Articles de revues : « Shear stress bed »

1

Guard, Paul Andrew, Peter Nielsen et Tom E. Baldock. « BED SHEAR STRESS IN UNSTEADY FLOW ». Coastal Engineering Proceedings 1, n^o 32 (31 janvier 2011) : 8. http://dx.doi.org/10.9753/icce.v32.sediment.8.

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Standard engineering methods of estimating bed shear stress using friction factors can fail spectacularly in unsteady hydrodynamic conditions. This paper demonstrates this fact using direct measurements of bed shear stresses under irregular waves using a shear plate apparatus. The measurements are explained in terms of the influence of the horizontal pressure gradient and the shear stresses acting on the surface of the plate. The horizontal fluid velocity at the edge of the boundary layer and the water surface elevation and slope were also measured. The paper demonstrates that the water surface measurements can be used to obtain accurate estimates of the forces on the bed, by employing Fourier analysis techniques or an innovative convolution integral method. The experimental results indicate that an offshore bed shear stress may be recorded while the free stream velocity remains onshore at all times. This demonstrates the failure of the standard engineering friction factor method in this scenario, since negative friction factors would be required. Important questions are raised regarding the appropriate definition for the bed shear stress when the vertical gradient of the shear stress is large. It is shown that it is problematic to define a single value for a “bed” shear stress in the presence of a strong horizontal pressure gradient. It is also argued that the natural driver for any model used to predict bed shear stress is the pressure gradient (or its proxy the free stream acceleration), rather than the velocity. This allows for accurate calculation of both acceleration effects (more rapid acceleration leads to a thinner boundary layer and higher shear stress) and also the direct action of the horizontal pressure gradient.

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Schönfeldt, Hans-Jürgen. « On the aeolian saltation bed shear stress and saltation roughness length ». Meteorologische Zeitschrift 15, n^o 3 (10 juillet 2006) : 307–15. http://dx.doi.org/10.1127/0941-2948/2006/0126.

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Cheng, Nian-Sheng, et Adrian Wing-Keung Law. « Fluctuations of Turbulent Bed Shear Stress ». Journal of Engineering Mechanics 129, n^o 1 (janvier 2003) : 126–30. http://dx.doi.org/10.1061/(asce)0733-9399(2003)129:1(126).

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Papa, M., S. Egashira et T. Itoh. « Critical conditions of bed sediment entrainment due to debris flow ». Natural Hazards and Earth System Sciences 4, n^o 3 (2 août 2004) : 469–74. http://dx.doi.org/10.5194/nhess-4-469-2004.

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Abstract. The present study describes entrainment characteristics of bed material into debris flow, based on flume tests, numerical and dimensional analyses. Flume tests are conducted to investigate influences of bed sediment size on erosion rate by supplying debris flows having unsaturated sediment concentration over erodible beds. Experimental results show that the erosion rate decreases monotonically with increase of sediment size, although erosion rate changes with sediment concentration of debris flow body. In order to evaluate critical condition of bed sediment entrainment, a length scale which measures an effective bed shear stress is introduced. The effective bed shear stress is defined as total shear stress minus yield stress on the bed surface. The results show that critical entrainment conditions can be evaluated well in terms of Shields curve using the effective bed shear stress instead of a usual bed shear stress.

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Seelam, Jaya Kumar, et Tom E. Baldock. « MEASUREMENT AND MODELING OF SOLITARY WAVE INDUCED BED SHEAR STRESS OVER A ROUGH BED ». Coastal Engineering Proceedings 1, n^o 33 (11 octobre 2012) : 21. http://dx.doi.org/10.9753/icce.v33.waves.21.

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Bed shear stresses generated by solitary waves were measured using a shear cell apparatus over a rough bed in laminar and transitional flow regimes (~7600 < Re < ~60200). Modeling of bed shear stress was carried out using analytical models employing convolution integration methods forced with the free stream velocity and three eddy viscosity models. The measured wave height to water depth (h/d) ratio varied between 0.13 and 0.65; maximum near- bed velocity varied between 0.16 and 0.47 m/s and the maximum total shear stress (sum of form drag and bed shear) varied between 0.565 and 3.29 Pa. Wave friction factors estimated from the bed shear stresses at the maximum bed shear stress using both maximum and instantaneous velocities showed that there is an increase in friction factors estimated using instantaneous velocities, for non-breaking waves. Maximum positive total stress was approximately 2.2 times larger than maximum negative total stress for non-breaking waves. Modeled and measured positive total stresses are well correlated using the convolution model with an eddy viscosity model analogous to steady flow conditions (nu_t=0.45u* z1; where nu_t is eddy viscosity, u* is shear velocity and z1 is the elevation parameter related to relative roughness). The bed shear stress leads the free stream fluid velocity by approximately 30° for non-breaking waves and by 48° for breaking waves, which is under-predicted by 27% by the convolution model with above mentioned eddy viscosity model.

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Zhang, Liyuan, Faxing Zhang, Ailing Cai, Zhaoming Song et Shilin Tong. « Comparison of Methods for Bed Shear Stress Estimation in Complex Flow Field of Bend ». Water 12, n^o 10 (2 octobre 2020) : 2753. http://dx.doi.org/10.3390/w12102753.

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Bed shear stress is closely related to sediment transport in rivers. Bed shear stress estimation is very difficult, especially for complex flow fields. In this study, complex flow field measurement experiments in a 60° bend with a groyne were performed. The feasibility and reliability of bed shear stress estimations using the log-law method in a complex flow field were analyzed and compared with those associated with the Reynolds, Turbulent Kinetic Energy (TKE), and TKE-w′ methods. The results show that the TKE, Reynolds, and log-law methods produced similar bed shear stress estimates, while the TKE-w′ method produced larger estimates than the other methods. The TKE-w′ method was found to be more suitable for bed shear stress estimation than the TKE method, but the value of its constant C2 needed to be re-estimated. In a complex, strong, three-dimensional flow field, the height of the measurement point (relative or absolute) should be re-estimated when a single point measurement is used to estimate the bed shear stress. The results of this study provide guidance for experimental measurement of bed shear stress in a complex flow field.

