Journal of Hydraulic Engineering

A theory is developed that is shown to give good prediction of the head‐discharge relationships for two designs of vortex drop structures. The first design is the widely used spiral guide wall design, and it is shown that control is shared between the critical section at the top of the shaft and the vortex inducing asymmetry of the guide wall inlet design. A second simplified and more space saving design, called the vertical slot vortex drop structure, is also analyzed. It is shown that, although the flow conditions and analysis are quite different, the resulting head‐discharge relationship is quite similar to the spiral guide wall design. For both designs the head‐discharge relationship is shown to be almost linear and the theoretical and measured discharges are in close agreement. The proposed vertical slot design produces a stable air core, similar to the spiral guide wall design, and consequently the flow is free from risk of surging to the pipe‐full flow condition. The compactness of the new design is especially suitable for underground installations.

The explanation of the hydraulics of junction boxes is poor in comparison with the corresponding explanation of the hydraulics of pipes. The results of a detailed experimental hydraulic model study of the hydraulic interactions in two common junction box configurations are presented. For a three‐pipe configuration a recently proposed empirically based model is shown to adequately represent both the lateral and longitudinal loss coefficients in a variety of flow conditions. The results from this configuration also indicate that substantial reductions in head losses at the box are possible through the incorporation of strategically placed deflector plates in the base of the junction box. The two‐pipe configuration results indicate that the benching of the box floor leads to the improved hydraulic efficiency of the box. Circulating mechanisms in the boxes are shown to be strongly related to corresponding head losses at the boxes. Both configurations demonstrate that the influences of Froude number and box submergence are not major, but nevertheless are important in some flow conditions.

The unsteady load and vibration behavior of vertical‐lift gates was investigated for different gate‐bottom geometries and disharge conditions in an open channel and at a conduit inlet. In all cases, vibrations occurred in specific ranges of a dimensionless velocity parameter whenever the flow fluctuated between complete separation and reattachment at the gate bottom. The excitation mechanism was attributed to the combined effect of shear‐layer instabilities and motion‐induced vortices shed at the leading edge of the gate bottom. Many puzzling features of in‐flow and cross‐flow vibration of gates with flow underneath can thus be clarified. In addition, it is shown that the slope of the mean lift curve acting on the gate bottom provides an effective means of predicting with reasonable accuracy the critical range of gate openings with respect to potential gate vibration. The prediction method is illustrated in a practical case.

Eleven existing, sediment‐transport formulas were tested against the field data measured at two United States Geological Survey (USGS) gauging stations along the Sacramento River in California, whose bed‐material sizes are classified as those ranging from fine sand to coarse gravel. They include the Ackers‐White Einstein‐Brown, Engelund‐Fredsoe, Engelund‐Hansen, Inglis‐Lacey, Karim, Meyer‐Peter and Mueller, Rijin, Schoklitsch, Toffaleti, and Yang formulas. General flow and sediment input variables required by each transport function, as well as dependent variables, are summarized and detailed, and measured hydraulic and sediment quantities including bed‐material size distributions are presented. The computed sediment discharges are tabulated for comparison, and are directly compared against the measured suspended‐sediment discharges. The computed values, however, are found to deviate significantly from the measured values except for a very few cases. The test results clearly demonstrate how difficult a task it is to predict sediment discharges in natural rivers.

The essentials of hydrodynamic model application techniques and necessary supporting data are given for the situation of a partially stratified water body. The presentation is aimed at improving numerical predictions of tidal hydrodynamics, and is oriented toward somewhat experienced modelers. So as to guide the modeler, the subjects covered are arranged in the order in which they ideally would be addressed during the execution of a study. These subjects fall broadly under the headings hydrodynamic classification, model selection, model adaptation, model testing and calibration, and model verification and application. Meaningful and quantitative measures are needed to establish model performance, and to provide a basis for model intercomparisons and possible future setting of standards. Various quantitative measures are suggested. To encourage their use, it is recommended that providing a quantitative measure when comparing model data with observed data be a condition for publication. A number of research needs are also identified.

