ABSTRACT
Mixing in estuaries is caused by turbulence produced both by the velocity shear resulting from bottom friction and by internal shear. The balance between the two varies, the former being dominant in well-mixed estuaries, and the latter in highly stratified estuaries. Whether the shear is sufficient to overcome the stability and cause mixing is represented by the Richardson number. When the gradient Richardson number, Ri, falls below a value of about one quarter, a parallel stratified shear flow becomes unstable. For estuarine mixing the important terms are those relating to vertical exchanges. Whenever the upward velocity fluctuation is correlated with a high density fluctuation, or vice versa, an upward vertical turbulent flux of salt is produced.
