Stabilising large grains in self-forming steep channels

Abstract

Abstract. It is understood that the interaction between sediment supply and discharge drives first-order behaviour of alluvial deposits. The influence of the grain size distribution over the mobility and resultant evolution is, however, unclear. Four experiments were conducted in a scaled physical model for two grain size distributions, analogous to a one-dimensional self-formed alluvial fan. We demonstrate the unsuitability of the median grain size as a predictor of deposit behaviour at flows when the material is not equally mobile. The results instead suggest, during conditions of unequal mobility, that the largest grains control the transport efficiency of the overall sediment mixture, and thus also the morphodynamics of the deposit and its tendency to store or evacuate material. Deposits appear to show a dependence upon the rate of material supply more strongly when the likelihood of its motion is less equally distributed (i.e. under partial transport conditions). If the coarse fraction (e.g. greater than 84th percentile) is instead mobile due to increased discharge or because of their relative size, transport rates will increase and the behaviour of the mixtures converge to a common state, with morphology influenced by the material's mobility.

Morphodynamics of an erodible channel under varying discharge

“…Circled points represent the first measurement taken during each phase largely inherited as the increase in shear stress was not sufficient to entrain larger-than-average grains located at bar-heads. This is consistent with studies that have identified larger-than-average grains controlling sediment transport and thresholds for channel deformation, rather than the average grain size (Booker & Eaton, 2020;MacKenzie & Eaton, 2017;MacKenzie et al, 2018).…”

Mechanisms for avulsion on alluvial fans: insights from high-frequency topographic data

“…Preliminary tests by Hamilton et al (2013) showed that using a more widely graded mixture produced channels that appeared more "realistic". Similarly, experiments by Booker and Eaton (2020), conducted in a narrow flume designed to represent the aggrading conditions on fans, demonstrated that a narrowly graded mixture failed to reproduce the morphodynamics of a widely graded mixture with the same D 50 . In addition, recent studies have highlighted how the largest grains in a distribution, while often only a small fraction of that distribution, have a disproportionately large influence on channel morphodynamics (MacKenzie and Eaton, 2017;MacKenzie et al, 2018;Williams et al, 2019).…”

Mechanisms for avulsion on alluvial fans: Insights from high‐frequency topographic data

“…Similarly, experiments by Booker and Eaton (2020), conducted in a narrow flume designed to represent the aggrading conditions on fans, demonstrated that a narrowly graded mixture failed to reproduce the morphodynamics of a widely graded mixture with the same D 50 . In addition, recent studies have highlighted how the largest grains in a distribution, while often only a small fraction of that distribution, have a disproportionately large influence on channel morphodynamics (MacKenzie & Eaton, 2017;MacKenzie et al, 2018;Williams et al, 2019).…”