The hydraulic and morphological response of a large coastal lake to rising sea levels

The hydraulic and morphological response of a large coastal lake system on the Australian east coast is assessed in regards to rising mean sea levels. The relative influence of sea level rise (SLR) in relation to other significant anthropogenic perturbation to the ocean inlet is investigated. Numerical modelling techniques are used to assist in predicting the likely behaviour of tides and estuarine flushing based on a hypothetical inlet channel configuration following sea level rise. The estuary examined in this paper is Lake Macquarie, a large coastal lake on the central coast of NSW. The analysis indicates that the tidal range within the lake could be expected to double by the year 2100 (for a SLR scenario of 0.91 m), and is associated with a 225% larger tidal prism. The predicted increase in the volume of the tidal prism is estimated to be equally attributed to SLR and the continuation of inlet scour currently occurring in response to entrance training. Mean levels in the lake are expected to rise by an amount similar to the predicted SLR. Lake flushing is indicated by modelling to be enhanced under sea level rise, possibly improving the water quality of the estuary but altering estuarine ecosystem characteristics. The findings of this paper have significant ramifications for the planning and management of this and other estuaries and associated floodplains in relation to both the built and natural environments. Specific examples include the extent of land subject to inundation risk, future navigational dredging requirements and the impact on fringing ecologies.