Abstract
Near the threshold of grain motion, sediment transport is “on-off”
intermittent, characterized by large but rare bursts separated by long
periods of low transport. Without models that can predict the presence
of intermittency, measurements of average sediment flux can be in error
by up to an order of magnitude. Despite its known presence and impact,
it is not clear whether on-off intermittency arises from the grain
activity (the number of moving grains) or grain velocities, which
together determine the sediment flux. We use laboratory flume
experiments to show that the on-off intermittency has its origins in the
velocity distributions of grains that move by rolling along the bed,
whereas grain activity is not on-off intermittent. Improved predictions
of sediment flux require that the types of intermittency we identify be
incorporated into stochastic models of sediment flux. Their recognition
opens the door to physically based uncertainty estimates of
time-averaged sediment flux.