Gravel threshold of motion: a state function of sediment transport disequilibrium?

• Main Author: J. P. L. Johnson
• Format: Article
• Language: English
• Published: Copernicus Publications 2016-08-01
• Series: Earth Surface Dynamics
• Online Access: http://www.earth-surf-dynam.net/4/685/2016/esurf-4-685-2016.pdf

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 (<i>τ</i>_c*) vary with many factors related not only to grain size and shape, but also with characteristics of the local bed surface and sediment transport rate (<i>q</i>_s). I propose a new model in which <i>q</i>_s-dependent <i>τ</i>_c*, notated as <i>τ</i>_{c(<i>q</i>_s)}*, evolves as a power-law function of net erosion or deposition. In the model, net entrainment is assumed to progressively remove more mobile particles while leaving behind more stable grains, gradually increasing <i>τ</i>_{c(<i>q</i>_s)}* and reducing transport rates. Net deposition tends to fill in topographic lows, progressively leading to less stable distributions of surface grains, decreasing <i>τ</i>_{c(<i>q</i>_s)}* and increasing transport rates. Model parameters are calibrated based on laboratory flume experiments that explore transport disequilibrium. The <i>τ</i>_{c(<i>q</i>_s)}* equation is then incorporated into a simple morphodynamic model. The evolution of <i>τ</i>_{c(<i>q</i>_s)}* is a negative feedback on morphologic change, while also allowing reaches to equilibrate to sediment supply at different slopes. Finally, <i>τ</i>_{c(<i>q</i>_s)}* is interpreted to be an important but nonunique state variable for morphodynamics, in a manner consistent with state variables such as temperature in thermodynamics.