在海洋湍流混合与沉积物输运研讨会（广州，2016年9月12-14日）上我的发言摘要为：

Scale effects of turbulence: revisiting the problem of sediment motion threshold

Shu Gao

State Key Laboratory forEstuarine and Coastal Research, East China Normal University, Shanghai 200062,China

E-mail:sgao@sklec.ecnu.edu.cn

Abstract: The definition of the threshold ofsediment motion is critical for marine sediment dynamics. The work by A.Shields laid the foundation for this research direction. Taking into account anumber of properties of fluid and sediment, the Shields curve is therelationship between the Shield number and the grain Reynolds number, whichexpressed implicitly the threshold for spherical particles with the samediameter. In the real world, sediments have different grain size distributions,with different shapes and roundness values. Thus, flume experiments werecarried out to include these factors. Using the grain size parameters and theshape factor values (representing both shape and roundness effects), the curvewas expanded to a family of Shields curves. This was thought to be stillinsufficient, because the phenomenon of cohesiveness associated withfine-grained materials was not considered. Subsequently, efforts had been madeto distinguish the cohesive and non-cohesive sediments. Some investigatorsattempted to link the threshold to the settling velocity of particles, but itappears that the problem of settling velocity itself is not simpler than thatof the threshold.

To add to this complexity, the scale effects ofturbulence also affect the threshold. Turbulence used to be considered as arandom process, but the improvement of measurement techniques soon revealedthat turbulence has both temporal and spatial structures: the magnitude of instantaneous velocity fluctuations varies in time and in location. Thisimplies that for the same mean current speed or near-bed shear stress, theaddition of the fluctuation magnitudes may influence the threshold. Forinstance, in flumes the limited water depth does not allow intense fluctuations,but in coastal waters with strong tidal currents the magnitude of fluctuationsmay be enhanced. As a result, the threshold measured by mean shear stress wouldbe higher in the flume. In the Jiangsu coastal waters, where there is a mega-tide condition, in situmeasurements of tidal currents and suspended sediment concentrations haverevealed such an effect. Resuspension takes place even if the shear stress iswell below the value predicted by the Shields curve method. In the future itwould be meaningful to undertake further process and mechanism studies, toimprove the theoretical framework for the threshold of sediment motion.