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Golpira, Amir, Fengbin Huang et Abul B. M. Baki. « The Effect of Habitat Structure Boulder Spacing on Near-Bed Shear Stress and Turbulent Events in a Gravel Bed Channel ». Water 12, n^o 5 (16 mai 2020) : 1423. http://dx.doi.org/10.3390/w12051423.

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This study experimentally investigated the effect of boulder spacing and boulder submergence ratio on the near-bed shear stress in a single array of boulders in a gravel bed open channel flume. An acoustic Doppler velocimeter (ADV) was used to measure the instantaneous three-dimensional velocity components. Four methods of estimating near-bed shear stress were compared. The results suggested a significant effect of boulder spacing and boulder submergence ratio on the near-bed shear stress estimations and their spatial distributions. It was found that at unsubmerged condition, the turbulent kinetic energy (TKE) and modified TKE methods can be used interchangeably to estimate the near-bed shear stress. At both submerged and unsubmerged conditions, the Reynolds method performed differently from the other point-methods. Moreover, a quadrant analysis was performed to examine the turbulent events and their contribution to the near-bed Reynolds shear stress with the effect of boulder spacing. Generally, the burst events (ejections and sweeps) were reduced in the presence of boulders. This study may improve the understanding of the effect of the boulder spacing and boulder submergence ratio on the near-bed shear stress estimations of stream restoration practices.

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8

Iverson, Neal R., Christian Helanow et Lucas K. Zoet. « Debris-bed friction during glacier sliding with ice–bed separation ». Annals of Glaciology 60, n^o 80 (décembre 2019) : 30–36. http://dx.doi.org/10.1017/aog.2019.46.

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AbstractTheory and experiments indicate that ice–bed separation during glacier slip over 2-D hard beds causes basal shear stress to reach a maximum at a particular slip velocity and decrease at higher velocities. We use the sliding theory of Lliboutry (1968) to explore how friction between debris particles in sliding ice and a rock bed affects this relationship between shear stress and slip velocity. Particle–bed contact forces and associated debris friction increase with increasing slip velocity, owing to increased rates of ice convergence with up-glacier facing surfaces. However, debris friction on diminished areas of the bed counteracts this effect as cavities grow. Thus, friction from debris alone increases only slightly with slip velocity, and for sediment particles larger than ~60 mm in diameter, debris friction peaks and decreases with increasing slip velocity. The effect on the sliding relationship is to steepen its rising limb and shift its shear stress peak to a slightly higher velocity. These results, which exclude the effect of debris friction on cavity size and debris concentrations above ~15%, indicate that the effect of debris in ice is to increase basal shear stress but not significantly change the form of the sliding relationship.

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9

Kiraga et Popek. « Bed Shear Stress Influence on Local Scour Geometry Properties in Various Flume Development Conditions ». Water 11, n^o 11 (8 novembre 2019) : 2346. http://dx.doi.org/10.3390/w11112346.

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Numerous approaches in sediment mobility studies highlighted the key meaning of channel roughness, which results not only from bed material granulation but also from various bed forms presence, caused by continuous sediment transport. Those forms are strictly connected with the intensity of particle transport, and they eventuate from bed shear stress. The present paper comprised of local scours geometric dimensions research in three variants of lengthwise development of laboratory flume in various hydraulic properties, both in “clear-water” and “live-bed” conditions of sediment movement. Lots of measurements of the bed conformation were executed using the LiDAR device, marked by a very precise three-dimensional shape description. The influence of the bed shear stress downstream model on scours hole dimensions of water structure was investigated as one of the key factors that impact the sediment transport intensity. A significant database of 39 experimental series, lasting averagely 8 hours, was a foundation for delineating functional correlations between bed shear stress-and-critical shear stress ratio and geometry properties of local scours in various flume development cases. In the scope of mutual influence of bed shear stress and water depth, high correlation coefficients were attained, indicating very good and good functional correlations. Also, the influence of bed shear stress and the total length of the scour demonstrated a high correlation coefficient.

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Maclean, Alastair G. « Bed Shear Stress and Scour over Bed‐Type River Intake ». Journal of Hydraulic Engineering 117, n^o 4 (avril 1991) : 436–51. http://dx.doi.org/10.1061/(asce)0733-9429(1991)117:4(436).

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11

Monsalve, Angel, Catalina Segura, Nicole Hucke et Scott Katz. « A bed load transport equation based on the spatial distribution of shear stress – Oak Creek revisited ». Earth Surface Dynamics 8, n^o 3 (29 septembre 2020) : 825–39. http://dx.doi.org/10.5194/esurf-8-825-2020.

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Abstract. Bed load transport formulations for gravel-bed rivers are often based on reach-averaged shear stress values. However, the complexity of the flow field in these systems results in wide distributions of shear stress, whose effects on bed load transport are not well captured by the frequently used equations, leading to inaccurate estimates of sediment transport. Here, we modified a subsurface-based bed load transport equation to include the complete distributions of shear stress generated by a given flow within a reach. The equation was calibrated and verified using bed load data measured at Oak Creek, OR. The spatially variable flow field characterization was obtained using a two-dimensional flow model calibrated over a wide range of flows between 0.1 and 1.0 of bankfull discharge. The shape of the distributions of shear stress was remarkably similar across different discharge levels, which allowed it to be parameterized in terms of discharge using a gamma function. When discharge is high enough to mobilize the pavement layer (1.0 m3 s−1 in Oak Creek), the proposed transport equation had a similar performance to the original formulation based on reach-averaged shear stress values. In addition, the proposed equation predicts bed load transport rates for lower flows when the pavement layer is still present because it accounts for bed load transport occurring in a small fraction of the channel bed that experiences high values of shear stress. This is an improvement over the original equation, which fails to estimate this bed load flux by relying solely on reach-average shear stress values.

