Estuary Flushing Time

Flushing times for 69 estuaries that are permanently open and 115 estuaries that are intermittently open were estimated using different methods. The classical tidal prism method of Dyer (1973) utilising the ratio of estuary volume to tidal prism, modified by a coefficient representing entrance exchange efficiency, was used for the 69 permanently open systems. For a further 69 systems intermittently closed for significant periods, a lower bound to flushing time was calculated based on the cumulative rainfall over a number of events, and accounting for evaporation, required to breach the entrance berm. For 46 systems that open under relatively minor rainfall, much of the runoff volume will flow through the estuary out to the ocean. In this case, the flushing time begins converging to the period between the size of rainfall event required to breach the entrance berm.; ; Flushing times were estimated to support the development of a new response-based estuary classification system based on dilution and flushing characteristics. The classification system was used to stratify estuaries in NSW for the development of reference conditions and the design of a sampling program of chlorophyll a, turbidity and associated water quality parameters to assess estuary health.