THE ORIGIN AND STABILITY OF SAND RIDGES ON SABLE-ISLAND BANK, SCOTIAN SHELF

Two models are used to predict the evolution of sand ridges on Sable Island Bank: the wave model of Boczar-Karakiewicz and Bona (1986a) and the sediment transport system: SED1DG (Davidson and Amos, 1985). The genesis of sand ridges is proposed to be the result of differential potential sediment transport (id) induced by 50-80 s period infragravity waves. Sand ridge crests are found where the spatial horizontal derivative of the transport rate (id) is negative, and troughs develop where this derivative is positive. These crests and troughs are orthogonal to the propagation direction of the infragravity waves, their consecutive distances vary in repetition length (Lr) in proportion with depth, and near the shelf break they are phase-locked to the shelf edge. Refraction of infragravity waves on Sable Island Bank is minor compared to refraction at the shelf edge. Sand ridge orientation is, therefore, largely determined by the orientation of the shelf edge over which the infragravity waves pass. The time scale for sand ridge generation, the degree of ridge asymmetry and the rate of migration are proposed to be the direct result of the rate and direction of shelf sediment transport which, for Sable Island Bank, is storm controlled and almost independent of the infragravity wave field. We predict that a typical time-scale for ridge formation in 40 m of water is 890 years with a time-averaged migration rate of 1.9 m y-1. © 1990.