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12

Kolerski, Tomasz, et Hung Tao Shen. « Possible effects of the 1984 St. Clair River ice jam on bed changes ». Canadian Journal of Civil Engineering 42, n^o 9 (septembre 2015) : 696–703. http://dx.doi.org/10.1139/cjce-2014-0275.

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This study examines the possible effect of the record ice jam of 1984 in the St. Clair River on river bed changes and conveyance. Numerical simulations were made to examine the flow and bed shear stresses during the jam formation and release periods. Simulation results indicate that the ice jam in the river did not cause a significant increase in bed shear stress compared to pre- and post-jam open water conditions. The insignificant change on bed shear stress during the ice jam period is the result of the large flow depth and the limited jam thickness. The bed shear stresses are much less than the critical shear stress for bed particle movement. This implies that the jam may not have had a significant impact on the channel conveyance.

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13

Wilson, Kenneth C. « Mobile‐Bed Friction at High Shear Stress ». Journal of Hydraulic Engineering 115, n^o 6 (juin 1989) : 825–30. http://dx.doi.org/10.1061/(asce)0733-9429(1989)115:6(825).

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Sarmiento, Oscar A., et Marco A. Falcon. « Critical Bed Shear Stress for Unisize Sediment ». Journal of Hydraulic Engineering 132, n^o 2 (février 2006) : 172–79. http://dx.doi.org/10.1061/(asce)0733-9429(2006)132:2(172).

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15

Oms, C., M. C. Gromaire, M. Saad, V. Milisic et G. Chebbo. « Bed shear stress evaluation in combined sewers ». Urban Water Journal 5, n^o 3 (septembre 2008) : 219–29. http://dx.doi.org/10.1080/15730620801924010.

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Zhang, Xiao-Feng, Wen-Ting Yang et Jun-Qiang Xia. « Bed shear stress in non-uniform flow ». Environmental Fluid Mechanics 16, n^o 4 (9 mars 2016) : 777–92. http://dx.doi.org/10.1007/s10652-016-9448-1.

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Kämpf, Jochen. « Extreme bed shear stress during coastal downwelling ». Ocean Dynamics 69, n^o 5 (18 mars 2019) : 581–97. http://dx.doi.org/10.1007/s10236-019-01256-4.

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Agrawal, Sunil K., et Jatinder K. Bewtra. « Modifications to the design procedure for grit chambers ». Canadian Journal of Civil Engineering 14, n^o 2 (1 avril 1987) : 216–20. http://dx.doi.org/10.1139/l87-033.

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A modified approach to the design of grit chambers has been suggested in this paper. This approach is based on the concept of critical shear stress at the bed rather than mean velocity as used by T. R. Camp. It is recognized that the relationship between critical bed shear stress and mean velocity in a channel is not constant, as assumed by Camp, but varies according to the flow conditions. Critical bed shear stress values, obtained in the laboratory for different particle characteristics, are given in this paper. The proposed method should provide a more rational and a better design procedure for grit chambers. Key words: grit chambers, scouring, bed shear stress, initiation of motion.

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Kabir, M. R., et H. Torfs. « Comparison of Different Methods to Calculate Bed Shear Stress ». Water Science and Technology 25, n^o 8 (1 avril 1992) : 131–40. http://dx.doi.org/10.2166/wst.1992.0187.

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Precise velocity profile measurements were carried out in a fully developed, subcritical and supercritical flow on a smooth and a rough bed in a flume with a rectangular cross section. In both cases different approaches were used to calculate the friction velocity, u*, and comparisons were made. The influence of the reference level (hypothetical bed level) has been discussed. Friction velocities calculated from the velocity profile are very sensitive to the hypothetical bed level. For a smooth bed the determination of the reference level does not present any difficulties. However, for a rough bed this level shows dependency on the channel slope.

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CHARRU, FRANÇOIS, et HÉLÈNE MOUILLERON-ARNOULD. « Instability of a bed of particles sheared by a viscous flow ». Journal of Fluid Mechanics 452 (10 février 2002) : 303–23. http://dx.doi.org/10.1017/s0022112001006747.

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The instability of a bed of particles sheared by a viscous fluid is investigated theoretically. The viscous flow over the wavy bed is first calculated, and the bed shear stress is derived. The particle transport rate induced by this bed shear stress is calculated from the viscous resuspension theory of Leighton & Acrivos (1986). Mass conservation of the particles then gives explicit expressions for the wave velocity and growth rate, which depend on four dimensionless parameters: the wavenumber, the fluid thickness, a viscous length and the shear stress. The mechanism of the instability is given. It appears that for high enough fluid-layer thickness, long-wave instability arises as soon as grains move, while short waves are stabilized by gravity. For smaller fluid thickness, the destabilizing effect of fluid inertia is reduced, so that the moving at bed is stable for small shear stress, and unstable for high shear stress. The most amplified wavelength scales with the viscous length, in agreement with the few available experiments for small particle Reynolds numbers. The results are also compared with related studies for turbulent flow.

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Zhong, Chun Xin. « Experimental Study on Critical Flow Conditions Causing Damage of Grass-Covered Revetment ». Applied Mechanics and Materials 71-78 (juillet 2011) : 1478–83. http://dx.doi.org/10.4028/www.scientific.net/amm.71-78.1478.

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Flexible vegetations supply great effect on river bank protection. It is important to study the critical flow conditions, including bed shear stress, scour duration and frictional velocity etc, causing damage of grass-covered revetment. Laboratory experiments prove that, destruction of turf can be estimated by observing the variation of water head. The experimental shear stress above vegetation zone has nonlinear relation with velocity square, which agrees with theoretic derivation. Before the turf destruction occurring, shear stress above vegetation zone increases with the scour duration and then goes to stabilization. This mentioned Shear stress will change suddenly when local scour damage happens, or keep stabilization while affine damage is on-going. Under the same flow conditions, the critical bed shear stress of reinforced turf is larger than that of natural turf. As far as the same-form revetments are concerned, the bed shear stress at breaking point increases with the flow velocity.

