Search

To investigate the complex hydrological, morphodynamic, and environmental processes in watersheds, a physically-based integrated two-dimensional (2D) surface and three-dimensional (3D) subsurface model for flow, soil erosion and transport, and contaminant transport in the surface-subsurface system is presented in this paper. The model simulates the rainfall-induced surface flow by solving the depth-averaged 2D diffusion wave equation and the variably-saturated subsurface flow by solving the 3D mixed-form Richards equation. The surface and subsurface flow equations are coupled using the continuity conditions of pressure and exchange flux at the ground surface. The model uses the concept of nonequilibrium in the depth-averaged 2D simulation of nonuniform total-load sediment transport in upland fields, considering detachments by rainsplash and hydraulic erosion driven by surface flow. The integrated 2D surface and 3D subsurface contaminant transport model takes into account the contaminant changes due to sediment sorption and desorption, as well as exchanges between surface and subsurface domains due to infiltration, diffusion, and bed change. The model applies the same set of surface equations of flow, sediment, and contaminant transport for describing both upland areas and streams, so that no special treatments are required at their interface. The established model has been evaluated by comparisons with published experimental, numerical, and analytical data and then applied in an agricultural watershed. The model is suitable for wetland areas and agricultural watersheds in which streams are not very narrow and deep, and meanwhile a relatively fine mesh that can distinguish the streams is preferred. DOI: 10.1061/(ASCE)HY. 1943-7900.0000116 CE Database subject headings: Numerical models; Integrated systems; Rainfall; Infiltration; Overland flow; Subsurface flow; Erosion; Pollution; Sorption; Watersheds.

A three-dimensional buoyancy-extended version of k--[epsilon] turbulence model was developed for simulating the turbulent flow and heat transport in a curved open channel. The density-induced buoyant force was included in the model, and the influence of temperature stratification on flow field was considered. The flow and temperature fields were simulated simultaneously. The model was validated by comparison with laboratory measurements, and the simulated fields were generally in good agreement with experimental data. A comparison of velocity fields in thermal and isothermal flow in curved open channel is presented and the effects of channel curvature and buoyant force on the velocity fields are also discussed. DOI: 10.1061/(ASCE)0733-9429(2009)135:7(554) CE Database subject headings: Temperature distribution; Open channel flow; Turbulence; Simulation; Heat transfer.

A one-dimensional model has been established to simulate the fluvial processes under dam-break flow over movable beds. The hydrodynamic model adopts the generalized shallow water equations, which consider the effects of sediment transport and bed change on the flow. The sediment model computes the nonequilibrium transport of bed load and suspended load. The effects of sediment concentration on sediment settling and entrainment are considered in determining the sediment settling velocity and transport capacity. In particular, a correction factor is proposed to modify the Van Rijn formulas of equilibrium bed-load transport rate and near-bed suspended-load concentration for the simulation of sediment transport under high-shear flow conditions. The governing equations are solved by an explicit finite-volume method with the first-order upwind scheme for intercell fluxes. The model has been tested in two experimental cases, with fairly good agreement between simulations and measurements. The sensitivities of the model results to parameters such as the sediment nonequilibrium adaptation length, Manning's roughness coefficient and the proposed correction factor have been verified. The proposed model has also been compared to an existing model and the results indicate the new model is more reliable. DOI: 10.1061/(ASCE)0733-9429(2007)133:1(48) CE Database subject headings: Dam failure; Flood routing; Sediment transport; Suspended load; Bed load; Erosion; One-dimensional flow; Shock waves.

A quasi-three-dimensional coastal area morphological (Q3DCAM) model has been developed using the process-based approach. It is for simulating complex multiscale coastal processes, primarily morphodynamic changes of the seabed. This software package has integrated three key submodels for simulating irregular wave deformations, nearshore currents, and morphological processes. The quasi-three-dimensional capability of the depth-averaged model has been developed to consider the vertical flow structure inside the surf zone and the cross-shore movement mechanisms of nearshore currents, e.g., undertow and mass flux. To this end, the calculations of the radiation stresses inside the surf zone have been improved by introducing the nonsinusoidal wave model for surface roller effects due to the breaking wave. To predict accurately the wave field and the morphological processes near coastal structures, the wave diffraction effects were included in a multidirectional spectral wave transformation model. The morphodynamic change was modeled by considering the sediment transport due to the combinations of waves and currents. These three submodels were validated by simulating three laboratory experimental cases in regard to: (1) irregular wave deformations over a shoal; (2) longshore currents in a wave basin; and (3) moveable bed evolutions around an offshore breakwater under attack of an incident wave. The numerical results of the morphological modeling confirmed that the Q3DCAM model consisting of the diffraction effects and the surface roller effects is capable of predicting waves, currents, and morphodynamic changes more accurately than before. Therefore, this validated model can be applied to simulate more realistic morphological processes in coastal zones including structures. DOI: 10.1061/(ASCE)0733-950X(2006) 132:6(462) CE Database subject headings: Coastal processes; Numerical models; Wave diffraction; Nearshore circulation; Coastal morphology; Three-dimensional models.

