Critical bed shear stress and threshold of motion of maerl biogenic gravel

Joshi, Siddhi; Duffy, Garret; Brown, Colin

doi:10.1016/j.ecss.2017.06.010

Cited by 21 publications

(13 citation statements)

References 48 publications

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“…The turbulent kinetic energy ( TKE ) was then estimated and used to calculate the shear stress due to currents (τ c , N/m 2 ), according to Andersen et al () and Shi et al () as follows:

T K E = ({u^{'}}^{2} + {v^{'}}^{2} + {w^{'}}^{2}) / 2

τ_{c} = C_{1} T K E

where C 1 is a constant (0.19; Kim et al, ; Pope et al, ; Stapleton & Huntley, ). Although this TKE method for estimation of τ c mentioned above has been applied widely in the published literatures (e.g., Biron et al, ; Bowden & Fairbairn, ; Joshi et al, ; Soulsby & Dyer, ; Williams et al, ), it is noteworthy that the calculation of τ c using equations and will be contaminated by wave action when waves are measurable at the sampling volume.…”

Section: Methodsmentioning

confidence: 99%

Erosion and Accretion on a Mudflat: The Importance of Very Shallow‐Water Effects

et al. 2017

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Understanding erosion and accretion dynamics during an entire tidal cycle is important for assessing their impacts on the habitats of biological communities and the long‐term morphological evolution of intertidal mudflats. However, previous studies often omitted erosion and accretion during very shallow‐water stages (VSWS, water depths < 0.20 m). It is during these VSWS that bottom friction becomes relatively strong and thus erosion and accretion dynamics are likely to differ from those during deeper flows. In this study, we examine the contribution of very shallow‐water effects to erosion and accretion of the entire tidal cycle, based on measured and modeled time‐series of bed‐level changes. Our field experiments revealed that the VSWS accounted for only 11% of the duration of the entire tidal cycle, but erosion and accretion during these stages accounted for 35% of the bed‐level changes of the entire tidal cycle. Predicted cumulative bed‐level changes agree much better with measured results when the entire tidal cycle is modeled than when only the conditions at water depths of >0.2 m (i.e., probe submerged) are considered. These findings suggest that the magnitude of bed‐level changes during VSWS should not be neglected when modeling morphodynamic processes. Our results are useful in understanding the mechanisms of micro‐topography formation and destruction that often occur at VSWS, and also improve our understanding and modeling ability of coastal morphological changes.

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“…The turbulent kinetic energy ( TKE ) was then estimated and used to calculate the shear stress due to currents (τ c , N/m 2 ), according to Andersen et al () and Shi et al () as follows:

T K E = ({u^{'}}^{2} + {v^{'}}^{2} + {w^{'}}^{2}) / 2

τ_{c} = C_{1} T K E

Section: Methodsmentioning

confidence: 99%

Erosion and Accretion on a Mudflat: The Importance of Very Shallow‐Water Effects

et al. 2017

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“…The threshold of deposition of marine biota varies significantly between species, since bioclastic particles derived from them have varied densities and shapes (Rieux et al ., 2019). Previous flume tank experiments found the shear stresses at threshold of motion for both broken and non‐fragmented bioclastic particles were an order of magnitude lower or similar to siliciclastic particles of the same sieve diameter, due to their irregular shapes (Komar & Li, 1986; Flemming, 2017; Joshi et al ., 2017; Fick et al ., 2020). Threshold stresses are also typically larger by up to a factor of three for more poorly sorted sediments (McCarron et al ., 2019).…”