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Lockwood, Kenneth, Patrick Grover et Ana Maria Ferreira da Silva. « Quantification of bed-load transport over dunes ». E3S Web of Conferences 40 (2018) : 02010. http://dx.doi.org/10.1051/e3sconf/20184002010.

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There is disagreement in the literature as to whether a shear stress-based approach can be used to accurately predict sediment transport over dunes. This study aims to address this disagreement. To this end, use is made of an experiment involving the study of naturally formed, fully developed dunes produced in a laboratory flume. The bed shear stress is estimated through a combination of velocity, Reynolds stress measurements, and results of a CFD RANS rough wall model. The validity of using Bagnold’s equation to predict the bed-load rate is subsequently analyzed. In contrast to what has been previously suggested by some authors, it is found from the present experiment that the bed-load rate correlates well with the bed shear stress, and that Bagnold’s equation yields realistic values of the bed-load rate over the stoss side of the dune downstream of the reattachment point. This work also highlights the difficulties in reliably estimating the bed shear stress in complex flows. Such difficulties are overcome in this paper through a combination of flow velocity measurements and modeled results.

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Blatter, Heinz, Garry K. C. Clarke et Jacques Colinge. « Stress and velocity fields in glaciers : Part II. Sliding and basal stress distribution ». Journal of Glaciology 44, n^o 148 (1998) : 457–66. http://dx.doi.org/10.3189/s0022143000001970.

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AbstractNumerical methods are used to examine the interaction between the spatial distribution of the basal shear traction and the corresponding basal velocity for an inclined slab geometry. In our improved treatment, we reject the common assumption that basal velocity is a simple function of local variables in favour of a non-local treatment that includes normal deviatoric stress and takes basal velocity to be an integrated response to spatially varying influences. Computationally, one must either iterate the basal velocity with a friction parameterization that relates basal shear traction to basal velocity or, alternatively, prescribe the basal shear traction that results from bed decoupling and substrate déformation.The average of basal shear traction over the entire bed of the ice mass is invariant under changes in sliding distribution and thus constitutes a useful reference; any local relative reduction of traction leads to basal movement, either sliding over the bed or moving with a deforming subglacial layer. The local stress réduction is accompanied by a concentration of traction up-and down-glacier of the moving base. Growth, decay and possible migration of basal stress concentrations may be closely related to short-lived sliding events and to surges.

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Blatter, Heinz, Garry K. C. Clarke et Jacques Colinge. « Stress and velocity fields in glaciers : Part II. Sliding and basal stress distribution ». Journal of Glaciology 44, n^o 148 (1998) : 457–66. http://dx.doi.org/10.1017/s0022143000001970.

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AbstractNumerical methods are used to examine the interaction between the spatial distribution of the basal shear traction and the corresponding basal velocity for an inclined slab geometry. In our improved treatment, we reject the common assumption that basal velocity is a simple function of local variables in favour of a non-local treatment that includes normal deviatoric stress and takes basal velocity to be an integrated response to spatially varying influences. Computationally, one must either iterate the basal velocity with a friction parameterization that relates basal shear traction to basal velocity or, alternatively, prescribe the basal shear traction that results from bed decoupling and substrate déformation.The average of basal shear traction over the entire bed of the ice mass is invariant under changes in sliding distribution and thus constitutes a useful reference; any local relative reduction of traction leads to basal movement, either sliding over the bed or moving with a deforming subglacial layer. The local stress réduction is accompanied by a concentration of traction up-and down-glacier of the moving base. Growth, decay and possible migration of basal stress concentrations may be closely related to short-lived sliding events and to surges.

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Pujara, Nimish, Philip L. F. Liu et Harry Yeh. « The swash of solitary waves on a plane beach : flow evolution, bed shear stress and run-up ». Journal of Fluid Mechanics 779 (18 août 2015) : 556–97. http://dx.doi.org/10.1017/jfm.2015.435.

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The swash of solitary waves on a plane beach is studied using large-scale experiments. Ten wave cases are examined which range from non-breaking waves to plunging breakers. The focus of this study is on the influence of breaker type on flow evolution, spatiotemporal variations of bed shear stresses and run-up. Measurements are made of the local water depths, flow velocities and bed shear stresses (using a shear plate sensor) at various locations in the swash zone. The bed shear stress is significant near the tip of the swash during uprush and in the shallow flow during the later stages of downrush. In between, the flow evolution is dominated by gravity and follows an explicit solution to the nonlinear shallow water equations, i.e. the flow due to a dam break on a slope. The controlling scale of the flow evolution is the initial velocity of the shoreline immediately following waveform collapse, which can be predicted by measurements of wave height prior to breaking, but also shows an additional dependence on breaker type. The maximum onshore-directed bed shear stress increases significantly onshore of the stillwater shoreline for non-breaking waves and onshore of the waveform collapse point for breaking waves. A new normalization for the bed shear stress which uses the initial shoreline velocity is presented. Under this normalization, the variation of the maximum magnitudes of the bed shear stress with distance along the beach, which is normalized using the run-up, follows the same trend for different breaker types. For the uprush, the maximum dimensionless bed shear stress is approximately 0.01, whereas for the downrush, it is approximately 0.002.

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Wen, Jiaqi, Yongcan Chen, Zhaowei Liu et Manjie Li. « Numerical Study on the Shear Stress Characteristics of Open-Channel Flow over Rough Beds ». Water 14, n^o 11 (30 mai 2022) : 1752. http://dx.doi.org/10.3390/w14111752.

