Lower-stage plane bed topography is an outcome of rarefied, intermittent
sediment transport
Abstract
Sedimentary bed configurations that are stable under weak fluid-driven
transport conditions can be divided into two groups: (1) meso-scale
features that influence flow and sediment transport through roughness
and drag partitioning effects (“mesoforms”), and (2) grain-scale
features that can effectively be ignored at the macroscopic scale
(“microforms”). In practice, these groups delineate ripples and dunes
from quasi-planar bed configurations. They are thought to be separated
by a transition in processes governing the relief of the bed; however,
the physical mechanisms responsible for this transition are poorly
understood. Previous studies suggest that planar topography is unstable
when interactions between moving particles lead to stabilized bed
disturbances that initiate morphodynamic pattern coarsening. This study
presents a kinetic interpretation of this hypothesis in terms of
parameters describing particle motion. We find that the
microform/mesoform transition corresponds to a critical transition in
particle behavior associated with increasing importance of particle
collisions. This transition also corresponds to the point where
continuum-based morphodynamic models are permissible at the most
unstable wavelength predicted from linear stability theory, providing a
link between descriptive and mathematical theories of bedform
initiation.