Much work is done to understand the mixing processes at a density interface. Laboratory studies commonly rely on experiments where entrainment is driven by either a mean shear or by a vertically oscillating grid. Although these experiments provide useful results, it is suggested that neither of these setups is completely representative of conditions in natural water bodies. The present study investigates a more general situation where the relative magnitudes of mixing generated by these two sources of energy are varied in order to better understand the differences in entrainment results reported for the two different kinds of experiments. A series of experiments is performed and a semiempirical model is developed to predict the entrainment measured in these tests. The model is based primarily on energy considerations and accounts directly for the mixing energy introduced by each of the mechanisms under consideration. The model agrees well with the data and also with data from a previous wind‐mixing experiment. Finally, application of the results to general entrainment modeling is discussed.

The power model is widely used in engineering as the structure for empirical models. The coefficients are fitted using a logarithmic transformation of the data. The logarithmic transformation leads to a biased model, which is not usually corrected for. Even when the traditional approach to eliminating the bias is used, only the intercept coefficient is changed; the other coefficients are not corrected, so they remain biased estimators. A numerical method for fitting the coefficients of the power model is discussed; the method enables the coefficients to be fit so they provide unbiased estimates and a minimum‐error variance in the y‐space, rather than the log y‐space. The numerical method is easily modified to fit the coefficients using an objective function based on the relative errors. Examples using actual engineering data are provided.

The Italian Hydrographic Service publishes yearly maximum rainfall depths with 1-, 3-, 6-, 12-, and 24-hr duration, recorded by recording rain gauges. It also publishes selected short-duration ($t<1$ hr) rainfall amounts. It is impossible to carry out a statistical analysis of the short-duration rainfalls for each recording rain gauge because the published data correspond to different record durations and to different recording rain gauges from year to year. Therefore, there is a practical need to derive empirical relationships between the 1-, 3-, 6-, 12-, and 24-hr duration amounts and shorter duration ($t<1$ hr) rainfall amounts. Such a relationship has been proposed by Bell and has been verified using rainfall data in the United States, South Africa, and Australia. In this technical note we study the applicability of Bell's rainfall-duration relationship in Sicily and Sardinia. Then, using mean values of the ratio between the depth of rainfall occurring in t min and that occurring in 60 min, we obtain, for each island, an exponential relationship between this ratio and duration t. The two exponential relationships give values quasi-equal to those calculated by Bell's relationship. This result seems to confirm the independence of short-duration rainfalls of geographical factors.

The necessary condition for, and the process of, evolution of a coarse layer over finer sediment underneath are presented. Flow regions for pavement and armor formation are identified. A new definition for armor and pavement is proposed. The relevation of the two flow regions contributes towards an improved understanding of the distinction between armor and pavement. With increasing shear velocity, the surface layer coarsens in the armor region and becomes smoother in the pavement region. The eroded material (EM) in the armor region becomes coarser with increasing shear velocity but remains finer than the parent-bed material (PBM). In the pavement evolution region, the EM is finer than PBM; and in the equilibrium pavement region, the composition of EM is identical to that of the PBM. The available experimental data are in agreement with the proposed hypothesis of evolution of the coarse surface layer.

Some recent investigations on outflowing jets indicate an increased interest in phenomena studied for the first time some centuries ago. The highly spatial flow character and the beauty of jets issued by vertical, rectangular orifices are herein documented by photographs. The main experimental observations are discussed, and the significant differences between jets with horizontal and vertical main jet axes are outlined. Effects of gravity, viscosity and surface tension on the jet flow are described.

Junctions are of considerable importance in the study of open‐channel flows. For dividing flow at right‐angled junctions of rectangular open channels, an estimate of the discharge ratio is obtained in terms of the Froude numbers in the main channel upstream and downstream of the junction. Experimental data is provided to validate the proposed model.