Journal of Hydraulic Engineering

Alluvial fans and fan-deltas are of three basic types: those built up primarily by the action of constantly avulsing river and stream channels, those constructed by sheet flows, and those resulting from the successive deposition of debris flows. The present analysis is directed toward the first two types. A mechanistic formulation of flow and sediment transport through river channels is combined with a simple quantification of the overall effect of frequent avulsion to derive relations describing the temporal and spatial evolution of mean (i.e., averaged over many avulsions) bed slope and elevation in an axially symmetric fan. An example of a fan formed predominantly by the deposition of sand is compared to a similar one formed predominantly by the deposition of gravel. In each example the case of channelized flow is compared to the case of sheet flow. The model is applied to the tailings basin of a mine in the companion paper.

The formulation of the companion paper describing the evolution of mean elevation in a radially symmetric alluvial fan is applied to the specific case of the tailings basin of an iron mine. The tailings basin provides a simple field-scale example of a fluvial fan-delta system. Calculation procedures are developed to determine the effect of changing water and sediment discharge on fan evolution. The analysis is used to evaluate options for the extension of the lifetime of the tailings basin.

An analytical model is developed to determine the influence of the bank stability on the stable width and depth of alluvial gravel-bed rivers with cohesive banks. The formulation of the model is based on the assumption that the stable width corresponds with an optimum condition that is equivalent to the maximum bed load transporting capacity. The optimum condition develops when the channel banks are at their limiting stability with respect to either mass failure or fluvial erosion. Two basic channel types are identified: bank-height and bank-shear constrained. Mass failure stability is estimated using a simple total stress approach. A method for estimating the critical bank shear stress based on model calibration is proposed. Analysis of field data indicates that the effect of the bank vegetation on bank stability can be expressed in terms of the critical bank shear stress. The average critical bank shear stress value calculated for riverbanks covered by vegetation with well-developed root networks was found to be approximately three times that obtained from rivers with weakly vegetated, grass-covered banks.

A high-speed video system was used to study saltation of sand in an open channel flow. High rates of image acquisition provided the information needed to resolve details of the Lagrangian properties of particle motion. Saltation of sand is described in terms of statistical properties of particle trajectories such as mean values and standard deviations of saltation length, height, and streamwise particle velocity. Results are compared with available empirical data. Saltation height seems to be independent of particle size when made dimensionless with the particle diameter; however, the dimensionless saltation length appears to increase as the particle size decreases. Particle collision with the bed is also analyzed. Observations of this process indicate that most of the interactions between saltating particles and the bed are of the collision-rebound type, which contradicts previous discussions on the subject. The estimated value of the dynamic friction coefficient is about half that proposed by Bagnold, in agreement with previous empirical evidence. Friction and restitution coefficients at collision are also estimated from the experimental observations. Nonvanishing values of the latter and values of the former lower than unity are obtained in agreement with previous work with gravel-size particles. Observations of particle resting time, particle reentrainment into saltation, particle rotation, and particle transverse motion during saltation are also presented, providing new insights on the physical processes associated with the saltation phenomenon.

The vertical distribution of suspended sediment concentration in open channel flows is usually described by the advection-diffusion equation. Hence, only the mechanisms of advection and diffusion affect the movement of sediment particles. However, recent publications show that the equation could not describe fully the sediment concentration distribution in open channels. A new model for particle migration in sediment-laden turbulent flow is presented. The model assumes there is an additional mechanism influencing the particle movement in sediment-laden flows. A new mechanism, which is called the drift, acts independently of advection and turbulent diffusion. Based on all three mechanisms, a new equation for the concentration distribution is derived. The equation describes the vertical distribution of sediment concentration more accurately than the advection-diffusion equation.

