Dynamics of gravity currents over a sediment bed
Post-doc project
Gravity currents are key processes that affect atmospheric, ocean and coastal circulation. Most of the gravity currents in nature occur over complex terrain, i.e. rough or mobile terrain with bottom sediments/particles. Avalanches are natural hazards that are observed quite frequently in winter and spring and which cause relevant damages to our infrastructures such as buildings, roads, electric power transmission, and can cost life to humans1, 2. The processes of entrainment from the bottom is also crucial for oceanic turbidity currents on continental shelf or for riverine inflows. In the nearshore for example, the breaking of large waves enhances sediment suspensions that may propagate off-shore as turbidity currents and contribute to the erosion of coastal regions, which are transitional environments playing a crucial role in reconceiving urban areas in view of the increasing sea level rise from climate predictions. Katabatic winds are also an example of an intermittent gravity flow and are important to determine the local air circulation in several regions.
Previous research on gravity currents have focused on dense currents descending a flat or uniform sloping smooth bottom3. Few studies have addressed simultaneously the feedback between the hydrodynamics of a gravity current and the geomorphological changes of a mobile bed. Hydrodynamic quantities such as turbulent and mean velocities, bed shear stress and turbulent stresses undoubtedly govern the processes of entrainment, transport and deposition. On the other hand, the incorporation of entrained sediment in the current may change its momentum by introducing extra internal stresses, thus provoking a feedback process. These two main questions are the object of investigation in this project. A good understanding of the influence of these parameters on the dynamic of gravity currents and related mixing will ultimately allow to deliver detailed information about (i) turbulent subgrid-scale processes needed in ocean, atmospheric and coastal numerical models for a correct parametrisation and (ii) potential zones of high pollution and of high risk in mountain areas and erosion of coastal regions.
REFERENCES
1 Hopfinger, EJ 1983 Snow Avalanche Motion and Related Phenomena. Ann. Rev. Fluid Mech. 15: 47-76.
2 Rastello, M. & Hopfinger, E.J. 2004 Sediment-entraining suspension clouds: a model of powder-snow avalanches. J. Fluid Mech. 509, 181206.
3 Simpson, J 1982 Gravity currents in the laboratory, atmosphere and ocean. Ann. Rev. Fluid Mech. 14: 213-234.
