Estimation Of Bed Shear Stress Research Articles

Coarse bedload transport in a mountain river

Coarse bedload transport dynamics are investigated utilizing hydrodynamic and sediment transport data obtained in an extensively instrumented study reach located in Squaw Creek, Montana, USA. During 1991 and 1992, a number of discrete bedload transport events associated with the daily rise and fall in stream discharge were investigated. Data show that initiation of sediment transport was accompanied by a reduction in bed roughness and by changes in bulk hydraulic parameters. For larger discharges, coarser fractions of the bed material mobilized, and bedload transport rates and average hydraulic parameters stabilized. As discharge reduced, mobile coarse particles became less frequent and deposited fine particles were removed, resulting in an increase in bed roughness. These observations are attributed to the downstream translation of bar sediments during the passage of a hydrograph. Bedload pulses were aperiodic but spatially variable. Flow turbulence and velocity profile data obtained during low flows allowed comparison between average bed shear stress and apparent bed roughness estimates obtained using different approaches. © 1998 John Wiley & Sons, Ltd.

Characteristics of velocity profiles along riffle-pool sequences and estimates of bed shear stress

Numerous velocity profiles were measured along a riffle-pool sequence for three different flow conditions. For each flow condition (ranging from baseflow to about 2 3 of bankfull depth), ten to fifteen velocity profiles were measured along the same transect. The objectives of the study were: (i) to differentiate between components of bed roughness and shear stress attributed to surface friction and large-scale bed undulations, and (ii) to investigate the effects of fluctuating discharges on the mean vertical flow structure in pools and riffles. No previous attempts have been made to investigate scales of bed roughness from velocity profiles along riffle-pool sequences and to determine how near-bed velocity gradients change with increasing flow discharge. At low flows (and as expected), riffles are characterized by higher near-bed velocity gradients than pools. Similarly, average bed shear stresses, as determined from measurements of flow depth and energy gradient, indicate a greater resistance and bed shear stress in riffle sections. Invariably, however, local shear stresses as determined from near-bed velocity gradients in pools and in riffles are much smaller than reach averages estimated from measurements of depth and slope. The shape of spatially averaged velocity profiles changes as discharges increases. The difference in terms of near-bed velocities between pools and riffles decreases as discharge increases which suggests that a velocity reversal may occur at or near bankfull. Further away from the bed surface, velocity differences between pools and riffles do remain significant. Finally, the curvature of the velocity profiles is more significant at high flows and surface roughness is maximized when flow conditions are intermediate, perhaps a result of increasing density of small-scale sedimentary structures.

Study on Flow Resistance and Sediment Transport on the Bed with Sediment Deposition in Pipe

Resistance of pipe flow with a stationary bed and with bed load on sediment deposition in pipes is discussed theoretically and experimentally, in which water flow area and hydraulic radius in a cross section of the pipe is divided into two parts: area and hydraulic radius influenced by smooth pipe wall and those by rough sediment wall. Experimental results about mean velocity and sediment transport rate in a pipe proved that the division of hydraulic radius of the water flow area into two is useful for the estimation of flow resistance and bed shear stress

Soft Mud Response to Water Waves

Motion within soft mud beds, wave‐induced bed shear stress, and wave attenuation under small amplitude waves are briefly examined using an analytic approach to gain insight into the mechanism by which coastal mud responds to water waves. The cohesive mud bed is discretized into layers of simple linear viscoelastic material characterized by constant density, viscosity, and shear modulus of elasticity, for approximately simulating depth‐varying bed properties as well as energy dissipation. Model results on wave orbital velocities, dynamic pressure, and wave attenuation are compared with limited data from wave flume tests on partially consolidated beds under relatively low deformations and show acceptable agreement. Wave attenuation coefficients exhibit dual dependence on the degree of bed consolidation and sediment composition. In the absence of adequate data on bed shear stress over mobile, partially consolidated mud beds, this method offers an approximate procedure useful for bed shear stress estimation r...