Aspects of aquatic ecology of the Hawkesbury estuary. I. Hydrodynamical processes

Abstract

The Hawkesbury estuary can be divided into a saline intrusion region and, further upstream, a freshwater region. Under wet weather conditions a thin freshwater lens is formed at the free surface of the saline intrusion region. This lens, which is rapidly flushed to the ocean, is composed of 'new' runoff water floating passively over the underlying layer of 'old' saline estuarine water. On the other hand, under dry weather conditions the saline intrusion region is sectionally well mixed by tidal turbulence although strong density currents are generated by the longitudinal gradients of salinity. A mathematical model is proposed for the tidal hydrodynamics of the estuary. The model is used to compute the dry weather distribution of concentration of contaminants (primarily suspended particles from industrial and other land-use activities, and nutrients from present and future municipal sewage treatment plants). It is suggested that the former already do and the latter may, by the year 2000, contribute significantly to pollution problems in the upper reaches of the estuary. A study of the effects of these contaminants on primary production is deferred until a future publication. It is suggested that a basin-wide study of Sydney municipal wastewater disposal alternatives, including ocean disposal, should be urgently undertaken.