Analysis of beach and foredune changes by aerial photography and topographic profiles, Tasmania, Australia

Beach foredunes following introduction of Ammophila arenaria have been shown to promote accretion and progradation, but after a few decades, large steep-faced foredunes develop that subsequently erode. Beach profile measurement combined with spatial change techniques have not been applied to investigation of A. arenaria foredune change before. This study investigated two adjacent beaches in Tasmania along 3.4km of coastline, one infested by A. arenaria and the other retaining native vegetation. Dune profile surveys were derived from topographic measurement and LiDAR data, and recent aerial imagery was analysed using the Digital Shoreline Analysis System to quantify net shoreline movement and rates of change. Results showed lack of progradation on the A. arenaria infested beach, with tall, steep-faced, concave foredunes that retreated up to 15m in 10 years. By contrast, the native vegetated beach showed continued progradation, with smaller convex-faced foredunes. The A. arenaria foredunes retreated particularly where the dune toe was lower in elevation. Sediment supply is likely reduced by the tall foredunes with dense vegetation-holding sand, causing storm erosion not to be replaced, hence a lowering beach and dune toe. Future erosion is likely to be a greater risk with sand supply locked into high volume A. arenaria-infested dunes, relative to native vegetated dunes.