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Bed shear stress is an important measure of benthic habitats since it is related to many ecological processes. In this study, we focused on the fluctuating characteristics of shear stress in rough-bed open-channel flows. The roughness element method was adopted to mimic natural rough beds and the Improved Delayed Detached Eddy Simulation (IDDES) model was used to obtain comprehensive information about shear stress near the rough bed. Three arrangement patterns of the roughness elements were simulated to compare their effects on flow structure and shear stresses. The arrangements of the roughness elements altered the Reynold stress and turbulent kinetic energy characteristics, due to the variance of blockage in lateral directions that led to flow detachment and changes in the flow directions. Quadrant analysis revealed the spatial variations of the instantaneous shear stress burst events at different locations in the wake. By using spectrum analysis, the accumulation of shear-stress energy from small to large vortex scales was estimated, which revealed that the instantaneous effect of the shear stress was significantly stronger than the effect of the time-averaged shear stress, especially on small scales. The results of this study suggest the significance of the fluctuation part of shear stress in further studies on ecological processes.

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Khuntia, Jnana Ranjan, Kamalini Devi, Sebastien Proust et Kishanjit Kumar Khatua. « Depth-averaged velocity and bed shear stress in unsteady open channel flow over rough bed ». E3S Web of Conferences 40 (2018) : 05071. http://dx.doi.org/10.1051/e3sconf/20184005071.

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Very few studies have been carried out in the past in estimating depth-averaged velocity and bed shear stress in unsteady flow over rough beds. An experiment is thus conducted to investigate the vertical and lateral velocity profiles under unsteady flow conditions in a rough open channel for various flow depths. One hydrogram is repeatedly passed through the rectangular flume with a fixed rigid grass bed. Using micro Pitot tube and Acoustic Doppler Velocimeter (ADV), the flow patterns are investigated at both lateral and longitudinal positions over different cross-sections. For two typical flow depths, the velocities in both the rising limb and falling limb are observed. Hysteresis effect between stage-discharge (h ~ Q) rating curve between rising and falling limbs is illustrated. Lateral distribution of depth-averaged velocity and bed shear stress are plotted at three different cross sections and compared with the steady flow conditions. In falling limb of an unsteady flow case, both depth-averaged velocity and bed shear stress distribution in the central region is higher than that of steady flow case. However, in the rising limb, the bed shear stress of unsteady flow is less than that of steady flow case. Further, in an unsteady flow, the magnitude of depth-averaged velocity is found to increase towards the downstream sections. Along the downstream positions, bed shear stress values increase for lower flow depths and decrease for higher flow depth cases.

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Dey, Subhasish. « Critical bed shear for initial movement of sediments on a combined lateral and longitudinal slope ». Hydrology Research 35, n^o 2 (1 avril 2004) : 153–64. http://dx.doi.org/10.2166/nh.2004.0011.

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An experimental study on critical bed shear-stress for initial movement of non-cohesive sediment particles under a steady-uniform stream flow on a combined lateral (across the flow direction) and longitudinal (streamwise direction) sloping bed is presented. The aim of this paper is to ascertain that the critical bed shear-stress on a combined lateral and longitudinal sloping bed is adequately represented by the product of critical bed shear-stress ratios for lateral and longitudinal sloping beds. Experiments were carried out with closed-conduit flow, in two ducts having a semicircular invert section, with three sizes of sediments. In laboratory flumes, the uniform flow is a difficult – if not impossible – proposition for a steeply sloping channel, and is impossible to obtain in an adversely sloping channel. To avoid this problem, the experiments were conducted with a closed-conduit flow. The critical bed shear-stresses for experimental runs were estimated from side-wall correction. The experimental data agree satisfactorily with the results obtained from the proposed formula.

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Matoušek, Václav, et Jan Krupička. « On equivalent roughness of mobile bed at high shear stress ». Journal of Hydrology and Hydromechanics 57, n^o 3 (1 septembre 2009) : 191–99. http://dx.doi.org/10.2478/v10098-009-0018-9.

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On equivalent roughness of mobile bed at high shear stressThe friction conditions at the top of a mobile bed are discussed for flows in the upper-plane-bed regime, i.e. for the flows with values of the bed Shields parameter larger than approximately 0.6. A special attention is devoted to flows of the bed Shields parameter bigger than 2. Experimental data for flows at high bed shear are collected from literature and new data are added from own measurements of flows of a 1.36-mm sand slurry in the 100-mm pipe loop of the Institute of Hydrodynamics. The database represents flows of very different solids and covers friction conditions within a broad range of Shields parameters up to the maximum value of about 23. The paper analyses the data in order to evaluate a relationship among the equivalent roughness of the top of the bed and other relevant parameters. A semi-empirical formula is proposed that relates the equivalent roughness to the bed Shields parameter, the ratio of flow velocity and particle settling velocity, and the ratio of flow hydraulic radius and particle diameter. The formula is applicable primarily to flows of combined load (contact- and suspended loads together).

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Pathirana, K. P. P., P. C. Ranasinghe et U. R. Ratnayake. « Bed Shear Stress in Unsteady Open Channel Flow Over Rough Beds ». Engineer : Journal of the Institution of Engineers, Sri Lanka 41, n^o 1 (30 janvier 2008) : 7. http://dx.doi.org/10.4038/engineer.v41i1.7078.

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Bockelmann-Evans, B. N., R. Davies et R. A. Falconer. « Measuring bed shear stress along vegetated river beds using FST-hemispheres ». Journal of Environmental Management 88, n^o 4 (septembre 2008) : 627–37. http://dx.doi.org/10.1016/j.jenvman.2007.03.044.

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Vongvisessomjai, Suphat. « TIME-DEPENDENT WAVE SHEAR STRESS ». Coastal Engineering Proceedings 1, n^o 21 (29 janvier 1988) : 81. http://dx.doi.org/10.9753/icce.v21.81.

