Bathymetric changes in three estuaries of the central New South Wales coast

Abstract

North American and northern European evidence suggests present infilling of estuaries with marine sediment as sea level has continued to rise. Along the New South Wales coast such infilling mainly has ended because sea levels have been stable since 6500 B.P. Bathymetric changes for the period 1868-1974 in three estuaries on the central coast of New South Wales show considerable contrast due to varying estuarine dynamics and sediment availability. Changes in Botany Bay have now stabilized or tended towards slight erosion (- 1332-1547 cm³ m^-2 year^-1) while changes within Port Jackson seaward of Bradley's Head (-7694 cm³ m^-2 year^-1) can be linked to dredging activities this century. In Broken Bay, substantial accretion has occurred at the ocean entrance (7684 cm³ m^-2 year^-1) and around Brisk Bay (5430 cm³ m^-2 year^-1). The former accretion can be attributed to movement of sediment landward of the 20-m contour under waves characteristic of the present wave regime, and to availability of sediment from adjacent semi-compartmentalized beaches. Such sediment movement and reservoirs do not exist at nearly as large a scale at the mouths of the other two Sydney estuaries. The latter accretion is coincident with the boundaries of marine sand intrusion up the estuary and of fluvial transport seawards. Because bathymetric change in areas of Broken Bay affected by ocean swell can be linked to wave hydrodynamics, changes in the wave climate such as increased storminess can lead to general bottom erosion. Such changes can also exacerbate erosion within Port Jackson and Botany Bay.