Journal of Hydraulic Engineering

Studies were conducted with a two-dimensional 1:20 scale model of a roller compacted concrete stepped spillway to evaluate the flow transition from the level spillway crest to the sloping steps, the energy dissipation of the steps, and performance of the proposed stilling basin. The steps were effective in dissipating energy. At maximum discharge, 14.5 m ³ /s/m (156 ft ³ /s/ft), the energy dissipation with the steps was 48% compared with an energy dissipation of 20% for the spillway with a smooth chute surface. At 5.81 m ³ /s/m (62.5 ft ³ /s/ft), the energy dissipation with the steps was 71% compared with 25% with a smooth chute surface. The energy dissipation caused by the steps significantly reduces the size of stilling basin required at the toe of the spillway compared to a conventional chute spillway.

A new mechanical device consisting of a set of safety membranes has been used in several small hydropower stations in China in lieu of a surge tank. Safety membranes are controlled weak points mounted on the penstock near the powerhouse. Whenever pressure on the membranes rises to the preset value, one or more membranes rupture, protecting the penstock and the unit effectively. The device is simple, reliable, and beneficial. This paper proposes a mathematical model for analyzing hydraulic transients of a hydropower plant where a set of safety membranes is mounted to replace an expensive surge tank. The prevailing method is employed in order to solve the momentum and continuity equations describing transient flow in closed conduits. An optimization procedure is developed to guarantee the convergence of the interative process for calculating pressure rises due to water hammer when Kaplan turbines are installed. Using this model, transients of generator load rejections are calculated in Linzhengqu hydropower station (Hebei Province, China). Computed results agree well with those of field tests.

The results of a series of 47 bed development experiments are combined with appropriate experimental results of other authors. The generation of initial bed waves, termed sand-wavelets, on a plane sand bed is found to be instigated by the occurrence of a random pileup of sediment, this pileup subsequently growing as further sediment is trapped by the pileup. When the height of the pileup approaches the bed roughness height, further pileups are generated at preferred spacings downstream. The wavelength l of the sand-wavelets so generated is relatively insensitive to the applied bed shear stress and primarily a function of the size d _g of the sediment. This wavelength can be described by the empirical formula $(l/d_g)R_{*c}^{0.2}$= 10 ^2.5. From a theoretical viewpoint, this wavelength is approximately described by a reassessment of the flow system instability analyses of Richards. Both ripples and dunes are found to develop from the sand-wavelets.

The relative magnitudes of the celerities at which disturbances on the water surface and the bed are propagated play a key role in the mathematical modeling of mobile bed flow. Earlier analyses assumed that the solids concentration of the flow was negligible. This assumption is inappropriate both for some natural streams and for many aqueous mine-waste disposal applications. Here, the relative celerities of mobile bed flow are determined for flows with finite solids concentrations. The solids concentration is assumed to be a function of the velocity and depth of flow. The analysis confirms and extends the results of earlier analyses. It is shown that the movement of the sediment and the water can be considered to be mathematically independent of each other only within very limited ranges of solids concentration and Froude number. This severely constrains the application of numerous existing mathematical models to mobile bed flows with relatively high solids concentrations.

A unique flume that measures the erosion of sediments at high shear stresses and with sediment depth has been designed, constructed, and used. Experiments in this flume, Sedflume, can be done either with reconstructed sediments or with relatively undisturbed sediments from the field. With either kind of sediment, two different types of experiments can be conducted. The first type determines the critical stress for erosion of the sediment as a function of depth, while the second type determines the erosion rate of the sediment as a function of both shear stress and depth. Measurements of the erosion rates of sediments from the Trenton Channel of the Detroit River in Michigan and the Lower Fox Fiver in Wisconsin were made by use of Sedflume at sediment depths up to 2 m. Representative results of these measurements are presented here. These results demonstrate the capabilities of Sedflume and also illustrate the large differences in erosion rates at different sites and the large variations in erosion rates with depth and as a function of shear stress.

