A comparison of Rippled Scour Depressions identified with multibeam sonar: evidence of localized sediment transport in inner shelf environments.

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Continental Shelf Research

Rippled scour depressions (RSDs) are distinct morphologic and textural features that have been observed in numerous nearshore environments as depressions containing coarse rippled sediments. The nature and significance of these features can be better understood with the use of multibeam sonar systems that acquire high-resolution bathymetry and co-registered backscatter intensity data and reveal both the small-scale characteristics of RSDs and larger-scale seabed patterns associated with them. In this paper, we compare RSDs identified in high-frequency (300 kHz) multibeam sonar surveys on the inner shelves of southwest Washington, northern California and Long Island, New York. RSDs at these sites are evident as distinct features that are generally up to 1 m deeper than the adjacent seabed and exhibit high acoustic backscatter intensity due to the presence of coarse sediments, typically gravel. Shore parallel oscillatory ripples (typically 1 m wavelength) exist within the coarse material associated with RSDs and are attributed to wave-orbital motion. We propose a classification of RSDs based on their morphology and textural characteristics as well as the larger-scale seabed patterns associated with them, which together provide insight about the processes that affect their development and evolution. The RSDs described here are apparently the result of localized flow that can redistribute significant volumes of sediment in the nearshore zone under a range of energetic regimes over relatively short time scales. One type of linear shore-normal RSD always corresponds to larger-scale low-amplitude transverse bedforms that appear to be related to longshore currents. Some of these low-amplitude bedforms migrated as much as 50–150 m over 2 years. Another type of elongate RSD, appears to be relatively stable over time, is characterized by bathymetric and textural asymmetry with one distinct upcurrent edge and one diffuse downcurrent edge, and is apparently a result of transverse flow. Other irregularly shaped nearshore RSDs that can be created and destroyed on time scales of less than 1 year, are the result of localized erosion and accretion of up to 1 m, and are distributed rhythmically along-shore in water depths of 8–16 m suggesting that they are related to hydrodynamics associated with the breaker zone. The characteristics of RSDs elucidated by high-frequency multibeam sonar data provide evidence of a variety of physical processes associated with RSDs and emphasize the importance of these features with respect to sediment transport on the inner shelf.