Climate Change Impacts on Entrance Processes of Intermittently Open/Closed Coastal Lagoons in New South Wales, Australia

Abstract

HAINES, P.E. and THOM, B.G., 2007. Climate change impacts on entrance processes of intermittently open/closed coastal lagoons in New South Wales, Australia. Journal of Coastal Research, SI 50 (Proceedings of the 9th International Coastal Symposium), 242 – 246. Gold Coast, Australia, ISSN 0749.0208. Intermittently open/closed coastal lagoons are a relatively unique feature of the south-east Australian coastline, and NSW in particular. Future climate change is expected to modify a number of climate variables that define the physical and chemical structure and ecological behaviour of the NSW coastal lagoons. These climate variables include mean sea level, wave climate and rainfall behaviour (total depth and storm intensity). Entrance morphodynamic processes are particularly vulnerable to climate change. These processes, which describe the opening and closing cycle of these lagoons, are controlled by dominant coastal (wave and sea level) conditions and episodic catchment runoff events. The magnitude of entrance impacts will depend on the particular characteristics and location of individual lagoons. An increase in mean sea level is likely to move the entrance sand berm upward and landward. An increase in south-easterly waves will cause minor rotations in beach planform alignments with expected net erosion from southern ends of beaches and net accretion at northern ends. For lagoon entrances at the southern end of beaches, this will result in a net loss of sand from the entrance and a shortening of the entrance channel. For lagoon entrances at the northern end of beaches, this will result in a net growth of the entrance berm, possibly offsetting the recession resulting from mean sea level rise. Increased typical lagoon water levels behind the berm are also expected. Marine flood tide deltas may accrete vertically and more landward. A decrease in total rainfall depth is likely to retard the cycle of filling and draining, and reducing the frequency of entrance breakout for mostly closed lagoons. For mostly open lagoons, however, an increase in rainfall storm intensity may actually increase scour through the entrance, thus increasing the longevity of ocean connections.