Holocene Beaches, Bedforms and Sediment Dynamics of Geographe Bay, Southwestern Australia
Authors: Gee R Dennis 
Published: January 2025

Geographe Bay is a large near-symmetrical north-facing bay in southwest Australia, characterised by a continuous arcuate sandy beach extending over 50 km, and a wide microtidal shallow inner continental shelf devoid of significant currents. It displays shoreline features in dynamic equilibrium with present metocean conditions, superimposed on relict bed-forms dating from the end of the Last Glacial Maximum. Records of historic shorelines, in places going back to 1941, show that about 90% of the bay’s beachline is accreting, whereas only about 10% is subject to natural erosion. Accretion is progressive, but not cyclic—estimates indicate the beach accumulates sand at about 100,000 m3/yr. Total long-shore littoral sand fluxes on both western and eastern arms of the bay are about 42,000 m3/yr, implying a remarkable addition of 142,000 m3/yr to the system. It is unlikely that such a volume was generated by coastal erosion, rivers draining into the bay, or erosion of offshore sandbars and ridges. Large, linear, stationary, sand ridges oriented at about 60° to the beachline are a distinctive bathymetric feature of Geographe Bay and extend to the 10 m isobath. These ridges are largely stabilized by seagrass meadows and are interpreted as relics from sometime in the last 7,000 years as their size, asymmetry and orientation are incompatible with present metocean conditions. They possibly formed due to intense high-pressure continental systems at the conclusion of the last Pleistocene glacial event after which a strong clockwise gyre may have operated in the bay. Small shore-attached sandbars, about 30° oblique to the beach, overprint the larger ridges to form a complex network bare of seagrass. Although locally reworked, they have been stationary for the past 80 years, and do not contribute to the sediment budget. The large Dunn Bay bars at the western end of the bay commenced building soon after sea-level recession from the Holocene highstand reached its present stillstand at about 3,770 yrs BP. A field of small, northeastly ripples, between the 6 m and 9 m isobaths, are evident on LIDAR bathymetry as fine lineaments that bulge shoreward into the seaward limits of the larger bedforms. They relate to refracted Southern Ocean swells that pass around Cape Naturaliste but are not actively importing sediment into the bay. The sand influx probably comes from seaward of seagrass meadows at depths of 10–15 m on the inner continental shelf. There is an inexhaustible supply of sandy sediment on the now-inundated post-glacial plain, derived from erosion of the Cretaceous Leederville Formation during the protracted sea-level falls from much higher levels through the Cenozoic. At present, northwesterly winter storm waves feed sediment into the bay where it is dispersed by seasonal wind-generated waves. Southwesterly winter fronts and summer sea breezes drive the major dispersal of ~35,000 m3/yr along the eastern arm of the bay, whereas a minor pathway of ~7,000 m3/yr along the western arm is driven by easterly winds. Under this regime, coastal erosion is not a major threat to Geographe Bay.