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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 64 (1980)

Issue: 5. (May)

First Page: 792

Last Page: 792

Title: Genesis of Sand Ridges on Storm-Dominated Shelves--Status Report: ABSTRACT

Author(s): Donald J. P. Swift, William Stubblefield, David W. McGrail

Article Type: Meeting abstract

Abstract:

Sand-ridge topographies are relatively common on storm-dominated shelves, as well as on tidal shelves. The definitive studies of fluid process and seafloor response have not yet been undertaken, but enough data are now available to construct reasonable models of ridge genesis and design experiments. Any model for ridge formation must explain the following observations. (1) Ridges form a 20 to 40° angle with the coast that opens into the prevailing direction of storm flow. (2) Sand waves on the sand ridges form 85° angles with the coast. (3) The coarsest sands are on the up-current side of sand ridges and the finest sands are on the down-current side.

A mean flow model is based on J. D. Smith's stability analysis of sand beds at low Froude numbers. Because of a phase shift between bottom topography and flow parameters, maximum shear stress occurs on the upcurrent slopes of bottom perturbations, hence their crests must aggrade. The skew with respect to the coast is explained as a shearing out of the bed form by the increasing efficiency of transport as the beach is approached and wave re-suspension of sediment intensifies.

A shear wave model attributes sand ridges to stationary, eddy-like instabilities in inner-shelf flow that result from an onshore-offshore velocity gradient over a sloping bottom. In this model, ridge orientation is determined by the orientation of the long axis of the eddy. Studies in progress should allow us to discriminate between these two models of sand-ridge formation.

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Copyright 1997 American Association of Petroleum Geologists