The dynamics of sand ridges, as observed on the innershelves and midshelves of various coastal seas, are investigated within the framework of an idealized model. These seabed undulations include shoreface-connected sand ridges and tidal sand banks. The new aspects in this study are the explicit incorporation of both steady and tidal currents, a shelf with a sloping bottom, and a statistical approach to describe the sediment transport during storms (linear in the current) and fair weather conditions (cubic in the current). The present model is used to gain understanding of the different ridge characteristics 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 is found that basically five different bottom modes can be excited: shoreface-connected sand ridges, Coriolis ridges, friction-induced bars, trapped tidal ridges, and tidal sand banks. Their growth, which is due to different physical mechanisms, strongly depends on the weather climate and the geometrical characteristics of the shelf. ¿ 2001 American Geophysical Union