Intermittency in bed load sediment transport

Bed load sediment transport near the threshold of motion is inherently noisy. Grains are moved by turbulent flow and are in nearly constant contact with other grains, resulting in sediment flux that is intermittent, displaying bursts in both space and time. Understanding the dynamical origin of these fluctuations, and their consequences for the morphodynamic evolution of alluvial channels, is a challenge. The first half of my talk will focus on the description and origins of so-called "on-off" intermittency observed in bed load sediment transport near the threshold of motion. We use a combination of laboratory flume experiments and a simple stochastic model to capture the on-off intermittent statistics and show that the on-off intermittency lies in the velocity of the grains which are rolling on the bed. The second part of my talk will introduce a work in progress: the development of an intermediate-complexity model, a cellular automaton, of bed load sediment transport with grain dynamics based on a simple set of rules. Mimicking collision interactions, and inspired by the concept of collective entrainment (where entrainment rate depends on the number of active grains), grains in our model are entrained by their active neighbors with a probability that depends on the local bed slope. Despite this simplified representation of grain dynamics, the model reproduces the intermittent statistics of grain activity observed in previous studies. A variant of the model with a basic flow parametrization and an avalanching condition reproduces some aspects of observed channel cross-sectional evolution, including widening in response to increased fluid discharge.