Transient mechanics of bed load sediment transport

Bed load transport, the movement of sediment by water or air in close proximity to the river bed or ground surface, is governed by complex interactions among fluid, particles, and evolving bed topography. Recognizing the limitations of deterministic, equilibrium approaches in capturing these interactions and accurately predicting bed load flux, recent studies have shown the advantages of adopting a stochastic approach, i.e., one that treats system components as random variables. One hallmark of bed load is the presence of unpredictable and often extreme temporal fluctuations in transport rates. The goal of this dissertation is to understand these transient processes, both as generated by the internal dynamics of a steadily-driven bed load system and in response to changes in external forcing. Field and experimental observations have been obtained to study the ensemble statistics of bed load transport over time scales ranging from milliseconds to hours and to consider how statistical metrics of bed load change with observational time scale. Flume experiments and field observations show how sand bed forms (ripples and dunes) respond in a lagged manner to changes in water discharge, and how this lagged response produces hysteresis in bed form dimensions through flood waves. Field observations of wind-blown sand demonstrate how turbulent fluctuations in wind stress drive lagged variations in sand flux that produce systematically time-scale biased estimates of the stress-flux relationship. Flume experiments that track gravel particle trajectories reveal the role of ballistic particle motion, intermittent transport, and heavy-tailed particle waiting times in producing particle dispersion that deviates from normal diffusion expectations; these experiments also indicate the potential for the statistical mechanical formalism to describe bed load dispersion. Idealized two-dimensional flume experiments demonstrate the importance of particle collisions for incipient motion and collective transport, and they show how the distribution of particle waiting times is related to the stochastic but mean-reverting tendencies of bed evolution. Together, these observations of stochastic bed load processes reveal characteristic time-scale signatures that can inform attempts to distinguish the origin of transport fluctuations arising from external forcing versus intrinsic fluctuating transport dynamics