Journal of Hydraulic Engineering

In 1983, Stedinger published a method, based on the noncentral t‐ distribution, for constructing approximate confidence intervals for quantiles of a Pearson type‐3 (P3) distribution when the coefficient of skewness is known. That method is extended to the case when the coefficient of skewness is estimated by either (1) The at‐site sample skew; (2) a generalized regional‐average skew; or (3) a weighted average skew. The confidence intervals perform satisfactorily in Monte Carlo simulations. Confidence intervals generated using a three‐parameter asymptotic method and the Water Resources Council method, which ignores uncertainty in the weighted skew, perform poorly. The actual confidence of confidence intervals for a design flood can be much lower than the target if uncertainty in the skewness coefficient is ignored. It is important to distinguish between the sampling variability in the sample skew coefficient at the site and the likely variation of the true skews in the region when estimating the precision of a generalized skewness coefficient for the region. Five approximations of frequency factors for the P3 distribution are also evaluated.

The multidimensional precipitation model proposed by Waymire et al. provides realistic representations of the rainfall process. The model, however, in its most simplified form requires the numerical evaluation of nine parameters. Islam et al. developed a parameter estimation procedure and they applied the methodology to a small experimental basin with a dense network of rain gauges. In this paper the same estimation procedure is attempted over a larger area where only a sparse rain‐gauge network is available. Parameter estimates are determined for cyclonic‐midlatitude and convective storms, which are characteristic of the winter and summer rain seasons over the test basin. Using the seasonal storm parameter estimates, multidimensional model storms are generated over the Brazos Valley in Texas, and the storms are assessed qualitatively.

A computer model RICE is developed for simulating ice processes in rivers. In the river‐hydraulics component, the flow condition is determined from one‐dimensional unsteady flow equations. In the thermal component, distributions of water temperature and ice concentration are determined from transport equations of thermal energy and ice. Effects of surface ice, skim‐ice, and border ice formations on the ice production are considered. The formation of ice cover is formulated according to existing equilibrium ice‐jam theories, with consideration to the interaction between the ice cover and the flow. The undercover ice accumulation is formulated according to the critical velocity criteria. The thermal growth and decay of the ice cover is simulated using a finite‐difference formulation applicable to composite ice covers consisting of snow, ice, and frazil layers.

The predictive capability of the sediment‐transport formulas of Ackers‐White, Engelund‐Hansen, and van Rijn at high flow velocities is investigated. Total‐load transport at high velocities over a fine‐sand bed (100‐400 μm) is studied by analyzing data from the literature and data from new flume and field measurements. Flume experiments were performed to measure flow‐velocity and sand‐concentration profiles in a self‐eroding flow initially free of sediment. The flow velocities were up to 2.7 m/s. A two‐dimensional vertical mathematical model for suspended sediment was applied to compute the equilibrium transport rates. Finally, predicted transport rates are compared with measured transport rates under flume and field conditions. The formulas of Engelund‐Hansen and van Rijn predict transport rates that are in reasonable agreement with the measurements.

At most bridges, wake vortex scour is insignificant and confluence scour does not exist. In clear‐water rivers flowing on fine sand, these two forms of local scour can be very large. The deep scour holes downstream from the large circular piers (10 and 15 m in diameter) at the Tahrir and Imbaba Bridges over the Nile River in Cairo, Egypt were produced by the conflicting velocity fields at the intersection of the wake vortex streams from adjacent piers. The depths of scour due to these wake vortices are now 8 to 11 m where the normal depth of flow is approximately 8 m. Confluence scour where the main and side channel join upstream from the Imbaba Bridge was 9 m in 1981 and 1987 and was aligned with the bisector of the intersection angle of the two channels. These depths of scour can be considered nearly clear‐water scour as the bridges are in the backwater of the Delta Barrages, the velocity is less than 1 m/s, and sediment transport is very low.

The transition from supercritical to subcritical flow in sloping channels is systematically investigated under a wide range of slopes. The hydraulic conditions for the formation of D‐jump, where the entire jump is formed on the slope, and B‐jump, where the jump is formed partly in the sloping portion and partly in the horizontal channel portion, are determined. Also, practical equations for the sequent depths and the length of the jumps are proposed. For the streamwise decay of the maximum velocity in the transition zone, the analogy with the wall jet is discussed.

Experiments are conducted on circular, vertical intakes drawing from two-layer stratified sources. Arguments are presented to suggest that current definitions of critical drawdown, as the condition under which the upper layer is on the point of being drawn into the intake, are unnecessarily conservative. Instead it is suggested that 1% mixing is acceptable. Experimental results for capped and uncapped intakes are presented to compare these two definitions of critical conditions. It is shown that significant differences in acceptable flows or depths may occur depending on which definition is used.

The starting point to determine optimal well design and operation should be that which maximizes net present worth. Optimal design is important because it maximizes the potential of the well; however, optimal operation of the well is also important and is the subject of this paper. Recent publications and ``rules of thumb'' still suggest operating wells based on noneconomic criteria. The purpose of this paper is to examine one of the more popular ``rules of thumb'' and show why following it can produce water at nonoptimal costs.