TABLE OF CONTENTS The assumption made in Equation 5.16 to arrive at the steady state equation for conservation of mass, requires that in the estuary, an equilibrium condition is reached between, on the one hand, advective salt transport through the downstream flushing of salt by the fresh water discharge, and, on the other hand, the full range of mixing processes induced by the tidal movement and the gravitational circulation. The time required for equilibrium to occur depends on 1) the rate at which the boundary conditions vary, in particular the rate of change of the fresh water discharge, and 2) the time required for the estuary system to adjust itself to a new situation. During the dry season, when the problem of salt intrusion is most acute, the variation of the fresh water discharge is generally slow. The question is: does the system react at the same pace as the boundary conditions, or does it need more time? In the latter case, the system lags behind steady state.
The estuary system reacts quite differently to an increase and to a decrease in the fresh water discharge. Generally the estuary reacts relatively quickly to an increase of the discharge. The new volume added at the upstream end of the estuary propagates as a mass wave through the system. However, the reaction to a decrease in the fresh water discharge is slow, since the process of salinization, gradually replacing the fresh water by saline water through mixing,...