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A knowledge of bed shear stress induced by waves is required to understand dynamic processes of nearshore morphologies as a results of sediment transport. However, the information on the stress is still incomplete due to lack of measured data. The study analyzes the unsteady horizontally averaged shear stresses measured over mobile beds in a water tunnel. It is found from the analysis that the presence of the third and fifth harmonics in the shear stress is in good agreement with the measured concentration of suspended sediments.

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HASHIMOTO, Susumu, Yoshitaka FUKUI et Hideo KIKKAWA. « Study on bed shear stress around channel constriction. » Doboku Gakkai Ronbunshu, n^o 357 (1985) : 115–21. http://dx.doi.org/10.2208/jscej.1985.357_115.

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OTSUJIMOTO, Tetsur, H. Antonio CARDOSO et Akira SAITO. « Open Channel Flow with Variable Bed Shear Stress ». PROCEEDINGS OF HYDRAULIC ENGINEERING 34 (1990) : 433–38. http://dx.doi.org/10.2208/prohe.34.433.

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Petit, François, Geoffrey Houbrechts, Alexandre Peeters, Eric Hallot, Jean Van Campenhout et Anne-Cécile Denis. « Dimensionless critical shear stress in gravel-bed rivers ». Geomorphology 250 (décembre 2015) : 308–20. http://dx.doi.org/10.1016/j.geomorph.2015.09.008.

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Maa, Jerome P. Y., C. H. Lee et F. J. Chen. « Bed shear stress measurements for vims Sea Carousel ». Marine Geology 129, n^o 1-2 (décembre 1995) : 129–36. http://dx.doi.org/10.1016/0025-3227(95)00102-6.

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Wilcock, Peter R. « Estimating Local Bed Shear Stress from Velocity Observations ». Water Resources Research 32, n^o 11 (novembre 1996) : 3361–66. http://dx.doi.org/10.1029/96wr02277.

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Paquier, Andre, et Saeed R. Khodashenas. « River bed deformation calculated from boundary shear stress ». Journal of Hydraulic Research 40, n^o 5 (septembre 2002) : 603–9. http://dx.doi.org/10.1080/00221680209499904.

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Iverson, Neal R., Robert W. Baker, Roger LeB Hooke, Brian Hanson et Peter Jansson. « Coupling between a glacier and a soft bed : I. A relation between effective pressure and local shear stress determined from till elasticity ». Journal of Glaciology 45, n^o 149 (1999) : 31–40. http://dx.doi.org/10.3189/s0022143000003014.

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AbstractTo predict the distribution of motion beneath glaciers on soft beds, the strength of the coupling between the ice and the bed and its variation with effective pressure must be known. A record of shear strain, acquired with a tiltmeter emplaced in till beneath Storglaciären, Sweden, indicates that fluctuations in water pressure cause variations in the local shear stress on the bed and that the bed deforms elastically in response to these variations. To estimate the shear stress from the elastic component of the total shear strain, the shear modulus of the till was measured in relaxation tests conducted in the laboratory with a ring-shear device. After accounting for the elastic compliance of the device, these tests yielded shear moduli of about 1000 and 1800 kPa at confining pressures of 85 and 280 kPa, respectively. These values are comparable to those of other granular materials undergoing recoverable shear strains of the same magnitude. The local shear stress on the till, calculated by applying the measured shear moduli to the tilt record, scales withPe1.7, wherePeis the effective pressure. This relation implies that asPedecreases at the ice/till interface, shear stresses on the till are reduced and concentrated elsewhere on the bed, perhaps where the till is absent or the glacier is frozen to the bed. When compared with the strength of the till determined from ring-shear tests, this relation also accounts for the lack of permanent deformation at depth in the bed during periods of lowPeand indicates that most basal motion was by sliding or ploughing.

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Afzalimehr, Hossein, et François Anctil. « Velocity distribution and shear velocity behaviour of decelerating flows over a gravel bed ». Canadian Journal of Civil Engineering 26, n^o 4 (1 août 1999) : 468–75. http://dx.doi.org/10.1139/l99-009.

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The behaviour of the bed shear stress in the presence of a decelerating flow over a gravel bed has been examined. The collected observations revealed that the velocity distribution in the outer region of the boundary layer may be described by a parabolic law. The results obtained by parabolic law are comparable to the bed shear stress estimated via the St-Venant method. At a specific cross section, shear velocities estimated by the parabolic and the St-Venant methods divert considerably from the estimation by zero pressure gradient method. For constant bottom slope and relative roughness, the estimated bed shear stresses based on the parabolic and the St-Venant methods are proportional to the flow discharge, whereas this tendency is not accounted for by the zero pressure gradient model.Key words: velocity distribution, shear velocity, decelerating flows, gravel bed, boundary layer.

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Shahmohammadi, Reza, Hossein Afzalimehr et Jueyi Sui. « Estimation of Bed Shear Stress in Shallow Transitional Flows under Condition of Incipient Motion of Sand Particles Using Turbulence Characteristics ». Water 14, n^o 16 (15 août 2022) : 2515. http://dx.doi.org/10.3390/w14162515.

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In this experimental study, using an ADV, experiments were performed in three different shallow water flows under hydraulically transitional flow condition to estimate the bed shear stress using turbulence characteristics. Vertical distributions of all shear and normal Reynolds stresses as well as TKE were evaluated and simplified in order to estimate bed shear stress under incipient motion of four groups of sand particles. To determine bed shear stress, as the main approach, the linear portion of the −u′w′ profiles were extended towards the channel bed. The necessity of the approach of the vector addition of −u′w′ and −v′w′ in this experimental study was examined. It was found that the bed shear stress can be effectively estimated by multiplying the values of u′20, v′20, w′20 and TKE0 by 0.17, 0.33, 1.24 and 0.2, respectively. However, it was found that these values were slightly proportional to the shear Reynolds number. Additionally, the one-point measurement approach was assessed. The TKE method which applies all three components of Reynolds normal stresses was preferred to the u′2, v′2 and w′2 methods. Results showed that, u′20, v′20 and w′20 have values of 60.5, 31.3 and 8.2 percent of the total, respectively.

