Flushing criteria in estuarine and laboratory experiments

Laboratory experiments have shown the range of densimetric Froude numbers for which a depression that is filled with saline water and located at the bottom of a watercourse will be flushed by the overflow of fresh water. If this densimetric Froude number, based on the water speed, the density difference between the saline water in the depression and the overflowing fresh water, and the depth of the depression, is greater than 1.5 or 2, then the saline fluid is purged very quickly in what is called the one-shot mode. (The range of Froude number depends on the cavity geometry and the manner of the initiation of the overflowing current). For lower values of Froude number the density interface also exhibits an internal wave that sweeps along the cavity ejecting fluid from it, and any liquid of high salinity that remains in the cavity is subsequently removed by turbulent entrainment. Data from a field test of an estuary in Chile show the occurrence of an internal wave that was formed at the upstream sill during ebb tide, the amplitude of which compares favorably with the results collected in the laboratory.