Author(s): Michel A. Verbanck
Linked Author(s): Michel Verbanck
Keywords: Lluvial regime; Antidunes; Flow-sediment-morphology system; Mobile bedforms; Vortex drag model; Separated and reattaching flow; Movable-bed resistance; Sediment transport capacity
Abstract: A new 1D approach is presented to compute the rate of dissipation of mean-motion energy in alluvial streams flowing over mobile bed forms. It treats flow over sandy alluvium as a combination of boundary-attached and-detached flow features, both necessary to explain how well-marked protrusions can develop spontaneously in the bed profile and be maintained in response to extreme water-sediment discharge and stream power conditions. Basing on the various bedform shapes, the influence they exert on alluvial channel resistance, sediment transport capacity and turbulence-damping is explicitly considered. Bedform types occurring in the upper alluvial regime are at the centre of the analysis. Based on the model and on confrontation with available experimental evidence, it is suggested that the antidune standing-wave flow configuration is associated with minimum possible drag, creating low-turbulence high-velocity conditions ideal to evacuate extreme river discharges. The boundary-attached component of the model introduces a generalized Froude number formulation in alluvial hydraulics, to represent topographicallyforced gravity waves occurring in non-shallow environments such as ripple fields. For the boundarydetached component of the model, we introduce a ‘control factor m’ (m≥1) which reflects a complex feedback-control loop process active in the separation cell immediately downstream of the bedform crest. Lift-up of bed particles (under the action of the corresponding vortices shedding out of the reattachment region) implies that control factor m should also play an important role in the further development of bursting-based conceptual models of non-cohesive sediment transport.
Year: 2004