Mixing and entrainment in developing stratified currents

The spatially developing region of a steady downslope current in transition from a hydraulically controlled, arrested wedge flow is examined through a set of laboratory experiments. The mixing and entrainment mechanisms at th e shear interface are explored with the use of imaging techniques. The init ial, unstable accelerating region, characterized by rapid development, low shear layer Richardson numbers and high entrainment rates, is limited by th e effects of stratification which suppress the large-scale overturning. A s ubsequent high Richardson number region is characterized by weak entrainmen t and the collapse of turbulence from the initial region. The flow approach es a quasi-stable state as the mixed layer draws energy from the accelerati ng density current. Observed large-scale structure is attributed to an inst ability of the marginally unstable shear layer. Entrainment rates are calcu lated within each region for the various slopes considered and estimates fo r the extent of the transition regions are obtained.