Search

The weighted usable area (WUA) is one of the popular metrics to quantify the stream physical habitat and the effects of instream structures. However, most habitat studies only considered existing or proposed channel geometries in their WUA computations and not how the channel will evolve over time. This study reports a numerical investigation for the effects of spur dikes on WUA along the Juanchiao levee, Touchien River, Taiwan. Seven virtual dikes were placed along the outer bend in the study reach and the Rhinogobius candidianus was selected for target species. The horizontal two-dimensional (2D) flow fields at various flows and the corresponding WUAs for R. candidianus were computed under four channel bathymetry configurations, including: (1) existing; (2) immediately after dikes are implemented; (3) after 10-year channel migration without dikes; and (4) 10 years after dikes are implemented. The WUA obtained from the first two bathymetry configurations aimed to investigate the short-term spur d...

The purpose of this paper is to develop an unsteady 2D depth-averaged model for nonuniform sediment transport in alluvial channels. In this model, the orthogonal curvilinear coordinate system is adopted; the transport mechanisms of cohesive and noncohesive sediment are both embedded; the suspended load and bed load are treated separately. In addition, the processes of hydraulic sorting, armoring, and bed consolidation are also included in the model. The implicit two-step split-operator approach is used to solve the flow governing equations and the coupling approach with iterative method are used to solve the mass-conservation equation of suspended sediment, mass-conservation equation of active-layer sediment, and global mass-conservation equation for bed sediment simultaneously. Three sets of data, including suspension transport, degradation and aggradation cases for noncohesive sediment, and aggradation, degra- dation, and consolidation cases for cohesive sediment, have been demonstrated to show the rationality and accuracy of the model. Finally, the model is applied to evaluate the desilting efficiency for Ah Gong Diann Reservoir located in Taiwan to show its applicability.

The characteristics method by using the cubic-spline interpolation is comparable to the Holly-Preissmann scheme in solving the advection portion of the advection-diffusion equation. In order to conduct a cubic-spline interpolation, an additional constraint must be specified at each endpoint. In general, four types of endpoint constraints are available, i.e., the first derivative, second derivative, quadratic, and not-a-knot constraints. The goal of this paper is to examine each type of endpoint constraints. Two hypothetical cases are used to conduct the investigation. Among the four types of constraints examined herein, the not-a-knot constraint and the first derivative constraint with high-order finite difference approximation yield the best results. However, as far as accuracy and simple implementation are concerned the not-a-knot constraint should be the best choice in solving the advection-diffusion equation

A numerical experiment is carried out to study the suitability of two-dimensional (2D) depth-averaged modeling for bend-flow simulation, in which the geometry of the studied channel is rectangular. Two commonly used 2D depth-averaged models for bend-flow simulation are considered in this study of which the bend-flow model includes the dispersion stress terms by incorporating the assumption of secondary-current velocity profile, and the conventional model neglects the dispersion stress terms. The maximum relative discrepancy of the longitudinal velocity, obtained from the comparison of these two models, is used as a criterion to judge their applicability for bend-flow simulation. The analysis of simulation results indicated that the maximum relative difference in longitudinal velocity is mainly related to the relative strength of the secondary current and the relative length of the channel. Empirical relations between the maximum relative difference in the longitudinal velocity, the relative strength of th...

An efficient hybrid finite-difference scheme capable of solving the dispersion equation with general Peclet conditions is proposed. In other words, the scheme can simultaneously deal with pure advection, pure diffusion, and/or dispersion. The proposed scheme linearly combines the Crank-Nicholson second-order central difference scheme and the Crank-Nicholson Galerkin finite-element method with linear basis functions. Using the method of fractional steps, the proposed scheme can be extended straightforwardly from one-dimensional to multidimensional problems without much difficulty. It is found that the proposed scheme produces the best results, in terms of numerical damping and oscillation, among several non-split-operator schemes. In addition, the accuracy of the proposed scheme is comparable with a well-known and accurate split-operator approach in which the Holly-Preissmann scheme is used to solve the pure advection process while the Crank-Nicholson second-order central difference scheme is applied to the pure diffusion process. Since the proposed scheme is a non-split-operator approach, it does not compute the two processes separately. Therefore, it is simpler and more efficient than the split-operator approach.

The purpose of this paper is to present a 2D depth-averaged model for simulating and examining flow patterns in channel bends. In particular, this paper proposes a 2D depth-averaged model that take...

The model performance of an engineering system is affected by many variables subject to uncer­ tainty. Point estimate (PE) methods are practical tools to assess the uncertainty features of a model involving multivariate stochastic parameters. Two PE methods have been developed for engineering applications. One is Rosenblueth's PE method, which preserves the first three moments of random variables and the other is Hare's PE method, which reduces the computations of Rosenblueth's method but only as appropriate for application to random variables with normal distributions. In this study, two algorithms are proposed to encompass the advantages of the two PE methods: computational practicality and the handling of mixture distributions. Through a numerical experiment, the proposed methods yielded more accurate estimations than those of Rosenblueth's method with about the same amount of computation as Hare's method. The two proposed methods were also applied to estimate statistical moments of a pier scouring model output to demonstrate their performance in an engineering application.

In hydraulic and hydrologic studies, engineers develop new models or calibrate existing models by various techniques. One is often concerned with the model validity regarding its ability to predict future events. Five potentially useful statistical validation methods are presented. For illustration, they were applied to examine the predictability of unit hydrographs derived by various methods in the framework of the least squares and its variations. It was found that storm-stacking (conventional multistorm analysis) together with storm-scaling yields the most desirable UH. The general framework of these validation methods can also be applied to a validation study of other hydrologic and hydraulic models.

As computation speed increases, Monte Carlo simulation is becoming a viable tool for engineering design and analysis. However, restrictions are often imposed on multivariate cases in which the involved stochastic parameters are correlated. In multivariate Monte Carlo simulation, a joint probability distribution is required that can only be derived for some limited cases. This paper proposes a practical multivariate Monte Carlo simulation that preserves the marginal distributions of random variables and their correlation structure without requiring the complete joint distribution. For illustration, the procedure is applied to the reliability analysis of a bridge pier against scouring.