Tidal anomalies at tide gauges on the open coast and close to the mouth of rivers on the East Coast of Australia have been investigated with the aim of identifying the causes of extreme tail-water levels which are the necessary basis for flood modelling. The study covers the period 1987 to 1997 which brought several tropical cyclones into the study area. The present study provides reliable statistics up to recurrence periods of the order 10 years. The tide gauge data show anomalies up to 0.8m and down to -0.4m during the study period. The reason for the positive anomalies showing greater magnitude than the negative ones is that some driving mechanisms, e.g., rainfall are only able to generate positive anomalies.
Spectral analysis of the anomalies reveals that most of the variance is at periods longer than 8 days and tidal anomaly spectra have their most significant peak at 20 days. This is somewhat surprising since visual
inspection of weather maps leads to the expectation that most of the variance would lie in the 2 to 5 day band: The typical time for the passage of a weather system. Correlation analysis and spectral analysis give hints but cannot decide on causality because most of the possible forcing agents are strongly correlated. That is, extreme barometric pressure, strong winds, heavy rainfall and large ocean waves all occur together with violent weather events. The East Australian Current, is the only "independent" forcing agent.
Remote extreme weather events can have strong effects on local tides via traveling continental shelf waves. On the East Coast of Australia these usually originate at Bass Strait and travel North. The coincidence of the trough of one such wave with the landfall of Cyclone Violet more or less cancelled the expected strong positive anomalies due to low barometric pressure, rainfall and strong onshore winds during this cyclone.
This study shows rather conclusively that wind waves, which may cause setup of the order 2m of the shoreline on beaches during storms, cause no measurable (<5 cm) setup in trained river entrances.