Gravel threshold of motion: A state function of sediment transport disequilibrium?
Abstract. In most sediment transport models, a threshold variable dictates the shear stress at which non-negligible bedload transport begins. Previous work has demonstrated that nondimensional transport thresholds (τ*c) vary with many factors related not only to grain size and shape, but also with characteristics of the flow and surrounding grains. Both a conceptual model and flume experiments suggest that τ*c should evolve as a function of local entrainment and deposition. Net entrainment preferentially removes more mobile particles while leaving behind more stable grains, gradually increasing τ*c and reducing transport rates. Net deposition tends to fill in topographic lows, progressively leading to less stable distributions of surface grains, decreasing τ*c and increasing transport rates. A new model is proposed for the temporal evolution of τ*c as a power-law function of net erosion or deposition, which shares some similarities with the Exner equation. Model parameters are calibrated based on flume experiments that explore transport disequilibrium. The τ*c-evolution equation is then incorporated into a simple morphodynamic model. The evolution of τ*c acts as negative feedbacks on morphologic change, while also allowing reaches to equilibrate to sediment supply at different slopes. Finally, τ*c is interpreted to be an important but nonunique state variable for channel morphology, in a manner consistent with the role that state variables such as temperature play in describing the evolution of thermodynamic systems.