Three-dimensional model for the formation of longshore sand structures on the continental shelf

A model is proposed for the formation and evolution of three-dimensional sedimentary structures such as longshore sand ridges on the continental shelf in water deeper than that of the shoaling region. Owing to the striking similarity between the bar spacing and the length scales in which interactions among the most energetic modes of shallow water waves take place, we argue that these bars are formed slowly by flows in the turbulent boundary layer generated by weakly nonlinear, dispersive waves. The model is based on the interaction between surficia! or internal, weakly nonlinear shallow water waves, having weak spanwise spatial dependence, and the bottom topography. While such underwater structures are not the result of a single foimative agent, it is argued that the mechanism proposed in this study does contribute significantly to their generation and evolution..