Natural Maintenance of Sand Ridges and Linear Shoals on the U.S. Gulf and Atlantic Continental Shelves and the Potential Impacts of Dredging

Ridge and swale topography is exceptionally well developed on the continental shelves of the Mid-Atlantic Bight and the northeastern Gulf of Mexico. In both cases, these linear ridges are oriented parallel to the predominant wave approach direction, suggesting a common process for both their origin and maintenance. Most researchers have concluded that ridges were derived from shorefaces of barrier islands as they retreated across the shelf in response to rising sea level and tides or storm-driven currents maintain them.The widely cited ridge formation theory of Huthnance (1982) requires a sufficient sand source, currents to move the sand, and an irregularity on the sea floor around which the ridges are initiated. McBride and Moslow (1991) postulated that one of the initial irregularities is a segment of an ebb-tidal delta abandoned by inlet migration. However, the search for other precursors continues. These theories of origin provide little information on how these features maintain their form once they are detached from the shore yet remain in a zone of active wave attack (i.e. in depths less than 20 m). Snedden et al. (1999) indicate that shoals in water depths less than approximately 20 m are migrating shoreward through the influence of Stokes Drift under fair-weather waves based on the work of McHone (1973). However, this model does not explain the maintenance of the form of linear shoal and ridge features.To assess the impacts of dredging on these features it is essential that a better understanding of the processes that maintain these features be developed. A new conceptual model presented in this paper demonstrates how waves shoaling and refracting up either side of a ridge off the coast of Maryland and Delaware result in convergence of sand transport over the crest of the ridge, thus maintaining the ridge even after it is detached from shoreface processes.The possibility that these ridges might deflate or disappear as a consequence of dredging, resulting in dramatic changes in wave conditions along the shore, is a major concern. The application of a spectral or phase-resolving wave model combined with two-dimensional hydrodynamic and sand transport models as applied in this paper represents a method to evaluate this potential impact of dredging.

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