An integral model of unsteady salinity intrusion in estuaries

In this paper a model of unsteady salinity intrusion in estuaries is developed using integration of the tidally averaged salinity balance equation to yield a nonlinear, first-order differential equation for the salinity intrusion length scale, L. This equation yields changes in L that are asymmetric with respect to flow increases and decreases, showing that the response time of salinity intrusion to flow changes is dependent on L and not directly on flow. An extension of this approach to an estuary for which the area varies exponentially with distance suggests that the simpler prismatic case embodies the essential physics of the response to flow changes. Hindcasts of the response of the salinity field in Northern San Francisco Bay to flow pulses based on the simple model are in good agreement with observations, suggesting that dynamical models of the type described here may be more accurate than ones that are purely empirical.