Modelling the Effect of Seasonal Flow Variations on the Position of Salt Wedge in a Microtidal Estuary

Abstract

The Swan River estuary, a microtidal estuary in the south-west of Western Australia, has a largely seasonal cycle of salinity variation due to the low tidal amplitude and strongly seasonal rainfall. In contrast, macro- and mesotidal estuaries may have semi-diurnal, diurnal or fortnightly periodicities in salinity variation. The temporal variation of the position of the salt wedge in the upper reaches of the Swan River estuary was analysed using field data and a laterally averaged, two-dimensional numerical simulation model for very dry (1993–94) and very wet (1973–74) years in the recent rainfall history. The salt wedge dynamics in the estuary are considered to have three phases; salt wedge dominated during summer and autumn characterized by very low freshwater discharge; salt wedge waning or absent in late winter and early spring with high flow; and salt wedge emplacement during late spring and early summer with low flow. During 1974, when the river inflow was greater than that of other years and remained substantial until early summer, the upstream salt wedge propagation during spring and early summer was strongly inhibited by river discharge. The correlation between the longitudinal location of the salt wedge and the inflow for 1994 (r2=0·86) further suggests that freshwater inflow is the most important mechanism affecting the salt wedge position in the Swan River estuary. When applied to other years this predictive tool gave reasonable predictions of the salt wedge position. The recorded positions of the salt wedge for the 4 years of study were compared against results of numerical model simulations, inflow correlations and the Hinwood formulation. Each method provided an adequate description of the salt wedge location apart from Hinwood's formulation for low river discharges, which greatly overpredicted upstream movement of the wedge.