Journal of Hydraulic Engineering

Pit 6 Dam, located on the Pit River in northern California, was constructed by Pacific Gas and Electric Company in San Francisco in 1965 for hydroelectric power generation. The gross head of the hydroelectric installation is 48.5 m for a rated generating capacity of 89 MW. Flow over the ogee spillway is regulated by two radial gates, and it discharges into a 33.5-m-wide and 19.5-m-long stilling basin. Since completion of the dam, there have been chronic problems with the floor blocks, some of which were dislodged more than once. To elucidate the primary cause of floor block failures and develop a new type of block, a 1:28-scale undistorted hydraulic model was constructed, and direct measurements of the longitudinal and lateral components of the forces acting on the model floor blocks were taken using a load cell. Extremely large fluctuating loads acting on the existing blocks in the lateral direction were found to be the primary cause of the block failures. Triangular hollow blocks, which substantially reduce the lateral loads, were then developed using the model.

This paper presents a practical method to predict depth-averaged velocity and shear stress for straight and meandering overbank flows. An analytical solution to the depth-integrated turbulent form of the Navier-Stokes equation is presented that includes lateral shear and secondary flows in addition to bed friction. The novelty of the present approach is not only its inclusion of the secondary flows in the formulation but also its applicability to straight and meandering channels. The analytical solution is applied to a number of channels, at model and field scales, and is also compared with other available methods such as that of Shiono and Knight and the lateral distribution method. The present formulation gives much better predictions of velocity and shear stress, particularly in those cases where the secondary flows are dominant.

A coupled ground-water/channel flow distributed model has been developed for continuous simulation in a 123-km² basin. The aim was to analyze the streamflow generation processes in natural vegetated environments. Finite-difference schemes have been used to solve conservation equations of the 2D saturated subsurface flow and the 1D kinematic surface flow. Because of the high hydraulic conductivity of the surface soil, only the saturation excess mechanism of runoff production has been considered. Parameter sensitivity analysis showed the overriding influence of soil storage capacity and conductivity. A grid discretization >100 m produces a hydraulic conductivity greater than physically meaningful, which considerably increases as the space-grid step increases. Results indicate that the model can satisfactorily simulate the water-flow behavior of the catchment after fitting the three parameters of surface hydraulic conductivity, effective porosity, and evapotranspiration losses. These are done after calculating the conductivity as a function of the height of the water table. The simulation efficiency has varied from 87% in the first 5-year calibration period to 85.8% in the subsequent 5-year validation period.

Characteristics of turbulence structure in quasi-2D flows with static and weakly mobile gravel beds are presented. Three sets of measurements with acoustic Doppler velocimeters in an irrigation canal were used: two with subcritical bed shear stress (static beds) and one with the bed shear stress τ_o close to critical τ_oc (weakly mobile bed). The analyses included vertical distributions of local mean velocities, turbulence intensities, turbulent shear stresses, velocity auto- and cross-spectra, the quadrant method, and high-order velocity moments. A number of properties of turbulence intensities, high-order moments, streamwise bursting parameters, and velocity spectra appeared to be similar for all three flows, but some properties were different. The most important one was an observed reduction in the von Kármán constant for the flow with weakly mobile bed. Comparison of these results with other studies and analogies with drag-reducing flows suggest that at τ_o/τ_oc ≈ 1 the drag on the bed for a given granular material should be minimized.

Particle saltating motions during bed load transport are dominated by the forces acting upon particles and the random process of the particles impacting on and rebounding from the channel bed. A real-time flow visualization technique is developed in this study to measure particle saltating trajectories, corresponding velocities, and impacting and rebounding angles. Based on the experimental data, regression equations for the dimensionless saltating length, height, and velocity were obtained. A numerical model that is able to simulate the continuous saltating process of a single particle was developed. The model was calibrated and verified with the experimental data, and the results were satisfactory. The model was also used to generate a series of synthetic data. Based on these data and the flume data collected by previous investigators, a bed load equation was derived.

Five versions of low Reynolds number k-ε models including the original one have been tested against the DNS data for 1D oscillatory boundary layers; one with sinusoidal and another with flat-crested free-stream velocity variation. The comparison has been made for the cross-stream velocity, turbulent kinetic energy, Reynolds stress, and wall shear stress. It is found that the original model is superior to the more recent models by virtue of its numerically desirable boundary conditions and accuracy of prediction for mean flow properties. A brief description about the wall-limiting behavior of important turbulent quantities is presented to recognize physically correct k-ε models.

This technical note discusses two questions related to three-tube (pitch or yaw) pitot probes, which are used for measuring the velocity vectors in 2D flows. When a pitch probe is used in flows in which the third component of the velocity is small but not zero, it is important to know the error this causes in the measurement of the velocity vector. Calibration experiments on a pitch probe showed that as long as the yaw angle is less than 14°, the relative error in the velocity measured by the pitch probe is less than 3% and the error in the pitch angle is less than 1°. Typically a pitch or yaw probe can only be used when the angle of attack is less than approximately 50°. The second part of this work addresses the practical issues associated with pretilting the probe to measure flows at angles greater than 50°.