Event

The Earth's surface is a granular-fluid interface, shaped by the feedbacks among water/air flow, topography and particle transport. A bewildering array of patterns arise due to the unstable nature of a sheared, granular free-surface; witness dunes, river networks and barrier islands, for example. For almost a century, research on understanding landscape patterns and predicting rates of sediment transport has focused on developing an ever-more detailed description of the turbulent fluid; the coupling to particle motion is often an afterthought. In this talk, I turn this approach around by asking "How much of sediment transport can be understood by treating it as a granular problem?", and outline recent results from our research group that seek to answer this question. I present evidence that granular phenomena such as jamming, creep, aging and segregation all occur in rivers. This motivates a new experimental study in which we create a laminar "river" in a coutte cell, and probe the dynamics of particle motion from the granular-fluid interface to deep inside the bed. We observe three phases of transport that exist simultaneously and are stratified in elevation: a dilute suspension at the top, which transitions to a dense granular flow, which then transitions to a creeping regime. A recently proposed unified granular rheology is found to describe the suspension and dense-granular flow regimes remarkably well. However, the transition to creep is not anticipated by the model or by the jamming hypothesis; creep is characterized by localized and intermittent particle motion indicative of a granular solid. I describe the nature of these regime transitions in the context of recent developments in soft-matter systems, and the implications of these findings for natural rivers. This work shows that much of fluid-driven sediment transport may be understood as a granular physics phenomenon, implying that the details of fluid flow are not important. However, the geophysical problem of a river presents a new geometry - a free-surface, in which there is slip between the applied fluid and the grains - that poses new challenges to our understanding of granular systems. I highlight results from our ongoing work examining some of these challenges, including the motion of grains at the free surface and also shear-induced segregation of mixed grain sizes.