Wave mechanisms responsible for grain sorting and non-uniform ripple distribution across two moderate-energy, sandy continental shelves

Abstract

Mechanisms responsible for observed ripple geometry and grain size trends at two open coast sites are considered. The field sites are two moderate-energy sandy shelves on the inner continental shelf out to 50 m depth at East Gippsland in southeastern Australia and at Pakiri in northeast New Zealand. Both are characterised by bands of medium sediments inshore and offshore, separated at 20–45 m depth by a zone with significantly coarser grain size (0.9 mm cf. 0.3 mm). Video observations of the seabed at 126 sites on the East Gippsland shelf revealed that the band was also characterised by larger wavelength ripples (1.0 m cf. 0.3 m). The size of the band stretching over an average of 20 km from 20–45 m depth and its presence at both field sites suggests that the observed characteristics are a long-term response to physical forcing, possibly in near-equilibrium with modern processes. Numerical modelling demonstrated that grain sorting on the shelf is initiated by the presence of maxima in predicted ripple activity and seabed roughness around 35 m depth. The local grain size evidently increases due to winnowing of the fines. Positive feedback, through dependence of ripple size on grain size, causes further lengthening of ripple wavelength, until the pattern has evolved to become highly pronounced on the shelf and self-sustaining, but limited by reducing sediment mobility as grain size increases.