Alluvial fans along mountain bases pose quite interesting problems for the design of hydraulic structures and highway crossings and for flood insurance studies. The Federal Emergency Management Agency (FEMA) alluvial-fan method is subject to uncertainties because it is an approximate method. In this paper, Rosenblueth's point-estimate is applied to the FEMA alluvial fan method to compute the mean and standard deviation for the 100-yr discharge at any point on the fan and the mean and standard deviation for the fan arc width. The mean and standard deviation for the 100-yr discharge are used to obtain the risk that the 100-yr discharge will exceed the discharge capacity of hydraulic structures on the fan. The mean and standard deviation for the fan arc width are used to estimate the risk that a given location on the fan is within the hazard flood zone. The HEC-1 rainfall-runoff computer model is used to compute inputs to the FEMA method. The proposed uncertainty and risk analyses are applied to an alluvial fan in north Scottsdale, Ariz.

River deltas are characterized by a very complicated and unsteady water regime. They are affected by changes in the river regime at the upper boundary of the delta, changes in the sea level at its lower boundary, and by hydrological and morphological processes within the delta itself. The most significant indicators of all processes affecting the water regime in the delta are changes in water discharges and water levels at various points of the delta hydrographic network as a function of water discharge in the delta head. These dependences are taken as a basis for hydrologic calculations in river deltas. This paper presents a physicostatistical method for plotting, assessing accuracy, and establishing the dynamics of water discharges and water levels in delta streams as a function of water discharge in the delta head. The automated technique allows for an operative solution of hydrologic tasks in river mouths. Recommendations to optimize hydrometric works in river deltas are suggested, which would take into account time-specific changes in delta hydromorphology and delta monitoring tasks.

Flow mechanisms causing salinity changes during various pumping conditions in a two-layer aquifer are presented and discussed on the basis of empirical observations. Theoretical models explaining the basic features of the observed behavior are proposed and applied in order to reproduce some of the observed phenomena. The models are based on simple assumptions regarding ground-water flow and mixing of the fresh and saline water. Qualitative predictions of the models proposed are acceptable. The models explain such phenomena as the following: (1) increase of salinity within the well during recovery; (2) initial increase of chlorides content after reintroduction of pumping and subsequent reduction of salinity; and (3) influence of the rate of pumping on the rate of chlorides content's reduction. The models proposed are “low-resolution” models that take into account only the basic features of phenomena observed.

Accurate calculation of horizontal diffusive transport is important in modeling temperature, salinity, sediment, and pollutant transport in estuaries and the coastal ocean. Many three-dimensional estuarine and coastal ocean models employ a so-called sigma coordinate transformation because of its useful applications with irregular bathymetry. However, for steep bottom slopes, the conventional definition of horizontal diffusion causes spurious diffusive transport. In this paper, a new formula is presented and tested for salinity diffusion in a quiescent-sloping basin under no external forces. Results indicate that this formula produces almost no spurious diffusion, so that the initial salinity field remains unchanged after a 30-d simulation.

Submerged vanes are shown to be effective in preventing bed load transport from entering water intakes. By generating a secondary circulation in the flow, the vanes change the magnitude and direction of the bed-shear stresses and cause a redistribution of the flow and sediment transport in the area affected by the vanes. As a result, the riverbed aggrades in one portion of the channel and degrades in another. The performance of two vane installations, designed according to guidelines developed earlier, are evaluated by comparing the bed topography before and after vanes were installed. The design guidelines, which utilize a numerical model developed earlier, are shown to be appropriate. The guidelines apply only when the intake flow is small enough that the withdrawal causes little change in the river flow velocity in front of the intake.

The Lacey, Chang, and Lacey-divergence formulas are chosen to represent different empirical, rational, and mixed approaches available for the design of alluvial canals. Lacey formulas are applicable for specific values of sediment concentration C and resistance of bank material to erosion τ_s, whereas Chang formulas are applicable for all C values but for a specific value of τ_s. Lacey-divergence formulas are of general application for all practical values of discharge Q , bed material size d₅₀ and τ_s. In the downstream direction along a canal system, the three operative conditions are (1) interdependence between P , R , and S , as indicated by the Lacey-divergence equation; (2) constancy of sediment concentration; and (3) minimum consumption of stream power. These three conditions remain the same in all canal systems, and, hence, in the exponential formulas for width, depth, and slope in terms of discharge, the indices 0.5, 0.33, and −0.167 remain the same; however, the coefficients change depending on the sediment concentration and strength of bank material, which vary from one canal system to another.