold range. The value of A f increases at a rate slower than the increase in S s , contrary to the trend previously reported by Bryant, Falk, and Paola in their experimental study on alluvial-fan dynamics. These results suggest that the relationship between A f and S s is dependent upon bed slope and that the response of A f to an increase in S s in unconfined braided rivers may be different than that on steep alluvial fans.

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

“…Sediment flux increased, the avulsion rate first increased and then stabilized as mass flows began to influence deposition, and the experiments also showed that less total sediment volume was needed to trigger an avulsion as the sediment supply rate increased. This is in contrast to the findings of Ashworth et al (2004Ashworth et al ( , 2006 who used Froude-scale modelling to observe avulsions in braided rivers and found that avulsion frequency increased with sediment supply but at a rate slower than the increase in the sediment feed rate. Ashworth et al (2004) concluded that the most probable cause of this difference was the dependency of sediment supply and avulsion frequency on bed slope; the steeper fan slopes of Bryant et al (1995) created multi-channel systems that are inherently less stable.…”

“…This is in contrast to the findings of Ashworth et al (2004Ashworth et al ( , 2006 who used Froude-scale modelling to observe avulsions in braided rivers and found that avulsion frequency increased with sediment supply but at a rate slower than the increase in the sediment feed rate. Ashworth et al (2004) concluded that the most probable cause of this difference was the dependency of sediment supply and avulsion frequency on bed slope; the steeper fan slopes of Bryant et al (1995) created multi-channel systems that are inherently less stable. The probability of avulsion is therefore likely to be greater with a smaller change in sediment supply, leading to the conclusion that alluvial fans are more prone to avulsion events than river systems.…”

Self-similar growth of a bimodal laboratory fan

“…Previous observations support this intuition, but there is no consensus yet about the slope's physical origin, which involves the response of a single channel to sediment transport and its destabilization into multiple threads (Whipple et al, 1998;Ashworth et al, 2004). We do not find any correlation between sediment discharge and slope in our experiment (Fig.…”

“…Producing experimental alluvial fans has become common in geomorphology (Schumm et al, 1987;Bryant et al, 1995;Whipple et al, 1998;Ashworth et al, 2004;Van Dijk et al, 2009;Clarke et al, 2010;Powell et al, 2012;Reitz and Jerolmack, 2012;Clarke, 2015). Here, we use a setup similar to that of, for example, Whipple et al (1998) to generate a radially symmetric fan over a horizontal basal surface (Fig.…”

“…Thus, as a consequence of the symmetry, the radius of the fan increases as (Q s t) 1 3 , where t is the time elapsed since the beginning of the experiment (Powell et al, 2012;Reitz and Jerolmack, 2012). Avulsions occur more frequently as the sediment discharge increases, showing that the internal dynamics of an experimental fan adjusts to the forcings (Bryant et al, 1995;Ashworth et al, 2004;Clarke et al, 2010;Reitz and Jerolmack, 2012). This adjustment allows the fan to maintain its conical shape, which, at first order and for a single grain size, is characterized by its slope only.…”