Influence of introduced Ammophila arenaria on coastal progradation: assessment by spatial analysis

Ammophila arenaria introduction is known to stabilise coastal dunes and promote sand accretion, but assessment using spatial analysis of its contribution to coastal progradation has been limited. This study assessed long term changes 1950-2016 in shoreline position using quantitative spatial analysis methods, of two adjacent beaches in north Tasmania, one infested during this period by A. arenaria and the other retaining native vegetation. Seven images from each decade were orthorectified, and the Digital Shoreline Analysis System was used to analyse 20m spaced shore perpendicular transects along 3.4km of coastline, to calculate net shoreline movement and digital linear regression rates. Historical ground photographs were also compared with present. Results showed that since the 1960s the A. arenaria-infested beach prograded substantially following introduction at maximum rates of 2.9m a-1, followed by a slowing of rate to reach a halt after 1994, with tall, steep and concave foredunes. The native vegetated beach also prograded, at lower rates of <1.5m a-1 that remained consistent over time, retaining convex incipient foredunes. Relative sea level rise also occurred over the period at equivalent to global eustatic rates, but coastal retreat was not evident. Future erosion may be a greater risk with sand supply locked into high volume A. arenaria-infested dunes, relative to native vegetated dunes.