Decomposition of Residual Circulation in Estuaries

The residual currents in estuaries are produced by a variety of physical mechanisms. To understand the contribution of each individual mechanism to the creation of residual circulation, it is necessary to separate the effect of one particular mechanism from the others. In this study, a method based on dynamics is developed to decompose the residual circulation into individual components corresponding to different forcing mechanisms. Specifically, residual flows are partitioned based on the separate contributions by river discharge, horizontal density gradient, internal tidal asymmetry, advection, semi-Stokes transport, and wind. The method includes the effects of the earth's rotation and can be applied for general conditions. Under the precondition that the ratio between width and length of the estuary is small, the continuity equation can be simplified such that the method only requires the data at a cross-estuary section to decompose residual currents. This makes the method practicable for real estuaries. Results from a generic numerical model are used to illustrate the decomposition method and to demonstrate its validity for weakly stratified estuaries.