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

Mechanisms responsible for observed ripple geometry and grain size trends a t two open coast sites are considered. The field sites an two moderate-ener gy sandy shelves on the inner continental shelf out to 50 m depth at East G ippsland in southeastern Australia and at Pakiri in northeast New Zealand. Both are characterised by bands of medium sediments inshore and offshore, s eparated 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 Ea st 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 phy sical forcing, possibly in near-equilibrium with modem processes. Numerical modelling demonstrated that grain sorting on the shelf is initiated by the presence of maxima in predicted ripple activity and seabed roughness aroun d 35 m depth. The local grain size evidently increases due to winnowing of the fines. Positive feedback, through dependence of ripple size on grain si ze, causes further lengthening of ripple wavelength, until the pattern has evolved to become highly pronounced on the shelf and self-sustaining, but l imited by reducing sediment mobility as grain size increases. (C) 1999 Else vier Science B.V. All rights reserved.