Revisiting the problem of sediment motion threshold

The definition of the threshold of sediment motion is critical for continental shelf sediment dynamics. The work by A. Shields laid the foundation for this research direction, leading to the well-known Shields curve. Here we review the most widely used threshold curves that have followed from the original Shields curve over the last 80 years, and propose that in terms of physical processes the threshold (critical Shields parameter) is a function of at least six variables, i.e. grain Reynolds number, grain size distribution, sphericity, roundness, particle cohesiveness and the scale effects of turbulence. Identifying these key factors, we paid a special attention to the role of the scale effects of turbulence. Turbulence was thought to be a random process, but the improvement of measurement techniques revealed that it has both temporal and spatial structures: the magnitude of instantaneous velocity fluctuations varies in time and in location, which can cause the deviation between in situ measurements and flume experiments. In coastal and shelf waters, in situ measurements of tidal currents and suspended sediment concentrations have revealed that resuspension takes place even though the bed shear stress is well below the Shields curve. Further process and mechanism studies are required to improve the theoretical framework regarding the turbulence structures and their interplay with sediment threshold. The scientific problems for future studies include the establishment of laboratory experiments, in situ measurements and process-based modelling under different water depths and hydrodynamic conditions to quantify the scale effects of turbulence; the development of new observation techniques for higher resolution and for extreme environments; development of new data processing methods, including big data methods to analyse turbulence structures; and the quantification of the effects of biological contributions and non-particle components on the family of Shields curves.