Recent progress in the study of granular materials, improvements in field instrumentation, and the increased capabilities of large-scale computer simulations have led to a better quantitative understanding of particle transport by a turbulent fluid, the interaction between transported grains and the bed, the development of surface feature on the bed, and their subsequent motion and interaction. Further progress will require advances in our understanding of particle-fluid interactions and the modification of particle-particle interactions in the presence of a fluid. This program is intended to accelerate these advances by bringing together physicists and experts on geological processes and morphology to address the questions that remain.

The focus of the program will be on the transport of particles by wind in air and gravity-driven flows in water. Examples are aeolian transport and turbidity currents, respectively. These systems have both common and unique features. They both involve turbulent fluid flow mitigated by dilute concentrations of particles through much of their body, important interactions between particles and particles and the fluid in denser concentration near and at the bed, and mobile beds that may be eroded, deposited upon, and deformed. They differ in the properties of the fluid with which the particles are interacting and the forces that drive the flows. Between them, they possess features of flows that transport sediment in rivers and along coastlines and volcanic ash and powder snow down mountainsides.

Because the study of such geophysical flows has typically been descriptive rather than predictive, the program has the potential to be transformational, i.e., it should lead to field measurements that are informed by theory. Because the length scales of the phenomena involved, field measurements of their features and flows around them are crucial to testing predictions. In the context of the program, geo-morphologists will have an opportunity to learn what can be predicted and what should be measured; physicists will learn what should be predicted and what can be measured. Together, they can fruitfully address the role of laboratory experiments.

A discussion of the relationship between turbulent flow, particle transport, and bed features observed in wind tunnels and water flumes and those measured in the field will necessarily raise issues of scaling. Scaling is also central to understanding the relationship between aeolian and aquatic transport and bed forms. As a consequence, characterization and comparisons between the measured and predicted scaling of geological features associated with air and water transport will be an important dimension of the program.