50 years of beach–foredune change on the southeastern coast of Australia: Bengello Beach, Moruya, NSW, 1972–2022

Abstract

A fifty-year time series of high-resolution field-surveyed beach profiles from a southeast Australian ocean beach was analysed to determine spatial and temporal variability and changes in beach–foredune morphology at annual to decadal time scales. Data from 452 surveys obtained at two to six-weekly intervals was available from four closely spaced (within 422 m) cross-shore profiles in the centre of a 6 km long beach. Each profile extended 110–130 m seaward from a dune datum at ~5 m Australian Height Datum (AHD) to mean sea level (MSL). Morphometric indicators examined included beach–foredune sand volume, four shore position contours at 0 m AHD (MSL) +1 m, +2 m, +3 m AHD, and across-shore topographic changes. Five phases of different beach/foredune conditions were identified, two in the 1970s, and three in the following four decades. After a brief accretion period (phase 1) major storms in May–June 1974 caused the loss of over 100 m3/m from each profile and a 50–60 m landward shift of the MSL shoreline. The substantial sand deficit lasted for the next five years with average beach volumes of 134 m3/m followed by incremental accretion of 120 m3/m from 1979 to 1982 (phase 2). In the next 12 years (phase 3, 1982–1994) the beach maintained high volumes averaging 268 m3/m. This declined by 45–60 m3/m in the mid 1990s (phase 4). Since 2002 (phase 5) sand stored above MSL has averaged 279 m3/m, ~10 m3/m greater than at any time since surveys began although the volume gradually declined in the decade prior to 2022. Morphological changes and shifts in shoreline position have accompanied volumetric changes. An incipient dune developed during the 1980s at the site of the 1970s foreshore later to become an established, vegetated foredune. Comparison of results with other long-term beach profile sites in Europe, Japan and USA showed some differences most notably Bengello exhibits no consistent seasonal summer-winter cycle in beach volume. General similarities include lack of clear beach change evidence of decadal–scale sea level rise, and all locations contend with the impacts of storms. Bengello averages 15 storms per year (H sig ≥3.0 m) and in the fifty-year period there have been 21 major events (H max >10 m) all of which have caused substantial erosion. Recovery times have varied from a few months to over 5 years. The net effect of these erosion-recovery events over 1972–2022 has not resulted in beach recession.