Journal of Hydraulic Engineering

This paper describes the influences of floating level ice covers on flow and bed-load transport in alluvial channels and proposes a semi-empirical procedure for calculating flow resistance in such channels. The procedure can be applied in conjunction with several existing methods for estimating flow resistance in alluvial channels under open-water conditions. Previously, cover influence was poorly understood, with the consequence that flow-resistance calculations for ice-covered alluvial channels entailed an inordinate amount of guesswork. The cover influences described herein were determined from the results of experiments conducted with a laboratory flume. The main findings of the flume experiments are that ice covers increase flow depth, decrease bulk flow velocity, increase dune length, and significantly decrease bed-load transport rate. Additionally, it was found that sediment transport relationships developed for open-water flow are applicable to ice-covered flow provided they are used in conjunction with estimates of actual bed shear stress. The proposed flow-resistance procedure enables such estimates to be made.

Most existing sediment routing models are one-dimensional models and hence they cannot be used to simulate lateral channel bed deformations. Conversely, a two-dimensional model is computationally time consuming, and is not suitable for long-term simulation. Using the stream tube concept, a one-dimensional model is extended to a quasi–two-dimensional model in this study. It is capable of simulating lateral variations of channel cross section through the adjustments of stream tube boundaries. The model has an option to treat the suspended load and bed load separately and hence is able to simulate the deposition behavior of the suspended sediment under a nonequilibrium process. The model also provides options to solve either de Saint Venant equation or energy equation and hence can be applied in both steady and unsteady flow conditions. An assessment of this model's performance has been conducted through a comparison to an analytical solution and a set of experimental data. The application of this model to the Keelung River and the Shiemen Reservoir in Taiwan also gave convincing results.

A stochastic theory is developed for longitudinal dispersion in natural streams. Irregular variations in river width and bed elevation are conveniently represented as one-dimensional random fields. Longitudinal solute migration is described by a one-dimensional stochastic solute transport equation. When boundary variations are small and statistically homogeneous, the stochastic transport equation is solved in closed-form using a stochastic spectral technique. The results show that large scale longitudinal transport can be represented as a gradient dispersion process described by an effective longitudinal dispersion coefficient. The effective coefficient reflects longitudinal mixing due to flow variation both within the river cross section and along the flow and can be considerably greater than that of corresponding uniform channels. The discrepancy between uniform channels and natural rivers increases as the variances of river width and bed elevation increase, especially when the mean flow Froude number is high.

To predict mudflow dynamics a numerical model based on shallow-water equations is developed. Previous work has shown that mudflow behavior can be well represented by a Herschel-Bulkley model. In a laminar regime, generally observed in practice, a wall friction force expression has been theoretically deduced for steady uniform flows. For unsteady flows the wall friction force is assumed to be equal to the resistance of a steady uniform flow with the same depth and mean velocity. The numerical model parameters, directly deduced from Herschel-Bulkley model, are determined independently by rheological measurements. Because of the lack of precise field data, experimental results obtained on a physical model are used in order to validate the numerical model. The main features of the experimental transient flows carried out in a laboratory flume are, for some identified conditions, in fairly good agreement with numerical model predictions, without any additional parameter fitting.

This paper presents the results of an extensive laboratory study aimed at improving the design of Denil fishways. For the standard design of the simple Denil, an equation has been developed between the dimensionless discharge Q* and the relative depth of flow d/b for d/b as large as 5.5. The normalized velocity distributions in the centerplane of the Denil were found to have certain shapes depending upon the d/b ratio. For the nonstandard designs of the Denil fishway, based on the results of about 660 experiments, a method has been found to predict not only the relation between Q* and d/b but also the normalized velocity profiles in the centerplane of the Denil. The coefficient of friction between the central stream in the Denil and the circulating water on the sides as well as the bottom has been evaluated along with the equivalent Manning's n for the Denil fishway. These results are believed to be important in extending the depth range of the standard Denils as well as making changes to the standard Denil for passing different species of fish.

