Mixing in gravity currents

We study entrainment in lock-release gravity currents using highly spatially-resolved optical transmission experiments and quantitative analysis of the available potential en- ergy of the flow. The principal results provide a resolution to the debate regarding the mechanism and degree of mixing in the head of gravity currents during the slumping phase. The nature of the complex internal mixing structure changes as a bore propagates from the tail to the head of the current during the slumping phase, and transitions to mixing at the leading edge of the flow as the bore extinguishes a billow at the trailing edge of the head and overtakes the current itself. We use quantitative methods to identify this connection between dynamics and entrainment and show that its manifestation as examined using different methodologies is the cause of previous contradictory experimen- tal findings. Therefore, we conclude that the two main perspectives previously considered at odds are in accord.