The Role of Grain Dynamics in the Onset of Sediment Transport This averaging procedure provides an ensemble-mean visualization of flow structures responsible for erosion “events”. Preliminary evidence indicates that surface stress peaks are associated with the passage of inclined, high-momentum regions flanked by adjacent low-momentum regions. . The Environmental Fluid Dynamics Lecture Series Presents a Seminar The Role of Grain Dynamics in the Onset of Sediment Transport Dr. Abe Clark Postdoctoral Associate Yale University School of Engineering and Applied Science New Haven, CT Tuesday, September 6, 2016 216 DeBartolo Hall 11 am-12 noon Water that flows over a bed of gravel or sand exerts a shear stress on the grains. A large collection of data, known as the Shields curve, shows that the threshold for grains to become entrained in the flow is primarily a function of the shear Reynolds number Re*. However, the explanation for why this is has remained a mystery. In this presentation, I will use the results from simple numerical simulations to demonstrate that the form of the Shields curve is primarily set by the dynamics of mobilized grains, which depend directly on Re*. Large grains at high Re* are significantly accelerated between collisions with the bed, while small grains at low Re* are not, allowing smaller grains to find more stable configurations. Thus, our results suggest that the onset of sediment transport is best described as when mobile grains can no longer stop. Abe Clark is a postdoctoral researcher in the Department of Mechanical Engineering & Materials Science at Yale. His research centers around describing the dynamics of weakly confined granular materials and applying the results to common problems in geophysics, engineering, and industry. He earned his Ph.D. in Physics from Duke University in 2014, where he studied the response of a granular material to a high-speed impact. At Yale, he primarily studies the onset of sediment transport in using simple computational models.