Shoreface-connected sand ridges on American and European shelves: A comparison

Abstract

Large scale sand ridges, 10 m or more in height and 2–4 km apart have long been accepted as characteristic of shelves experiencing strong tidal flow. However, they also occur on inner continental shelves whose strongest flows are wind-induced. The Middle Atlantic Bight of North America is the best known example, but inner shelf ridge topographies occur extensively on other Atlantic shelves. We wish to call attention to similarities and differences among the inner shelf ridges of North America, South America, and Europe, as a necessary step prior to the framing and testing of hypotheses concerning the hydrodynamics of ridge formation. Inner shelf ridge fields occur primarily on low, unconsolidated coasts whose substrates provide an ample supply of sediment for ridge formation. The constituent materials are commonly coarse to fine sand, but on the Guiana coast of South America, similar ridges appear to be forming in a mud substrate. Here ridges have much greater spacing (tens of km versus km), less relief, and much flatter slopes (1,3000 versus 1,500). Inner shelf ridges are characteristically aligned obliquely to the shoreline, with acute angles opening into the prevailing flow direction. Downcurrent slopes tend to be steeper and finer grained. The ridges tend to migrate downcoast and offshore, extending their crestlines so as to maintain contact with the shoreface. Where ridges are nearly shore-parallel (current parallel), the movement is expressed as downcurrent growth of ridges and downcurrent extension of swales between ridges. The textural and morphologic asymmetry of inner shelf sand ridges can be explained in terms of existing models for sand wave formation. The oblique orientation with respect to the shoreline is also possibly compatible with these models. An alternative, or perhaps complementary explanation for sand ridge genesis requires helical flow structure in the shelf flow field during peak events, but both theory and observation are less well developed for this model. p]Inner shelf sand ridges appear to be responses to periods of intense flow induced by the passage of storms. The extent to which a given shelf sector develops ridge topography may depend on the efficiency with which the local water mass responds to storm passage. Ridge behavior, and especially ridge migration rates, are necessary information for marine environmental management, since ridges are most active in the nearshore zone where the shelf floor is used for sewage outfalls, deep water ports, nuclear reactor sites, and other structures requiring a measure of sea floor stability.