Section: Discussionmentioning

confidence: 99%

Wave‐influenced deposition of carbonate‐rich sediment on the insular shelf of Santa Maria Island, Azores

et al. 2022

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Formulae for sediment thresholds of motion are commonly based on flume experiments on rounded quartz particles and it is unclear how well they predict thresholds in natural settings. Here, sediment threshold shear stresses were calculated from one such formula using surface grain‐size data from 112 sites around Santa Maria Island, Azores. To compare with those stresses, a Simulating Waves Nearshore model was run for three typical winter months to predict shelf stress maxima due to waves. As wind‐driven and other circulations also increase stresses, the model predictions are under‐estimates. Comparison of the two stress estimates suggests that the whole shelf of the island was mobile during extreme conditions. However, three forms of evidence contradict this. First, 129 rollovers of sandy clinoforms lying in 30 to 200 m water depths around the island were identified from boomer seismic data. It has been suggested that such rollovers mark depths at which hydrodynamic stresses fall beneath the sediment threshold of motion. Second, differences in grain‐size diversity between carbonate‐free and whole sediment indicate where carbonate particle fragmentation occurs. Third, seabed images reveal variations in ripple character and presence. The combined data suggest that deposition has occurred in the middle and outer shelf, overlapping where the model predicts sediment mobilization. However, by decreasing the model bottom shear stress or increasing the shear stress at threshold of motion by a factor of two to three, deposition is predicted to have occurred immediately deeper than the shallow active rollovers. Therefore, in practice, the ratio of wave‐imposed shear stress to stress at threshold of motion is two to three times smaller than predicted. This is speculated to be due to the presence of widespread hard substrates and other features shielding particles between them from wave stresses. Alternatively, the threshold of motion is higher than expected from the formulae for these sediments dominated by bioclastic particles.

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“…). Confidence intervals of the slope and intercept (two standard deviations) were computed to derive the statistical errors of the u * and z 0 parameters (Wilkinson, ; Joshi et al ., ). Critical bed shear stress is derived from u * :

τ_{0 cr} = ρ u_{*}^{2}

where ρ is the mass density of water.…”

Section: Methodsmentioning

confidence: 97%

“…The estimations of bottom shear stress between these techniques can show important differences of up to 40% (Biron et al ., ). In this study, critical bed shear stress values were derived from mean velocity profiles in the boundary layer, by using the law of the wall technique, for two reasons: (i) mean velocity measurements are not sensitive to the sampling frequency of the Vectrino Profiler, and less sensitive to noise and signal decorrelation along the profile, compared to turbulence estimation based on velocity deviation; and (ii) most studies on bioclastic sediment (Paphitis et al ., ; Weill et al ., ; Joshi et al ., ) report bed shear stress derived from law of the wall regression on mean velocity profiles, and the same method should be applied in order to compare the results of this study.…”

Section: Methodsmentioning

confidence: 99%

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Threshold of motion and settling velocities of mollusc shell debris: Influence of faunal composition

et al. 2018

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Bioclastic particles derived from mollusc shell debris can represent a significant fraction of sandy to gravelly sediments in temperate and cool-water regions with high carbonate productivity. Their reworking and subsequent transport and deposition by waves and currents is highly dependent on the shape and density of the particles. In this study, the hydrodynamic behaviour of shell debris produced by eight mollusc species is investigated for several grain sizes in terms of settling velocity (measurements in a settling tube) and threshold of motion under unidirectional current (flume experiments using an acoustic profiler). Consistent interspecific differences in settling velocity and critical bed shear stress are found, related to differences in shell density, shell structure imaged by scanning electron microscopy and grain shape. Drag coefficients are proposed for each mollusc species, based on an interpolation of settling velocity data. Depending on the shell species, the critical bed shear stress values obtained for bioclastic particles fall within or slightly below empirical envelopes established for siliciclastic particles, despite very low settling velocity values. The results suggest that settling velocity, often used to describe the entrainment of sediment particles through the equivalent diameter, is not a suitable parameter to predict the initiation of motion of shell debris. The influence of the flat shape of bioclastic particles on the initiation of motion under oscillatory flows and during bedload and saltation transport is yet to be elucidated.

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Critical bed shear stress and threshold of motion of maerl biogenic gravel

Cited by 21 publications

References 48 publications

Erosion and Accretion on a Mudflat: The Importance of Very Shallow‐Water Effects

Erosion and Accretion on a Mudflat: The Importance of Very Shallow‐Water Effects

Wave‐influenced deposition of carbonate‐rich sediment on the insular shelf of Santa Maria Island, Azores

Threshold of motion and settling velocities of mollusc shell debris: Influence of faunal composition

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