Sediment transport patterns in the San Francisco Bay Coastal System from cross-validation of bedform asymmetry and modeled residual flux
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Bedform
Seafloor Spreading
Bedform
Flume
Hyperconcentrated flow
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Acoustic observations of bedform migration and suspended sediment transport at the New Jersey LEO-15 site revealed that bedload and near-bottom suspended load could be the dominant sediment transport mode over wave-formed ripples on a sandy bottom. Bedforms observed during storms using a rotary sidescan sonar were found to be wave orbital scale ripples which migrated in the onshore direction, forced by asymmetrical wave orbital velocities. Estimates of suspended sand transport were calculated from observations of acoustic backscattering and water velocity profiles. Suspended sand transport was also forced by asymmetrical wave velocities, and was found to occur primarily during the weaker offshore phase of wave motion. This net offshore suspended sediment transport was an order of magnitude less than the flux associated with onshore ripple migration. Thus it is hypothesized that ripple migration is most likely forced by unobserved bedload and near-bottom suspended transport. Spatially resolving bedload transport from the stationary bed is difficult since the motion occurs within a few grain diameters of the bed. Acoustic Doppler-based techniques, which are ideal for this type of measurement since the sediment velocity can be resolved from the stationary bed in the frequency domain, are being developed.
Bedform
Suspended load
Acoustic Doppler velocimetry
Ripple marks
Surf zone
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Bedforms are common features in shallow marine environments, and their presence evokes questions regarding the spatial and temporal stability of the seafloor. Though observation of bedform dynamics from multibeam bathymetry and its derived products enhances understanding of seafloor stability, the ability to successfully detect bedform migration depends on (1) the survey resolution and positioning uncertainty, and (2) the establishment of an optimum survey-repetition rate.
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Seafloor Spreading
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Abstract Bedload transport is an important mechanism for sediment flux in the nearshore. Yet few studies examine the relationship between bedform evolution and net sediment transport. Our work contributes concurrent observations of bedform mobility and bedload transport in response to wave dominant, current dominant, and combined wave‐current flows in the nearshore. Bedload sediment flux from migrating bedforms during combined wave‐current conditions accounted for at least 20% more bedload transport when compared with wave dominant flows and at least 80% more than current‐dominant flows. Bedforms were observed to transport the most sediment during periods with strong currents, with high‐energy skewed waves, and while bedform orientation and transport direction were aligned. Regardless of flow type, bedform migration rates were directly proportional to the total kinetic energy contained in the flow field. Eleven bedload transport models formulated to be used in combined flows (both shear and energetics based) were compared with sediment flux estimated from measured bedform migration. An energetics based sediment transport model was most representative for our data.
Bedform
Energetics
Energy flux
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Bedload transport on Spratt Sand is a result of migrating bedforms and a lack of sediment clearly limits the growth and development of these bedforms. Maximum bedload transport based on dune-tracking is observed for a combination of waves and currents, probably due to an increase in migration rates rather than an increase in ripple dimensions.
Bedform
Ripple marks
Suspended load
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