This paper presents the results of a laboratory study on the erosion of noncohesive sediment beds made of three different mixtures by deeply submerged plane turbulent wall jets. The sediment mixtures had median sizes of 6.75, 1.32, and 1.62 mm, and geometric standard deviations of 1.32, 2.02, and 3.13. The main objectives were to study the effect of sediment gradation on the asymptotic scour size and determine an effective size for the sediment mixtures for predicting erosion. The experimental observations and analysis presented in this study established that the sediment nonuniformity has a significant effect on the size of the scour hole produced by the jet. The effective size of the sediment mixture for obtaining a good correlation for the depth of scour was found to be d₉₅ rather than d₅₀ for defining the densimetric particle Froude number, F_o. The average reduction due to armoring in the maximum scour depth and dune height was found to be ∼60%, and it was found to be 50% for the scour hole length and the distance of the dune.

Plunging jets from overtopping dams have the potential to erode dam foundations and abutments. The erosive potential of the jet is related to the energy of impact and the rate of energy dissipation in the plunge pool. A method was sought to predict the velocity decay of a free-falling, highly turbulent, rectangular jet in a plunge pool. Two types of jets were evaluated: a highly turbulent, fully air entrained developed jet; and a highly turbulent, nonaerated undeveloped jet. A model of a plunging jet was constructed to simulate these free-falling, highly turbulent rectangular jets. Data were collected from 14 tests, from which empirical equations were derived for the two jet classifications to predict jet velocity decay through the plunge pool. The jet velocity decay was determined to be a function of jet impact velocity with the plunge pool surface, jet density at impact with the plunge pool surface, the density of the water, gravity, and plunge depth. An expression was derived to compute jet breakup length to determine the jet condition at impingement with the plunge pool.

The concept of “feedback” is introduced into potential free surface flow to derive a new and simple iterative method termed the residual pressure feedback method that is used for computing free surface profile. The residual pressure that vanishes when the convergence is achieved is fed back in order to adjust the residual pressure at the free surface and the free surface elevation continuously during the process of iteration. The use of the proposed method is demonstrated through a typical example of flow under sluice gates. The calculated results agree satisfactorily with those obtained from other methods and experimental data. This method features a simple iterative process, an easy-to-formulate computer code, and rapid convergence. Unlike some iterative methods, it is not necessary to greatly modify the computer program when the method is used to calculate different examples.

Numerous formulations have been proposed linking friction factors to the gravel-bed river downstream hydraulic geometry. Data from 280 rivers show that existing gravel-bed power and semilogarithmic formulations fail to correctly predict the friction factors. A new gravel-bed semilogarithmic formulation is developed using nonlinear statistical tools. It considers the influence of the relative roughness, the Froude number, the sediment mobility parameter, and a cross-sectional form factor. The best fit is achieved by modifying the adjustment parameter and the interior of the logarithmic term of the universal velocity distribution equation.

In this paper the stratification dynamics of a river channel are described and modeled using a two-dimensional, primitive equation, circulation model that incorporates a turbulence closure scheme. The model is applied to a weir pool on the Murrumbidgee River and to an open section of the River Murray, Australia. Excellent agreement between measured and simulated temperature profiles under a wide range of river discharge and meteorological forcing is obtained in the two systems. The net surface heat flux and the river discharge are the two most important parameters determining the stratification status of these rivers with little effect from direct wind mixing.

The purpose of this technical note is to investigate the applicability of the Principle of Maximum Entropy in hydrodynamics, particularly in velocity distribution in open channels, by comparing the predictions of two 2-D models with measured data. Both models use a curvilinear coordinate system, but whereas the first uses logarithmic law for velocity distribution, the second is derived by the principle of maximum entropy. The computed values are then compared to 1,832 point velocity measurements by means of laser-Doppler velocimetry in five experiments with rectangular smooth-wall flumes. The results show that the entropy model performs better than the logarithmic model not only in general terms (mean absolute error 12.8% versus 19.1%), but in practically all flow regions, especially in those near the channel bed (absolute error 23.8% versus 55.3%). Because the models disregard secondary flow, velocity predictions in the upper region of near-side walls are relatively poor.