Sediment eddy diffusivities in the nearshore zone, from multifrequency acoustic backscatter

Sheng, Jinyu; Hay, Alex E.

doi:10.1016/0278-4343(94)e0025-h

Cited by 28 publications

(13 citation statements)

References 22 publications

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“…Using acoustic backscatter sensor (ABS) data to invert (3), Vincent and Downing [1994] reported that eddy diffusivity profiles, under combined waves and currents, increased linearly from the bed level to ∼20 cm above the bed and decreased above that level. Other authors have also found linearly increasing eddy diffusivity near the bed to be scaled by the characteristic shear velocity [ Sheng and Hay , 1995; Vincent and Osborne , 1995]. The vertical length scale of the coherent diffusivity profile and its behavior above the linear region are subject to further research and first‐hand discussion on the subject can be found in the work of Sheng and Hay [1995].…”

Section: Introductionmentioning

confidence: 89%

Examination of diffusion versus advection dominated sediment suspension on the inner shelf under storm and swell conditions, Duck, North Carolina

2002

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A benthic boundary layer tripod supporting six current meters and three profiling acoustic backscatter sensors (ABS) documented storm and swell conditions during the fall of 1996 at a depth of 13 m on the inner shelf off Duck, North Carolina. Sediment concentration was higher in the wave boundary layer (WBL) during storm conditions but higher ∼40 cm above the bed (cm ab) during swell conditions. To test the applicability of a diffusive balance during storm versus swell, ABS data were used to invert the vertical diffusion equation and solve for eddy diffusivity from 1 to 50 cm ab. During the storm period, diffusivity derived from the ABS up to ∼40 cm ab agreed well with viscosity derived above the WBL from observed current profiles and from the Grant‐Madsen‐Glenn (GMG) model. During the swell period, diffusivity derived from the ABS up to ∼40 cm ab did not agree with observed mean current shear above this level nor with the GMG model. Diffusivity did agree with viscosity derived from shear stress due to waves within the WBL extrapolated to a height greater than the modeled WBL. We speculate that during swell conditions, shedding vortices enhanced mass and momentum exchange, extending the eddy viscosity associated with waves above the predicted WBL; during storm conditions, strong currents prevented vortices from penetrating beyond the predicted WBL. Rouse diffusion models with two‐ and three‐layered eddy diffusivity and combined diffusion‐advection models with one and three‐layer were applied to the observational data set. During the storm the two‐ and three‐layered Rouse models including multiple grain sizes and bed armoring reproduced the observed concentration well. During swell (weak current conditions) all the models considered underpredicted the observed concentration if applied with a standard WBL thickness. To correct this, enhanced vertical exchange was represented by a thickened WBL whenever mean currents were weak relative to the estimated jet velocity associated with wave‐induced vortex shedding. The two‐layer Rouse model then reproduced the concentrations observed during swell remarkably well. This implies that mean sediment suspension dominated by wave‐induced advection may still be approximated by a diffusion‐like process under some circumstances.

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Section: Introductionmentioning

confidence: 89%

Examination of diffusion versus advection dominated sediment suspension on the inner shelf under storm and swell conditions, Duck, North Carolina

2002

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“…The first two of the normalizations have been used by previous authors [e.g., Sheng and Hay , 1995] and the latter two were chosen here on the basis of the theoretical expressions in section 2. The scale thickness of the wave boundary layer δ w has been taken here as: …”

Section: Experimental Arrangement and Data Analysismentioning

confidence: 99%

Observations and analysis of sediment diffusivity profiles over sandy rippled beds under waves

Thorne¹,

Davies²,

Bell³

2009

J. Geophys. Res.

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[1] Acoustic measurements of near-bed sediment diffusivity profiles are reported. The observations were made over two sandy rippled beds, classified as ''medium'' and ''fine'' in terms of sand grain size, under slightly asymmetric regular waves. For the medium sand, the ripples that formed had relatively steep slopes, while for the fine sand, the slopes were roughly half that of the medium sand. In the medium sand case, the form of the sediment diffusivity profiles was found to be constant with height above the bed, to a height equal approximately to the equivalent roughness of the bed, k s , while above this the sediment diffusivity increased linearly with height. For the case of the fine sand there was no constant region; the sediment diffusivity simply increased linearly with height from the bed. To understand the difference between the respective diffusivity profiles, advantage has been taken of the high temporal-spatial resolution available with acoustic systems. Using intrawave ensemble averaging, detailed images have been built up of the variation in concentration with both the phase of the wave and also height above the bed. These intrawave observations, combined with measurements of the bed forms and concepts of convective and diffusive entrainment, have been used to elucidate the mixing mechanisms that underlie the form of the diffusivity profiles observed over the two rippled beds. These mechanisms center on coherent vortex shedding in the case of steeply rippled beds and random turbulent processes above ripples of lower steepness.

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“…In recent years acoustic backscatter systems [11][12][13][14][15] have been developed which can make detailed observations of near-bed suspended sediment processes. To extract suspended sediment parameters from the acoustic data entails an inversion procedure which is based on an assumed knowledge of the scattering characteristics of marine suspended sediments.…”

Section: Introductionmentioning

confidence: 99%

Measurements and analysis of acoustic backscattering by elastic cubes and irregular polyhedra

Thorne

Sun

Zhang

et al. 1997

The Journal of the Acoustical Society of America

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Underwater acoustic studies of backscattering by submerged targets have generally focused on bodies with spherical and cylindrical symmetry. However, there are interests in scattering by objects which may be characterized by more angular features, with surfaces that tend to be composed of facets and edges. To investigate the scattering properties of such bodies, the backscattering by a number of elastic cubes, and irregularly shaped polyhedra, have been studied. Data were collected by measuring the band limited impulse response of the scatterers, using a broadband transducer, which operated as a transceiver, both transmitting and receiving signals. To present the scattering measurements nondimensionally a form function definition has been employed to normalize the backscattered signals. The normalized frequency has been expressed as ka, where k is the acoustic wave number, and a is a characteristic dimension of the scatterer. The cube observations covered a broad ka range, kaϭ3 -34, thereby encompassing the lower ka scattering region where the cube can be considered to be nominally rigid, through to higher ka values where it would be expected that the elastic properties of the cube become important. The measurements on irregular polyhedra are more limited in extent, however, the results are presented for comparison with the cube data. To compare the observations with predictions, computations of acoustic backscattering for both rigid and elastic cubes are presented.

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Sediment eddy diffusivities in the nearshore zone, from multifrequency acoustic backscatter

Cited by 28 publications

References 22 publications

Examination of diffusion versus advection dominated sediment suspension on the inner shelf under storm and swell conditions, Duck, North Carolina

Examination of diffusion versus advection dominated sediment suspension on the inner shelf under storm and swell conditions, Duck, North Carolina

Observations and analysis of sediment diffusivity profiles over sandy rippled beds under waves

Measurements and analysis of acoustic backscattering by elastic cubes and irregular polyhedra

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