Have we misunderstood the Shields curve?

The Shields curve, which compiles measurements of fluvial sediment transport thresholds in terms of the nondimensionalized threshold fluid shear stress and shear Reynolds number, is a standard reference in geophysics and hydraulic and coastal engineering and commonly thought to describe the critical flow conditions that are required for flow-driven entrainment of bed sediment. However, recent findings from several independent research groups have challenged this belief on various grounds: (i) particle-bed impacts predominately trigger sediment entrainment [1]; (ii) such impact-triggered entrainment can sustain continuous transport even when flow-driven entrainment events are (almost) completely absent [2, 3, (4)]; and (iii) extrapolating measurements of the transport rate of such impact-sustained continuous transport to zero yields transport thresholds that still fall on the Shields curve [5]. The question that thus emerges from these findings is, if not flow-driven entrainment, then what is the physics behind the thresholds shown in the Shields curve? We will try to give an answer to this question based on our latest research.

 

[1] Vowinckel et al., Journal of Hydraulic Research, 2016, doi: 10.1080/00221686.2016.1140683

[2] Pähtz & Duran, Physical Review Fluids, 2017, doi: 10.1103/PhysRevFluids.2.074303

[3] Lee & Jerolmack, Earth Surface Dynamics, doi: 10.5194/esurf-6-1089-2018

[4] Heyman et al., Journal of Geophysical Research: Earth Surface, 2016, doi: 10.1002/2015JF003672