A model for sand ridges on the shelf: Effect of tidal and steady currents

The dynamics of sand ridges, as observed on the innershelves and midshelves of various coastal seas, are investigated within the framework of an ideal ized model. These seabed undulations include shoreface-connected sand ridge s and tidal sand banks. The new aspects in this study are the explicit inco rporation of both steady and tidal currents, a shelf with a sloping bottom, and a statistical approach to describe the sediment transport during storm s (linear in the current) and fair weather conditions (cubic in the current ). The present model is used to gain understanding of the different ridge c haracteristics in different geographical regions. This is done by analyzing the initial growth of small bottom perturbations, which evolve on a basic state describing a longshore uniform flow over a reference topography. It i s found that basically five different bottom modes can be excited: shorefac e-connected sand ridges, Coriolis ridges, friction-induced bars, trapped ti dal ridges, and tidal sand banks. Their growth, which is due to different p hysical mechanisms, strongly depends on the weather climate and the geometr ical characteristics of the shelf.