Journal of Hydraulic Engineering

Engineers and scientists must be prepared to meet challenges by developing techniques necessary to address the demands of society. In addition to the application of engineering principles, the ability to consider numerous complexities involving geomorphology, biology, environmental issues, economics, and the possibility of hearings and litigation both pre‐ and postproject, are increasingly important. Crucial targets include the safety, life, and rehabilitation of dams and reservoirs; the relicensing and expansion of hydropower; the maintenance, upgrading, and improvement of our transportation system with particular reference to waterways and bridges; meeting future demands for river transportation; formulating improved flood‐control measures; and meeting the present and future needs in terms of water supply and water quality. Challenges to the profession are as follows. The water‐related engineer must become better educated in terms of the physical responses of systems to development, as well as in the assessment of associated multidisciplinary impacts. Finally, it will be increasingly important that engineers and scientists be able to judge, based upon interaction, experience, and knowledge of the physical processes, the utility of mathematical models.

Analytical methods are used to obtain criteria for wave‐motion stability in canals with automatic feedback controllers. With this purpose the Saint‐Venant wave‐motion differential equations are linearized, as well as the equation of the discharge through check gates of the canal and the boundary conditions determined by the selected control method. The analysis is done for the COPI‐BIVAL water level automatic control method, which is more general than the BIVAL water‐level control method, well known in France. According to the Nyquist criterion a characteristic function F(s) of the closed‐loop system is defined, using the Laplace transform of the governing wave‐motion equations. The criterion of the wave‐motion stability is obtained for the canal with one and more poles, taking into account the influence of the friction forces. The authors also present the study results of the wave‐motion stability in canals obtained by numerical simulation, using the nonlinear equations. A comparison between the numerical simulation results and the analytical Nyquist criterion shows good agreement.

When a problem of hydraulic transients with entrapped air in pipelines and/or with water‐column separation is tackled, the variation of the length of the liquid column is not usually taken into account. In this paper (due largely to the assumption of a system with a cross section at the air‐water interface in a horizontal pipeline), it is shown that in a system with entrapped air and without changes of potential energy at the air‐water interface (horizontal pipe) the final pressure obtained in a mass oscillation in which the kinetic energy of the fluid and the liquid column length variation are neglected is the same as the final pressure obtained from the consideration of both effects. This peak pressure is also independent of the assumed polytropic exponent for the entrapped‐air compression. Nevertheless, the instant when the maximum is achieved is different.

The effects of sediment size gradation ${\sigma }_i$ and sediment loading ratio m on the aggradation‐degradation process and related flow characteristics in an alluvial channel were investigated experimentally. Three different size gradations of sediment were used. For each gradation, experiments were carried out for four different sediment loading conditions, namely, initial equilibrium, overloading, underloading, and clear water. Hydraulic characteristics, sediment transport rates, and gradations were measured during experiments. A linear model of sediment transport under overloading and underloading conditions was developed. Experimental results show that with nonuniform sediment, the aggradation‐degradation cycle is found to be irreversible. Under a given loading ratio, speeds of the aggradation wave and degradation wave increase with increasing ${\sigma }_i$ and with decreasing m. The recovery ratios of bed elevation and bed slope decrease as ${\sigma }_i$ increases. Regression relations have also been derived from the experimental data obtained.

For rivers in flood and beaches under wave attack, the shear stress at the top of the stationary granular bed can set up a sheared layer of contact‐load (bed‐load) particles. This type of motion also occurs in pressurized ponduits, where more accurate observations can be obtained. For high shear stress $\left(Y>0.8\right)$ the shear layer has a thickness of several grain diameters. Experimentation and analysis demonstrated that mobile beds at high shear stress obey neither the smooth‐boundary nor the rough‐boundary law, but follow a different friction relation of their own, which does not depend on particle size. This analysis has now been significantly extended using a curvilinear concentration profile based on previous experiments. This profile is now applied to the evaluation of boundary friction and the transport rate of contact‐load solids. The present analysis provides the theoretical background to explain both existing experimental results and new data presented here.

A new total sediment-load sampler is designed and tested in a flume. The new instrument consists of a bed-load sampler and a suspended-load sampler, both attached to a supporting frame. The suspended-load sampler consists of seven intake nozzles, which are connected to plastic hoses and operated by pumps. The bed-load sampler consists of a nozzle connected to a 150 μm or 240 μm-nylon bag. At the upper side of the bag, a 500 μ nylon patch (0.1×0.15 m²) is present to reduce the blocking effect by fine particles. The bed load sampler is connected to a swing arm, which can move upward and downward to allow good contact between the sampler nozzle and the bed. The hydraulic coefficient of the bed-load sampler is found to be very close to unity for filling percentages of the bag up to 50%. The sampling efficiency of the bed-load sampler is found to be somewhat larger than unity (1.2 to 1.4) at low transport rates (velocity smaller than 0.5 m/s).

A physical model of an urbanized storm-drainage conveyance system was conducted to identify the location and nature of channel overtopping. The drainage system is characterized by supercritical flow (F = 1.30 to 1.76) and a heavy debris load. A trash rack became the study focus. Trash rack orientations of 1:2 and 3:1 slopes were tested and evaluated for self-cleaning operations. The 1:2 slope rack tended to clog, while the 3:1 slope rack exhibited self-cleaning tendencies in supercritical-flow conditions. A blockage analysis was conducted, indicating that the 3:1 slope rack would cause localized flooding with only 41% blockage. Further, blockage near the bottom of the channel (nonfloating debris) resulted in more severe localized flooding than blockage placed near the water surface (floating debris). The present study suggests that the traditional procedure for evaluating system-design adequacy by assuming 50% blockage of the trash rack is not necessarily conservative for supercritical applications. It is recommended that a 40% blockage be assumed for trash-rack analysis in supercritical flow.