Wave Setup in River Entrances - Field Surveys of Wonboyn Lake Estuary and Tallebudgera Creek Estuary, Australia

Abstract: It is unclear whether wave setup exists in river entrances and previous studies have shown that it is virtually non-existent in trained river entrances. However, conclusive studies have not been made on semi-trained and untrained river entrances. In this study, two field surveys were conducted in Australia; one at Wonboyn Lake Estuary, New South Wales, and one at Tallebudgera Creek Estuary, Queensland. At Wonboyn Lake, water surface elevations were recorded along the open coastline and inside the lagoon at a number of locations. Current data was also recorded along with topography and bathymetry. The data showed that there were significant elevation differences (100-140 mm) between the river entrance and offshore water levels, despite the measurement points being close to one another and the net flow in the entrance being zero. A similar dataset was recorded at Tallebudgera Creek, but due to problems with the weather, the data was not complete. Two models, one analytical and one numerical, were used to analyze whether wave setup may have occurred at the river entrance in Wonboyn Lake. The analytical model was originally designed to calculate wave setup along the open coast, but an attempt was made to apply it to a river entrance. The resulting output from the river entrance was uncertain due to this, but showed there may have been a setup of approximately 70 mm at the aforementioned point in time when the difference was 100-140 mm. The numerical model (HEC-RAS) was employed using bathymetry and topography data recorded in the Wonboyn Lake area. The model showed that the driver of the lagoon water level was the higher water level in the entrance and not the offshore MWL. The conclusion of the Wonboyn Lake analysis was that there may have been wave setup, or some other phenomena, that raised the level of the water in the entrance from the offshore water level, since the water level in the entrance was the driver of the system. However, since the analytical model was too uncertain to be conclusive it was not possible to confirm or discard the occurrence of wave setup in the river entrance. It was not possible to conduct a reliable analysis of the Tallebudgera Creek data, since it was too fragmented to apply any of the two previously mentioned models to it. Finally, it is recommended that more research is made on the subject of wave setup, if it is to be concluded whether it exists in river entrances or not.