From Foreshore to Foredune: Foredune Development Over the Last 30 Years at Moruya Beach, New South Wales, Australia

Abstract

Between 1974 and 1976, a series of east-coast cyclones in the western Tasman Sea resulted in extensive coastal erosion along southeastern Australia. In many beach compartments, the backshore and incipient foredune were completely removed, and the sea cut back into the swale and/or second dune ridge. This occurred at Moruya Beach, where a profile monitoring program had been established in 1972—a program that continues to this day. Here we report field evidence describing the initial condition of the beach, its subsequent erosion (such that the position of the initial backshore became the foreshore), and how this foreshore became reconstituted as a backshore ultimately developing into the present foredune. Critical to the formation of the frontal dune was the presence of a broad backshore berm at an elevation of 2.3 to 2.8 m above local mean sea level (MSL). Achievement of this elevation did not, by itself, guarantee foredune development. Rather, there is also a width threshold to the berm, which at Moruya is at least 30 m. While the berm reached either this elevation (2.3 to 2.8 m) or width (>30 m) on several occasions prior to the formation of the incipient foredune, it was only when both conditions were satisfied that the embryo foredune developed into an incipient foredune. This was in the late 1970s and early 1980s. Incremental vertical growth of the foredune took place over the next 15 years, from an elevation of ca. 3.0 m up to 5 m above MSL. Initially, the position of the newly accreted foredune was well seaward of its prestorm (1972) position, but in the last few years it has tended to migrate inland, though the geographic position of the mean and high water level intercepts have not migrated with it.