Some effects of suspended sediment stratification on an oceanic bottom boundary layer

Adams, Charles E.; Weatherly, Georges L.

doi:10.1029/jc086ic05p04161

Cited by 148 publications

(86 citation statements)

References 19 publications

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“…Here, the estimations are done based on A = 4.7 and A = 14.7 [5][6][7][8]. A = 4.7 reduced the slope of the comparison between the log-law and Reynolds stress estimates from 7 to 2.8; A = 14.7 reduced it to 1.2 ( Figure 7a; green dots and cyan line).…”

Section: Discussionmentioning

confidence: 99%

“…However, the log-law is difficult to apply in weak flow conditions and it does not account for stratification effects on flow. Near-bed stratification due to sediment resuspension can reduce the bottom drag [5][6][7][8]. On the other hand, as shown by field measurements [9] and model predictions [10], surface wave-induced turbulence within the relatively thin wave boundary layer near the bed nonlinearly interacts with steady current flow and enhances bottom friction, which affects the vertical structure of flow throughout the water column.…”

Section: Introductionmentioning

confidence: 99%

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Variability of Bed Drag on Cohesive Beds under Wave Action

Safak

2016

Water

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Drag force at the bed acting on water flow is a major control on water circulation and sediment transport. Bed drag has been thoroughly studied in sandy waters, but less so in muddy coastal waters. The variation of bed drag on a muddy shelf is investigated here using field observations of currents, waves, and sediment concentration collected during moderate wind and wave events. To estimate bottom shear stress and the bed drag coefficient, an indirect empirical method of logarithmic fitting to current velocity profiles (log-law), a bottom boundary layer model for combined wave-current flow, and a direct method that uses turbulent fluctuations of velocity are used. The overestimation by the log-law is significantly reduced by taking turbulence suppression due to sediment-induced stratification into account. The best agreement between the model and the direct estimates is obtained by using a hydraulic roughness of 10 −4 m in the model. Direct estimate of bed drag on the muddy bed is found to have a decreasing trend with increasing current speed, and is estimated to be around 0.0025 in conditions where wave-induced flow is relatively weak. Bed drag shows an increase (up to fourfold) with increasing wave energy. These findings can be used to test the bed drag parameterizations in hydrodynamic and sediment transport models and the skills of these models in predicting flows in muddy environments.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Variability of Bed Drag on Cohesive Beds under Wave Action

Safak

2016

Water

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“…These rederivations build directly on previous investigations of the effects of near-bottom stratification by suspended sediment [e.g., Adams and Weatherly, 1981;McLean, 1992]. A correction for very weak stratification is presented which reduces to an additional term in the least squares fit applied to classic log profiles; an approach for stronger stratification is also applied which requires and Wright, 1995].…”

Section: They Likewise Observed Concave Downward Profiles Andmentioning

confidence: 99%

Sensitivity of bottom stress and bottom roughness estimates to density stratification, Eckernförde Bay, southern Baltic Sea

Friedrichs¹,

Wright²

1997

J. Geophys. Res.

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Abstract. Thermohaline density stratification may significantly alter the classic nearbottom logarithmic velocity profile in many weak to moderately energetic, partially mixed estuaries. Results from Eckemf6rde Bay suggest fits to log profiles which neglect thermohaline stratification may lead to overestimates of bottom stress and roughness of the order of 130 % and 600 %, respectively. Measurements of velocity obtained at four heights within 1 m of the seabed are input to theoretical models for velocity shear derived via dimensional arguments for the "overlap" layer. Previous investigators applying dimensional arguments to thermohaline stratification in estuaries have assumed buoyancy flux to be independent of height within the overlap layer. This may be a poor assumption since there is no significant source or sink of thermohaline buoyancy at the sediment-water interface. In this paper, dimensional arguments which do not assume constant buoyancy flux are used to reduce estimates of the drag coefficient and bottom roughness to below the unreasonably high values predicted by simple log profiles. Formulations assuming very weak and relatively strong stratification are applied, and estimates of buoyancy frequency derived from fits to velocity profiles are compared with independent estimates of stratification. Estimates of bottom stress and roughness derived from velocity profiles are also found to be sensitive to fluid acceleration, uncertainties in instrument settling, and limitations in current meter accuracy, but these latter effects appear secondary in Eckemf6rde Bay to the impacts of thermohaline stratification.

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“…stratification (Weatherly and Martin, 1978;Adams and Weatherly, 1981;Smith and McLean, 1977), bed-load sediment transport (Smith and McLean, 1977;Grant and Madsen, 1982; Gust and Southard, in press), and acceleration, either temporal (Grant andMadsen, 1979, 1982;Grant, 1982) or spatial (Zilker et al, 1977;Zilker and Hanratty, 1979;Yaglom, 1979). All of these are common in field settings.…”

Section: 3mentioning

confidence: 99%

Flow and skin friction over natural rough beds

Paola¹

1983

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When a boundary layer develops over a bed that is hydrodynamically rough at a length scale or scales larger than the grain size (a macrorough bed), as is usually the case where bed forms are present, it is necessary to distinguish among total boundary shear stress and its components, form drag and spatially averaged skin friction. It is known that the mean-velocity field reflects the composite boundary shear stress. Above about one roughness height above the tops of the roughness elements, the velocity does not vary horizontally. Its vertical profile is semilogarithmic and scales with the total friction velocity u*t and total roughness length zot. This region is here called the integrated logarithmic layer (ILL). Below the ILL the velocity varies horizontally in response to the irregular boundary; this region is called the surface layer.In the first of two sets of experiments reported here, skin-friction measurements were made with an array of flush-mounted hot films at four points on the stoss slope of one of a field of two-dimensional immobile current ripples. Total boundary shear stress was also measured, as were mean-velocity profiles in the ILL and the surface layer. The ILL behaves as described above. Although surface-layer velocity profiles are semilogarithmic, their semilogarithmic slope is not proportional to the local skin-friction velocity, so they do not locally obey the law of the wall. Rather, the velocity field can be decomposed into a spatially averaged rotational component and a local inviscid perturbation. The measured skin-friction field is consistent with a simple model for sediment transport over the bed forms except near reattachment, where the fluctuating skin friction is important. The data are also consistent with the drag-partition theories of Einstein and Barbarossa (1952) and Engelund (1966). Normalized skin-friction spectra do not vary with streamwise position but do vary with Reynolds number; skin-friction probability density functions show significant increases in skewness and kurtosis near reattachment but do not vary strongly with Reynolds number.In the second set of experiments the skin-friction vector field was measured around isolated hemispheres, with model sedimentary tails one and four obstacle heights long and without tails. The measured skin-friction fields are not consistent with deposition along the obstacle-flow centerline downstream of reattachment, which occurs about two obstacle heights downstream of the trailing edge of the hemisphere. This applies for local bed-load erosion and deposition and for general deflation of the bed, and is not substantially altered by the presence of either tail. Measurements were also made of skin friction, total boundary shear stress and ILL velocity profiles over h,B-rough arrays of hemispheres with and without tails four roughness heights long, at two areal densities. The skin-friction field in the denser array is significantly distorted from that around an isolated element. The measured skin friction in both arrays is signif...

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Some effects of suspended sediment stratification on an oceanic bottom boundary layer

Cited by 148 publications

References 19 publications

Variability of Bed Drag on Cohesive Beds under Wave Action

Variability of Bed Drag on Cohesive Beds under Wave Action

Sensitivity of bottom stress and bottom roughness estimates to density stratification, Eckernförde Bay, southern Baltic Sea

Flow and skin friction over natural rough beds

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