This paper presents the results of an exploratory laboratory study on the hydraulics of fish resting pools that are built between two Denil fishways, making a full turn or arranged in a folded-back pattern. These experiments show that the flow from the Denil entering the pool diffuses as a surface jet, with an increased growth rate, possibly because of the circulation and turbulence in the pool. This diffusing jet impinges on the backwall and dives into the pool. The flow formation in the vicinity of the outflowing Denil appears to occur in a relatively small region. The energy dissipation in the pool is significant. To provide some resting areas for fish ascending the multiple Denils, it is necessary to provide some depth below the common invert of the two Denils. Some suggestions have been made for determining the size of these resting pools.

Numerical investigations of flow over side weirs in U-shaped channel are reported. Numerical models are obtained from energy principles and solved by a finite-difference method. The results are presented in graphical form. The study covers suband supercritical flow regimes. Derived expressions for the side weir discharge and water surface profiles in U-shaped channels for these regimes are compared with experimental results. Necessary charts are presented for practical engineering use.

Alluvial channels readily adjust their morphology, hydraulics, and sedimentology in response to disturbances. Knowledge of the spatial and temporal trends of channel adjustment is central to the protection and maintenance of bridges. In this study the reliability of a bridge during the process of erosive channel adjustments following channel modifications is quantified. An interdisciplinary approach is used to determine the stage of channel evolution following the disturbance and the dominant processes during each stage, and to compute the probability of bridge failure or reliability. An actual case study in west Tennessee is provided. Information from this technical note can be used to determine the effect of channel modifications on highway structures and the vulnerability of bridges to erosional processes over the life of the bridge, and to assess maintenance and mitigation needs.

The Preissmann scheme, often referred to as the four-point scheme, is a bidiagonal implicit finite-difference method for solution of the de St. Venant equations. It is unconditionally stable and extremely robust, and thus is one of the most widely used methods in free-surface one-dimensional subcritical numerical modeling. The purpose of this technical note is to discuss the limitations of Preissmann scheme when applied to transcritical flow. In particular, the analysis presented shows that the Preissmann scheme cannot be used to simulate transcritical flow using the through (shock capturing) method.

Available experimental data on friction in smooth rectangular channels were analyzed by a researcher and arrived at the relationship $1/\mathit{f}\prime$= 2 log $R\mathit{f}\prime$− 0.8, where R= 4VR/v and f′=f$\dot{s}$Ψ_p(P/R) for channels and f′=f for smooth pipes. Ψ_p(P/R) is a shape parameter. Values of P/R when b/y< 2 were indicated by an asterisk, and a relationship was obtained between P/R and Ψ_p(P/R). Two researchers analyzed the same data that were used by another researcher and concluded that two functions Ψ₁(P/b) =$\mathit{P}/\mathit{b}$ and Ψ₂(b/y) are required to reflect the shape parameter. However, they combined and obtained Ψ_K=Ψ₁/Ψ₂=f/f′. Ψ₂(b/y) was calculated and plotted against b/y. They noted that Ψ_K≡ 1/Ψ_p, but maintained that there is no good relationship between Ψ_p and P/R. In the present study, it is shown that based on the curve, the equation connecting Ψ_p(P/R) and P/R could be obtained, which fits the curve within 1% error. A plot P/R versus Ψ_p(P/R) is given for ready use.

Tayfur and Kavvas in their 1994 paper presented a method for spatial averaging of flows over rills and interrill areas while explicitly accounting for the interaction between the overland-flow and rill-flow dynamics. The local scale consists of a single rill that is fed by an overland-flow section on each side. The analysis at this local scale is a crucial step in obtaining field-scale averages. The purpose of this technical note is to present analytical solutions for steady-state conditions at this local scale based on the model of Tayfur and Kavvas. These solutions have important implications for large-scale stochastic averaging of flow equations. The influence of the interaction between rills and interrill areas is clearly demonstrated. Additionally, such an analysis shows that equivalent sections are not available, unless the interaction between rills and interrill areas can be neglected.