Gravity currents: a personal perspective.

Gravity currents, driven by horizontal differences in buoyancy, play a central role\ud in fluid mechanics, with numerous important natural and industrial applications.\ud The first quantitative, fluid-mechanical study of gravity currents, by von K´arm´an in\ud 1940, was carried out before the birth of this Journal; the next important theoretical\ud contribution was in 1968 by Brooke Benjamin, and appeared in this Journal more\ud than a decade after its birth. The present paper reviews some of the material that has\ud built on this auspicious start. Part of the fun and satisfaction of being involved in\ud this field is that its development has been based on both theoretical and experimental\ud contributions, which have at times been motivated and supported by field observations\ud and measurements

Gravity currents: a personal perspective.

Gravity currents, driven by horizontal differences in buoyancy, play a central role in fluid mechanics, with numerous important natural and industrial applications. The first quantitative, fluid-mechanical study of gravity currents, by von K´arm´an in 1940, was carried out before the birth of this Journal; the next important theoretical contribution was in 1968 by Brooke Benjamin, and appeared in this Journal more than a decade after its birth. The present paper reviews some of the material that has built on this auspicious start. Part of the fun and satisfaction of being involved in this field is that its development has been based on both theoretical and experimental contributions, which have at times been motivated and supported by field observations and measurements

Energy balances for propagating gravity currents: homogeneous and stratified ambients

The exchange of energy for an inviscid gravity current which is released from a lock and then propagates over a horizontal boundary is considered. Attention is focused on effects due to stratification in the ambient. The investigation uses both a one-layer shallow-water model and Navier–Stokes finite-difference simulations. There is good agreement between these two approaches for the energy of the dense fluid (the current). The results indicate that with respect to the behaviour of energy as a function of time we can distinguish between: (a) currents propagating at supercritical speed (with respect to the fastest internal wave in the ambient), including a nose propagating into an unstratified ambient; and (b) currents propagating at subcritical speed, including the strongest effective stratification for which the density at the base of the ambient is equal to that of the current. The stratification enhances the accumulation of potential energy in the ambient and reduces the energy decay (dissipation) of the two-fluid system. The interaction of the internal waves with the head of the current in the subcritical case has no significant influence on the energy balance of the current

Viscous gravity currents inside confining channels and fractures

Geodynamics geophysics and tectonics; heh1; DAMPT

Extreme natural hazards: population growth, globalization and environmental change.

Mankind is becoming ever more susceptible to natural disasters, largely as a consequence of population growth and globalization. It is likely that in the future, we will experience several disasters per year that kill more than 10 000 people. A calamity with a million casualties is just a matter of time. This situation is mainly a consequence of increased vulnerability. Climate change may also be affecting the frequency of extreme weather events as well as the vulnerability of coastal areas due to sea-level rise. Disastrous outcomes can only increase unless better ways are found to mitigate the effects through improved forecasting and warning, together with more community preparedness and resilience. There are particular difficulties with extreme events, which can affect several countries, while the largest events can have global consequences. The hazards of supervolcanic eruptions and asteroid impacts could cause global disaster with threats to civilization and deaths of billions of people. Although these are very rare events, they will happen and require consideration. More frequent and smaller events in the wrong place at the wrong time could have very large human, environmental and economic effects. A sustained effort is needed to identify places at risk and take steps to apply science before the events occur

Flow of buoyant granular materials along a free surface

© 2018 Cambridge University Press. We study experimentally the flow of light granular material along the free surface of a liquid of greater density. Despite a rich set of related geophysical and environmental phenomena, such as the spreading of calved ice, volcanic ash, debris and industrial wastes, there are few previous studies on this topic. We conduct a series of lock-release experiments of buoyant spherical beads into a rectangular tank initially filled with either fresh or salt water, and record the time evolution of the interface shape and the front location of the current of beads. We find that following the release of the lock the front location obeys a power-law behaviour during an intermediate time period before the nose of beads reaches a maximum runout distance within a finite time. We investigate the dependence of the scaling exponent and runout distance on the total amount of beads, the initial lock length, and the properties of the liquid that fills the tank in the experiments. Scaling arguments are provided to collapse the experimental data into universal curves, which can be used to describe the front dynamics of buoyant granular flows with different size and buoyancy effects and initial lock aspect ratios