Journal of Hydraulic Engineering

This paper describes a somewhat alternative approach to combining observations and numerical model results in order to produce a more accurate forecast routine. The approach utilizes artificial neural networks to analyze and forecast the errors created by numerical models. The resulting hybrid model provides very good forecast skills that can be extended over a forecasting horizon of considerable length. The method has been developed for the purpose of operational forecasting of current speeds in the Danish ∅resund Strait. The forecast system was used as a planning tool during the construction of the 16 km-long fixed link across the ∅resund Strait, linking the countries of Denmark and Sweden.

Experiments were performed to measure the mixing characteristics of turbulent momentum jets discharged horizontally into a linearly-stratified, stationary environment. These flows can occur when sewage is discharged into water bodies such as lakes. The centerline dilution was found to follow the results for an unstratified jet up to the point where the jet begins to collapse under the influence of the stratification. The distance at which this occurs is slightly longer than that reported previously from visual observations. The dilution then continues to increase with distance, but more slowly. The results are interpreted in terms of stratified turbulence collapse, and a model is proposed for the initial and final collapse of the turbulence in the jet. Implications for mixing zone models are discussed.

Turbulence generated by a vertically oscillating grid in a water tank was investigated using the digital particle image velocimetry technique. The statistical turbulence characteristics computed based on the experimental data agree well with the two main findings reported in the literature: (1) The turbulence decays following the power law; and (2) the integral length scale increases linearly with the distance from the grid. In addition, the flow structure near the grid was observed in detail with the advantage of the planar measurements. It was found that the velocity fluctuations in the region near the grid vary depending on the grid geometry. The fluctuations immediately over the bar position are significantly different from those over the grid openings. Turbulence with the highest intensity occurs above the intersection of the square bars that constitute the grid. The results imply that shear flow clearly exists near the grid and homogeneity of the turbulence can only be achieved at a distance from the grid greater than about three mesh sizes. In addition, it was found that at least 400 vector maps should be taken to ensure the accuracy of the measured velocity fluctuations.

A 3D numerical model has been developed to study the deposition patterns for sediment dumping in ambient water with cross-flow. The model formulation is based on the governing equations for the conservation of mass, momentum, and density excess, assuming the discrete particles can be represented by a continuous field of density difference with a specified settling velocity. To model the turbulence generated by the particles, a buoyancy extended k-ε model is employed. Numerically, the governing equations are split into three parts in the finite-difference solution: advection, dispersion, and pressure propagation. The advection part is solved by a characteristics-based scheme, the dispersion part is solved by the central difference method, and the pressure propagation part is solved implicitly by using the Gauss-Seidel iteration method. The computed results show that two different deposition patterns exist. One is oblong, and the other is horseshoe-shaped, depending on the ratio of the initial negative buoyancy flux of the sand discharge and the horizontal momentum flux of the flow. The computed results are in satisfactory agreement with experiments. The existence of the two different deposition patterns is explained by using the concept of bifurcation of buoyant plumes.

The scouring process in a plunge pool of loose bed with uniform bed-materials due to a two-dimensional plane impinging jet was simulated computationally. The finite-element-based unsteady three-dimensional model, CCHE3D, with k-ε turbulence closure was employed to solve the flow field. It has long been recognized that the unsteady behavior of the turbulent jet fluctuation plays an important role in scouring and transporting sediment in the plunge pool. In order to model this phenomenon realistically, one has to consider the effects of both shear stress and the life force on sediment particles due to pressure fluctuation. The latter has been taken into account by using empirical relationships of flume data. Both of these effects have been incorporated in the nonequilibrium sediment transport model consisting of sediment pickup rate and step length adopted for the jet scour problem. The model constant relating to the fluctuating lift force was calibrated using an empirical equation to predict the quasi-equilibrium scour depth. The results simulated by the model proposed here agree reasonably well with experimental data.

It is shown how some basic probability tools may help in the characterization of the turbulent processes involved in the mechanics of sediment transport. Both the initiation of sediment motion and the threshold for sediment suspension are analyzed in a stochastic frame of work. Isorisk curves are constructed based upon some minimum experimental information. The well-known Shields' curve is shown to correspond to an erosion risk of approximately 40%. These concepts are believed to be of importance for the design of movable-bed channels as well as for future experimental investigations, which are indeed necessary for computing the risk for higher values of the particle Reynolds number. Finally, an approximate methodology is proposed for computing the risk of erosion.

The Manning roughness factor varies with the hydraulic radius in grass-lined channels. The functional relationship between the roughness factor and the hydraulic radius is highly nonlinear. This necessitates a lengthy and tedious trial-and-error method to solve the Manning formula in the different stages of the traditional design procedures for grass-lined channels. Various charts are presented in this technical note that can be used in designing grass-lined channels without a need for trial-and-error. The charts were developed using predetermined solutions to the Manning formula. They are presented in terms of dimensionless parameters and can be employed with any consistent unit system. The use of the charts is demonstrated through an example.