Critical Bed Shear Stress for Unisize Sediment

The overall, spatially averaged, mean magnitude of local, spatially averaged (over a small area enclosing the particles’ projected area), instantaneous, critical Shields shear-stress parameters required for incipient motion of uniform-sized sand grains, independent of the bed shear-velocity particle Reynolds number, equal to 0.16, is obtained from calibration of a theory for bed load sediment transport, by minimizing the sum of the squares of the deviations between theoretical and experimental bed load rates. Additionally, optimized expressions for a proposed probability density distribution of the bed shear stresses, for its standard deviation, for finite, maximum, and minimum bed shear stresses, and a bed load rate are obtained. In terms of the mean fluid shear stress, a dimensionless, critical, shear-stress parameter equal to 0.0513 is obtained. Investigation of the probability density distribution of the spatially varying, critical shear stresses would allow a more accurate formulation for the case of low transport rates.     

Data Interpretation for In Situ Measurements of Cohesive Sediment Erosion

Methods for interpreting data from in situ flume measurements of cohesive sediment dynamics are evaluated and a technique for estimating various erosion parameters using in situ measurements is proposed. There is currently a lack of uniformity in analysis techniques for cohesive erosion data collected in flumes and with in situ instruments and the proposed technique resolves some of these inconsistencies. The data set used in this study was derived from field experiments conducted with a straight benthic in situ flume in different aquatic environments in New Zealand. The experiments with stepwise increases in flow velocity revealed that peaks in the erosion rate at the beginning of each velocity step are most likely associated with heterogeneous bed structure, as transient hydrodynamic effects due to the experimental procedure were found to be insignificant. The field data showed an exponential decay of the erosion rate with time that is indicative of depth-limited erosion. These data are used to illustrate methods for the parameterization of the proposed semiempirical erosion equation, taking into account the time dependency of the erosion process.     

Shallow Turbulent Flows by Video Imaging Method

Shallow turbulent flows were produced in a tank of small thickness to study the friction effects on large-scale turbulent motion of small depth. The tank was constructed of two parallel walls. The space between the parallel walls (4.4, 1.57, and 0.59 cm) was small compared with the height (107 cm) and the width (212 cm) of the tank, and was varied during the experiments for different friction effects. Turbulent flows were produced by the injection of water in the form of starting jets into the tank filled with water. The large-scale turbulent flow in the small space between the walls of the tank is confined to essentially two-dimensional motion, and the motion is retarded by the force of friction. Dye was injected with the source fluid as the tracer for the highly unsteady and quasitwo-dimensional turbulent motion. From the initiation of the turbulent motion at the source to the final interaction of the jets with the tank bottom, the entire sequence of events was recorded by a pair of video cameras. The depth-averaged concentration of the dye was analyzed using the recorded video images.     

Critical Bed-Shear Stress for Cohesive Sediment Deposition under Steady Flows

Results of two laboratory experiments on the cohesive sediment deposition behavior are presented. Data indicate that an annular flume, either with or without the channel bed and the top ring rotating in opposite directions to minimize the secondary circulation, can be used to study the deposition behavior of cohesive sediments. Direct observations on when and where the bed is formed suggest that deposition only occurs when the local bed-shear stress (τb) is less than a critical value. Secondary circulation in the flume, which produced upward current near the inner corner and downward current near the outer corner, did not prevent deposition near the inner corner (because of the small τb) nor promote deposition near the outer corner (because of the relatively large τb).     

3D Numerical Simulation for Water Flows and Sediment Deposition in Dam Areas of the Three Gorges Project

This paper presents a three-dimensional (3D) mathematical model for suspended load transport in turbulent flows. Based on the stochastic theory of turbulent flow proposed by Dou, numerical schemes of Reynolds stresses for anisotropic turbulent flows are obtained. Instead of a logarithmic law, a specific wall function is used to describe the velocity profile close to wall boundaries. The equations for two-dimensional suspended load motion and sorting of bed material have been improved for a 3D case. Numerical results are in good agreement with the measured data of the Gezhouba Project. The present method has been employed to simulate sediment erosion and deposition in the vicinity of the Three Gorges Dam. The size distribution of the deposits and bed material, and flow and sediment concentration at different times and elevations, are predicted. The results agree well with the observations in physical experiments. Thus, a new method is established for 3D simulation of sediment motion in the vicinity of dams.     

Mean Flow and Turbulence around a Laboratory Spur Dike

The three-dimensional turbulent flow field around a spur dike in a plane fixed-bed laboratory open channel was studied experimentally using a microacoustic Doppler velocimeter. Mean and turbulence characteristics in all three spatial directions were evaluated at upstream and downstream cross sections near the dike. Results showed that the primary flow separated in both lateral and vertical directions. Two counter-rotating flow circulations, consisting of the lateral and vertical velocity components, originated at the dike section. Downstream of the dike, the circulation in the flow-separation zone is stronger than the one in the contracted primary flow zone. The maximum bed-shear stresses estimated using Reynolds stresses is about three times as large as the mean bed-shear stress of incoming flow.     

Using Hue, Saturation, and Value Color Space for Hydraulic Excavator Idle Time Analysis

Accurate analyses of equipment idle time are crucial for the efficient utilization of construction equipment in large construction projects. The less idle time the equipment has, the higher productivity it can achieve. However, it is not feasible for field personnel to visually observe the operation of construction equipment all day. An image processing-based methodology is presented in this paper to automatically quantify the idle time of hydraulic excavators. The image color space (hue, saturation, and value), which shows significant advantages over the red, green, and blue color space in identifying and tracing hydraulic excavators, is used as the base for image segmentation and tracing algorithms. The changing centroid coordinates of an excavator in successive images taken at constant time intervals are used as indicators of movement. Experimental results show that the presented methodology has a promising application potential for effective equipment management in construction projects.     

Experimental Investigation of the Hydraulic Erosion of Noncohesive Compacted Soils

This paper presents the results of a laboratory investigation whose purpose was to evaluate the effects of compaction on the erodibility of cohesionless soils. By means of a recently developed flume experiment, sediment erosion rates and incipient motion, as a function of shear stress, average velocity, and dry density, have been determined for three compacted sand and gravel mixtures. A preliminary comparison of the incipient motion values shows that granular soils compacted at the Proctor optimum have a higher resistance to free surface flow erosion than those compacted at lower and higher densities. This leads one to infer that the Proctor optimum, generally used as a standard for construction, might also be an optimum for hydraulic resistance and stability. Additional comparison of the experimental data with two commonly used incipient motion criteria also suggests that Yang’s criterion is a better predictor of soil detachment than the Shields-Yalin criterion.     

Formula for Sediment Transport in Rivers, Estuaries, and Coastal Waters

The aim of the present study is to develop a formula for the relationship between flow strength and sediment discharge. The appropriate definition of energy dissipation rate E in the theorem of Bagnold in 1966 is discussed and it is found that the sediment transport rate gt in unidirectional flows can be well predicted when E is defined as the product of bed shear stress τ0 and near bed velocity u*′. Then the linear relationship between u*′E and the sediment transport rate is examined using measured data. The good agreement between measured and predicted values indicates that the phenomena of sediment transport can be reasonably described by the near bed flow characteristics. As the hydrodynamic modelers are able to calculate the bed shear stress and near bed velocity in various cases now, thus the new relationship may provide numerical modelers a tool to calculate the sediment transport in rivers, estuaries and coastal waters. To prove this, the simplified analytical expressions of E and u*′ in wave-current flows and coastal waters are derived, the results are checked with the available data over a wide range of flow conditions; and good agreements are achieved, indicating that the presumption is valid in the cases investigated.