HR: 1330h
AN: OS23B-01    [Abstracts]
TI: Ripple and Morphologic Behavior of Sorted Bedforms
AU: * Trembanis, A C
EM: atrembanis@whoi.edu
AU: Traykovski, P A
EM: ptraykovski@whoi.edu
AB: Complex sorted bedforms or rippled scour depressions (RSDs) are common inner shelves features, yet understanding of their morphodynamic behavior is limited. Seabed measurements from the Martha's Vineyard Coastal Observatory (MVCO) are used to field test several widely used bedform models including Nielsen (1981); Wikramanayake (1993); and Wiberg and Harris (1994). Deployed quadpods supported acoustic sensors and provided time series on boundary layer processes, sediment transport, and bedform dynamics. Observations indicate that sorted bedforms form as windows through thin blankets of fine sand into underlying coarse sand. Elongate wave oriented narrow windows form and fade in response to wave events. Their location is closely tied to transitional boundaries between coarse and fine sand. A conceptual model of the evolution of such windowed features is presented. Alternating patches of coarse and fine sand beds characterized the study area. Patterns of bed roughness were mapped using autonomous underwater vehicle (AUV) side-scan sonar sonograms, diver observations, grab sampling, short cores, and acoustic images from stationary rotary sonar systems. The exposed coarse patches supported large wave orbital ripples. The fine sand areas possessed smaller anorbital ripples during calm periods, while during storm conditions occasionally developed into complex irregularly spaced low relief hummocky bedforms or cross ripples. During other storm events, flat bed conditions prevailed. The tested ripple models predicted some, but not all of the observed bedform variation between the coarse and fine sand sites. Specifically, the models tended to account for most of the evolution of large wave-orbital ripples within coarse sand deposits, but accounted for less of the evolutionary diversity of bedforms within the fine sand deposits. In particular, none of the models predicted that the hummocky bedforms would replace ripples on the fine sand bed during the storm. Instead the models all suggest that sheet flow conditions would wipe out the ripples leaving a plane bed. The ripple models tested each qualitatively predicted a strikingly distinct class of bedforms to exist between the coarse and fine sand sites such that only suborbital and anorbital ripples were predicted to develop over the fine sand site, whereas only orbital and suborbital ripples were predicted to develop over the coarse sand site. Continued observations and new models of bedform behavior particularly during storm events are critical in order to advance our understanding of bedform dynamics associated with heterogeneous sediment systems.
DE: 3045 Seafloor morphology and bottom photography
DE: 4546 Nearshore processes
DE: 4558 Sediment transport
SC: Ocean Sciences [OS]
MN: 2005 Joint Assembly