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Krishnappan, Bommanna G. « Erosion behaviour of fine sediment deposits ». Canadian Journal of Civil Engineering 31, n^o 5 (1 octobre 2004) : 759–66. http://dx.doi.org/10.1139/l04-054.

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Erosion characteristics of fine sediment deposits were studied experimentally using a rotating circular flume in the laboratory. The influence of the rate of application of bed shear stress and the structure of sediment beds was investigated. When the shear stress was applied suddenly on a bed formed by placing a thick slurry of kaolin in the flume, the concentration of the eroded sediment first increased to reach a maximum value, and then it started to decrease and finally attained a steady state value that was significantly lower than the maximum concentration (the steady state concentration value was one third of the maximum concentration value). When the shear stress was applied gradually, the behaviour was similar, but the difference between the values of the maximum concentration and steady state concentration was very small (<10%). When the shear stress was applied gradually on a bed formed by depositing a dilute suspension, the concentration increased monotonically and reached the steady state concentration without exhibiting the hump. Such a behaviour was attributed to the sorting of flocs in the settling of dilute suspension in which stronger flocs deposit first followed by weaker flocs in succession.Key words: kaolin, erosion rate, bed shear stress, bed structure, rotating circular flume, flocculation, size distribution.

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Wang, Rui, et Guoliang Yu. « Experimental study on incipient condition of fluidized bed sediment in oscillatory ». E3S Web of Conferences 81 (2019) : 01014. http://dx.doi.org/10.1051/e3sconf/20198101014.

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In this paper, the incipient condition of the fluidized bed sediment with different sizes and water contents were experimentally studied in an os- cillatory tunnel made of acrylic boards. One-hundred experimental runs were performed with sediment samples by varying the yield stress to determine the relationship between the critical condition of incipient motion and the rheolog- ical properties of the cohesive sediments. Experimental results showed that the yield stress of the bed sediment decreased as the fluidization level increased. When the yield stress is no longer changed, the bed sediment was considered completely fluidized. In oscillatory flow, the critical shear stress decreases with the increase of fluidization level. When the bed sediment reaches the full flu- idization state, the critical shear stress of the bed sediment at the bottom re- mained constant. For cohesive sediments, in the case that particle size and bulk density were known, the relationship between the yield stress and the critical shear stress was analyzed, and the incipient condition of the cohesive sediment under oscillatory flow action was determined.

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Jewel, Arif, Kazunori Fujisawa et Akira Murakami. « Evaluation of Incipient Motion of Sand Particles by Different Indirect Methods in Erosion Function Apparatus ». Water 13, n^o 8 (19 avril 2021) : 1118. http://dx.doi.org/10.3390/w13081118.

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An experiment was carried out in an acrylic glass-sided re-circulating closed conduit with a rectangular cross section, which is similar in construction to an erosion function apparatus. An adjustable sand box, made of acrylic glass, was attached to the bottom of the conduit as the sand zone or the test section. The hydraulics of the flow in the erosion function apparatus is complicated due to the limited part of the non-smooth and erodible soil surface attached to the closed conduit. As the bed shear stress changes with the bed roughness, even though the flow velocity does not change, establishing a method to estimate the incipient motion is an important challenge for an erosion function apparatus. The present study was conducted to explore the incipient motion of sands from bed shear stress estimated by four different indirect methods on both the sand bed and the smooth bed installed in the erosion function apparatus. In the experiment, particle image velocimetry (PIV) was used to investigate flow dynamics and incipient motion in terms of dimensionless critical bed shear stress. The experimental results show that the bed shear stress estimated from the log-law profiles in the sand zone and the smooth zones are relatively higher than those of the other indirect methods. The dimensionless critical bed shear stress of threshold condition evaluated by all indirect methods was found in good agreement with those of previous results in both zones. The Manning roughness and Darcy–Weisbach friction coefficients were evaluated based on the critical shear velocity at the incipient motion. Although these coefficients were found slightly greater in the smooth zone than in the sand zone, in both zones, they showed good agreement with previous studies.

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Geremew, Africa M. « Erosion characteristics and stochastic nature of bed shear stress in underwater mine tailings ». Canadian Journal of Civil Engineering 44, n^o 6 (juin 2017) : 426–40. http://dx.doi.org/10.1139/cjce-2016-0319.

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The erosion of mine tailings was investigated by examining the physical processes during the initiation of motion of the tailings. Erosion experiments were conducted on mine tailings samples and natural soils in a Plexiglas laboratory annular column under 50 cm water cover. Resuspension was introduced with a Teflon stirrer and the bed shear stress was estimated from the measured near-bed velocity field and the pressure change in the boundary layer. Two modes of initiation of motion of cohesive mine tailings that showed cohesive behaviour was noticed: pitting erosion and line erosion and the modes of initiation of motion changed mainly with percentage of fines. At incipient motion of the tailings that showed cohesive behaviour, the pore water pressure distribution showed a relative sudden peak and a decline when the aggregated tailings burst. A four order of magnitude difference was observed between the undrained shear strength and critical shear stress for surface erosion of the tailings. The stochastic nature of the bed shear stress was explained by the Rayleigh distribution that provides an approach for correcting the critical shear stress estimated from the near-bed velocity. This correction is necessary to achieve a conservative estimate of the critical shear stress for design purposes.

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Wilson, Robert J., et Michael R. Heath. « Increasing turbidity in the North Sea during the 20th century due to changing wave climate ». Ocean Science 15, n^o 6 (6 décembre 2019) : 1615–25. http://dx.doi.org/10.5194/os-15-1615-2019.

