Interaction between an inclined gravity current and a pycnocline in a two-layer stratification

A series of laboratory experiments were conducted to investigate the characteristics of a dense gravity current flowing down an inclined slope into a quiescent two-layer stratification. Such flows may be observed as brine effluent from desalination plants are released into the near-coastal environment. The presence of the pycnocline causes the gravity current to split and intrude into the ambient at two distinct levels of neutral buoyancy, as opposed to the classical description of gravity currents in stratified media as being either a pure underflow or interflow. The splitting behavior is observed to be dependent on the Richardson number (Ri) of the gravity current, formulated as the ratio of the excess density and the ambient stratification. For low Ri, underflow is more dominant, while at higher Ri interflow is more dominant. As Ri increases, however, we find that the splitting behavior eventually becomes independent of Ri.

At longer times, reflections of the underflow and the interfacial waves continue to induce mixing, which may be more representative of confined basins in nature. We find that mixing due to reflections is more prevalent in higher Ri regimes, mainly due to breaking internal waves that lead to boluses that travel far upslope from the original pycnocline position. At lower Ri, the final distribution of the gravity current is overall less affected by mixing induced by reflections, even as the gravity current fluid at the base of the water column continues to get mixed due to the pressure gradients caused by the internal waves. Both wave-wave and wave-current interactions are observed, along with shear instabilities driven by the movement of the ambient fluid, which is triggered by the release of the gravity current.