Journal of Hydraulic Engineering

Field data obtained from Oak Creek are used to study the bedload transport in gravel‐bed streams. A similarity approach is used to delineate a functional relationship obtained for the bedload transport rate based on a dimensional analysis reasoning. A bedload relation, which allows for sediment grading effects and is valid for low Shields stresses $\left({\varphi }_{50}<1.4\right)\text{,}$ is developed. A new bedload formula valid for the whole range of Shields stresses is proposed. This new formula is used to predict the possible variation of the median size of the surface layer and bedload material with Shields stress. The expression for the reduced hiding function obtained here suggests a dependence on Shields stress in addition to its dependence on grain size $(D_i/D_{50})\text{.}$

Lake Anna, a reservoir of 3,900 ha and 21 m maximum depth, provides cooling water for the two‐unit North Anna Nuclear Power Station in Virginia. Using the flux‐gradient method, vertical diffusivities in the lower layers of Lake Anna are computed from seven years of temperature data dating back to before plant operation. Results indicate strong dependence of vertical mixing on condenser flow rate with average summertime diffusivities below the surface layer ranging from $0.46-0.68m^2/\mathrm{day}$ for conditions of two‐unit operation and from $0.06-0.14m^2/\mathrm{day}$ for conditions of zero to one unit operation. Vertical diffusion is parameterized as a calibrated function of condenser flow rate, wind speed, and a characteristic vertical density difference. A numerical model employing this parameterization gives good agreement between measured and predicted vertical temperature profiles during separate validation tests.

The motion of sediment in oscillatory flow over a flat bed is investigated both theoretically and experimentally. The theory makes use of results for the relationship between stress and strain in steady flow. The experiments were performed in two different oscillatory flow water tunnels with two different sediments. They were designed to measure the variation of velocity with height in the moving layer of sediment under conditions of intense sediment transport. Although certainly not perfect, the agreement between theory and experiment is reasonable, provided the stress ratio is chosen correctly. Calculated values of the mean sediment transport rate during a half‐cycle are compared with the predictions of currently available formulas.

An extensive series of experiments on the characteristics of inclined and vertical dense jets discharged into a uniform crossflow of various speeds and directions is reported. The inclined jets were maintained at 60° to the horizontal and the results for terminal rise height, and dilutions at the terminal rise height and impact points are compared to those for vertical jets. For discharges into stagnant ambients, it is found that the effect of source volume flux should not be neglected for jet Froude numbers less than about 25. Empirical equations to predict dilution and rise height based on dimensional and length scale arguments are presented. The dilution of an inclined jet increases as the angle to the current increases. Dilutions for inclined jets discharging into the crossflow are lower than for a vertical jet and dilutions for discharges with the crossflow are generally higher. Applications to design are discussed, and it is found that the inclined jet is generally preferable to the vertical jet. This is because of the lower rise height of the inclined jet, the much higher dilution under stagnant conditions, and the horizontal momentum given to the wastefield.

A method of analysis of tidal flap doors under oscillatory flow conditions is presented. Hydrodynamic data for flap doors is given for use in numerical simulation of door movement. Application of the data is illustrated by considering the case of a pumping station overflow that includes twin tidal flap doors. Boundary conditions for the numerical model are the inflow to the pumping station and the time history of pressure at the outfall determined from wave‐climate measurements offshore. Computations were undertaken with a threefold purpose: to establish peak water levels in the pumping station for selection of safe machinery levels; to determine the closure rate of flap doors to assess risk of slamming; and to quantify airflow rates to and from access chambers to provide adequate venting arrangements.

A nonlinear parabolic model for aggradation processes in alluvial rivers or channels is presented. A general exponential formula for the sediment transport relation is postulated to account for the nonequilibrium process. An indirect method for evaluating the coefficients of the sediment transport relation is described. The technique is illustrated by numerical experiment and comparison with data. Analytical expressions obtained from perturbation solutions of the characteristic parameters relevant to the aggradation process are derived. The validity of the model is assessed by comparing the analytical and numerical results with available experimental data. Good agreement is obtained.

The concept of critical stress for the initial motion of noncohesive sediment beds under turbulent flow conditions is reviewed. Observational definitions of incipient motion are many and not entirely compatible. Some laboratory flume observations of sediment movement suggest that no true threshold exists. Current understanding of turbulent fluid motion at a sediment bed suggests that some particle movement must occur at all nonzero time‐mean velocities. A combined model for flume flow and sediment transport having no threshold is seen to explain features of data that have been previously used to support the threshold concept.

This work lists important references on the issue of critical stress for incipient motion and erosion, principally for noncohesive, abiotic sediments. It emphasizes publications subsequent to the bibliography compiled in 1966 by the Committee on Sediment Transport Mechanics. These references refer to the paper ``Do Critical Stresses for Incipient Motion and Erosion Really Exist?'' by J. William Lavelle and Harold O. Mofjeld, Journal of Hydraulic Engineering, March, 1987.

A flume study is conducted to determine culvert shape influence on the scour geometry at culvert outlets. Twenty-six experiments are performed in which flows discharged for 316 minutes through circular, arch, square and rectangular shaped culverts and the resulting scour holes are documented. The scour hole dimensions for the arch, square and rectangular shaped culverts are normalized and compared to the scour hole dimensions for the circular culvert. Culvert shape significantly influenced the scour dimensions compared to the scour dimensions determined using the circular shape culvert. A series of expressions are developed correlating the dimensionless scour hole parameters of depth, width, length and volume to the modified discharge intensity. The denominator of the modified discharge intensity parameter, termed the culvert shape factor, compensates for shape differences for comparison.