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Abstract. Data on Secchi disc depth (the depth at which a standard white disc lowered into the water just becomes invisible to a surface observer) show that water clarity in the North Sea declined during the 20th century, with likely consequences for marine primary production. However, the causes of this trend remain unknown. Here we analyse the hypothesis that changes in the North Sea's wave climate were largely responsible by causing an increase in the concentrations of suspended particulate matter (SPM) in the water column through the resuspension of seabed sediments. First, we analysed the broad-scale statistical relationships between SPM and bed shear stress due to waves and tides. We used hindcasts of wave and current data to construct a space–time dataset of bed shear stress between 1997 and 2017 across the northwest European Continental Shelf and compared the results with satellite-derived SPM concentrations. Bed shear stress was found to drive most of the inter-annual variation in SPM in the hydrographically mixed waters of the central and southern North Sea. We then used a long-term wave reanalysis to construct a time series of bed shear stress from 1900 to 2010. This shows that bed shear stress increased significantly across much of the shelf during this period, with increases of over 20 % in the southeastern North Sea. An increase in bed shear stress of this magnitude would have resulted in a large reduction in water clarity. Wave-driven processes are rarely included in projections of climate change impacts on marine ecosystems, but our analysis indicates that this should be reconsidered for shelf sea regions.

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47

Iverson, Neal R., Robert W. Baker, Roger LeB Hooke, Brian Hanson et Peter Jansson. « Coupling between a glacier and a soft bed : I. A relation between effective pressure and local shear stress determined from till elasticity ». Journal of Glaciology 45, n^o 149 (1999) : 31–40. http://dx.doi.org/10.1017/s0022143000003014.

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AbstractTo predict the distribution of motion beneath glaciers on soft beds, the strength of the coupling between the ice and the bed and its variation with effective pressure must be known. A record of shear strain, acquired with a tiltmeter emplaced in till beneath Storglaciären, Sweden, indicates that fluctuations in water pressure cause variations in the local shear stress on the bed and that the bed deforms elastically in response to these variations. To estimate the shear stress from the elastic component of the total shear strain, the shear modulus of the till was measured in relaxation tests conducted in the laboratory with a ring-shear device. After accounting for the elastic compliance of the device, these tests yielded shear moduli of about 1000 and 1800 kPa at confining pressures of 85 and 280 kPa, respectively. These values are comparable to those of other granular materials undergoing recoverable shear strains of the same magnitude. The local shear stress on the till, calculated by applying the measured shear moduli to the tilt record, scales with Pe1.7, where Pe is the effective pressure. This relation implies that as Pe decreases at the ice/till interface, shear stresses on the till are reduced and concentrated elsewhere on the bed, perhaps where the till is absent or the glacier is frozen to the bed. When compared with the strength of the till determined from ring-shear tests, this relation also accounts for the lack of permanent deformation at depth in the bed during periods of low Pe and indicates that most basal motion was by sliding or ploughing.

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Shimozono, Takenori, Akio Okayasu et Teppei Mishima. « ON THE BOTTOM SHEAR STRESS DURING LONG WAVE RUNUP AND BACKWASH ». Coastal Engineering Proceedings 1, n^o 32 (26 janvier 2011) : 47. http://dx.doi.org/10.9753/icce.v32.currents.47.

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Laboratory experiments were performed to examine flow characteristics during runup and backwash of a breaking solitary wave. Boundary layer flows were measured by LDV at two locations landward of the still shoreline under different bed conditions. Temporal variations of the bottom shear stress were estimated from nearbed velocity profiles with different methods. The shear stress determined through the logarithmic fitting roughly agreed with that deduced from a depth-integrated momentum balance during turbulent flow phases. The friction factors during the active phases of the runup and backwash were similar in magnitude except in the leading tip where the value sharply increased. A movable bed experiment was also carried out under the same geometry to discuss relations between the bottom shear stress and sediment transport. The result indicated clear dependencies of bed deformations on the magnitude and duration of the exerted shear stress.

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Sumiadi, Sumiadi, Bambang Kironoto, Djoko Legono et Istiarto Istiarto. « Bed-Shear Velocity Measurement in Curved Open Channel ». Civil and Environmental Science 004, n^o 01 (1 avril 2021) : 093–105. http://dx.doi.org/10.21776/ub.civense.2021.00401.9.

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Generally, the condition of the rivers in Indonesia are alluvial rivers which had meanders, where the change in the river bed topography often occur. One of the parameters associated with changes in the river bed topography is bed-shear velocity, or Reynolds stress. The bed-shear velocity can be calculated by the Reynolds stress distribution method and the Clauser method which commonly used in straight channels. In fact, on natural channel there is a curve and even a meandering channel. With more complex flow conditions, the use of the Clauser method in curved channels can be questioned, is it still accurate or not. In this paper, both methods will be discussed by comparing the measurement data in the laboratory using 180 curved channel with flat bed. The results of data analysis show that the use of these two methods in curved channels produces an average difference of around 19.81%, where the Clauser method gives greater results and better tendencies. Apart from the differences in the results given, it can be said that the Clauser method as well as the Reynolds stress distribution method can still be used to calculate the bed-shear velocity in the curved channel

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Henry, Pierre-Yves, Alf Tørum, Øivind Artsen, Dag Myrhaug et Muk Chen Ong. « PROBABILITY OF EXCEEDING THE CRITICAL SHEAR STRESS FOR SAND MOTION IN SPECIFIC WAVE AND CURRENT CONDITIONS ». Coastal Engineering Proceedings 1, n^o 33 (15 octobre 2012) : 4. http://dx.doi.org/10.9753/icce.v33.sediment.4.

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This study is focusing on the threshold of sand motion under random waves combined with a following current. The analysis is based on some flume experiments realized over a natural sand bed for different flow conditions (waves and currents). The main result comes as a map of the probability to exceed the threshold of sand motion, as a function of a wave and a current mobility parameter. These observations are compared to methods predicting the bed shear stress using an equivalent monochromatic wave, and links between the probability of exceeding the critical shear stress for initiation of sand motion and the calculated maximum bed shear stress are found.

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