Bed Shear Stress

Whether or not a sediment particle is entrained from a channel bed is associated with both average bed shear stress and shear stress fluctuation, the latter being flow-dependent and also related to bed irregularities. In the first part of this study, a preliminary analysis of possible fluctuations induced by bed roughness is presented for the case of an immobile plane bed comprised of unisized sediments. The result shows that the roughness-induced variation is generally comparable to that associated with near-bed turbulence, and both variations can be approximated as log-normal in terms of probability density distribution. The bed particle mobility is then analyzed by considering the effects of shear stress fluctuations. The relevant computations demonstrate that with increasing shear stress fluctuations, the probability of the mobility of a bed particle may be enhanced or weakened. For the case of low sediment entrainment, the probability is increased by turbulence. However, the probability may be reduced by the shear stress fluctuation if the average bed shear stress becomes relatively higher. This study shows that the critical condition for initial sediment motion could be overestimated if the Shields diagram is applied for the condition of flows with high turbulence intensities.

New experimental measurements of bed shear under solitary waves and solitary bores that
represent tsunamis are presented. The total bed shear stress was measured directly using a
shear cell apparatus. The solitary wave characteristics were measured using ultrasonic
wave gauges and free stream velocities were measured using an Acoustic Doppler
Velocimeter. The measurements were carried out in laminar and transitional flow regimes
(¡«104 < Re < ¡«105). This sort of data is sparsely available in literature. In the absence of
direct measurements, shear stress is indirectly estimated using velocity profiles or is
inferred using standard friction factors. However, this indirect method has its limitations,
e.g., under unsteady hydrodynamic conditions and relatively large roughness the
assumptions of both approaches are no longer valid. More than 168 experimental runs
comprising solitary waves and bores were carried out over a smooth flat bed with wave
height to water depth ratio varying between 0.12 and 0.69. Analytical modeling was
carried out to predict shear stresses using Fourier and convolution integration methods.
This paper presents comparison of the measured and predicted bed shear stress or skin
friction stress, together with estimates of traditional wave friction factors. Overall, the
models can predict the bed shear stress with a satisfactory degree of accuracy.