An optimal flow control methodology based on adjoint sensitivity analysis for controlling nonlinear open channel flows with complex geometries is presented. The adjoint equations, derived from the nonlinear Saint-Venant equations, are generally capable of evaluating the time-dependent sensitivities with respect to a variety of control variables under complex flow conditions and cross-section shapes. The internal boundary conditions of the adjoint equations at a confluence (junction) derived by the variational approach make the flow control model applicable to solve optimal flow control problems in a channel network over a watershed. As a result, an optimal flow control software package has been developed, in which two basic modules, i.e., a hydrodynamic module and a bound constrained optimization module using the limited-memory quasi-Newton algorithm, are integrated. The effectiveness and applicability of this integrated optimal control tool are demonstrated thoroughly by implementing flood diversion controls in rivers, from one reach with a single or multiple floodgates (with or without constraints), to a channel network with multiple floodgates. This new optimal flow control model can be generally applied to make optimal decisions in real-time flood control and water resource management in a watershed. CE Database subject headings: Open channel flow: Sensitivity analysis; Floods; Geometry; Optimization.

A three-dimensional numerical model was developed for simulating the phosphorus concentration in shallow lakes. In this model, the computational domain was divided into two parts: the water column and the bed sediment layer. The processes of mineralization, settling, adsorption, desorption, bed release (diffusion), growth, and death of phytoplankton were taken into account, and the concentration of organic phosphorus, phosphate, and related water quality constituents was simulated. The concentrations of adsorbed (particulate) and dissolved phosphate due to adsorption-desorption were calculated using two formulas derived based on the Langmuir equation. The release rate of phosphorus from the bed sediment layer was calculated by considering the effects of the concentration gradient across the water-sediment interface, pH, temperature, dissolved oxygen concentration, and flow conditions. The adsorption and desorption of phosphate from sediment particles, as well as its release from bed sediment, were verified using laboratory experimental data. The model was calibrated and applied to Deep Hollow Lake in the Mississippi alluvial plain. The simulated trends and magnitudes of phosphorus concentration were compared with field observations. The simulation results show that there are strong interactions between sediment-related processes and phosphorus concentration. CE Database subject headings: Phosphorus; Numerical models; Lakes; Sediment.

Several existing formulas for the initial porosity and settling velocity of sediment have been tested by using extensive data collected from different countries and regions, and modified to achieve better reliability or convenience in use. CE Database subject headings: Sediment; Sediment deposit; Porosity; Settling velocity; Particle size; Shape.

This paper presents a two-dimensional (2D) shallow-water numerical model, which is based on the resolution of the Saint-Venant equation using the unstructured finite-volumes method, combined with Green's theorem technique. The model has been validated by several benchmarks. The numerical results obtained from the model are in good agreement with the analytical or experimental ones. The paper also presents an application of this model to flood diversion from the Red River into a water-retention zone for the purpose of reducing flood threat at Hanoi, capital of Vietnam. DOI: 10.1061/(ASCE)0733-9429(2006) 132:3(258) CE Database subject headings: Two-dimensional models; Numerical models; Shallow water; Floods.

To improve navigation conditions for barges passing through river channels, many submerged weirs (SWs) have been installed along the bendways of many waterways by the U.S. Army Corps of Engineers. This paper presents results from three-dimensional numerical simulations that were conducted to study the helical secondary current (HSC) and the near-field flow distribution around one SW. The simulated flow fields around a SW in a scale physical model were validated using experimental data. The three-dimensional flow fields around a SW, the influence of the SW on general HSC, and the implication of effectiveness of submerged weirs to realign the flow field and improve navigability in bendways were analyzed. The numerical simulations indicated that the SW significantly altered the general HSC. Its presence reduced a skewed pressure difference cross its top and a triangular-shaped recirculation to the downstream side. The over-top flow tends to realign toward the inner bank and therefore improves conditions for navigation. DOI: 10.1061/(ASCE)0733-9429(2005) 131:8(682) CE Database subject headings: Numerical analysis: Flow simulation: Secondary flow: Turbulent flow: Three-dimensional models: Channel bends: Hydraulic structures: Weirs.

An upwind conservative scheme with a weighted average water-surface-gradient approach is proposed to compute one-dimensional open channel flows. The numerical scheme is based on the control volume method. The intercell flux is computed by the one-sided upwind method. The water surface gradient is evaluated by the weighted average of both upwind and downwind gradients. The scheme is tested with various examples, including dam-break problems in channels with rectangular and triangular cross-sections, hydraulic jump, partial dam-break problem, overtopping flow, a steady flow over bump with hydraulic jump, and a dam-break flood case in a natural river valley. Comparisons between numerical and exact solutions or experimental data demonstrated that the proposed scheme is capable of accurately reproducing various open channel flows, including subcritical, supercritical, and transcritical flows. The scheme is inherently robust, stable, and monotone. The scheme does not require any special treatment, such as artificial viscosity or front tracking technique, to capture steep gradients or discontinuities in the solution. DOI: 10.1061/(ASCE)0733-9429(2004)130:10(977) CE Database subject headings: Open channels; Unsteady flow; Dam failure; Numerical models.

In this study, the proposed one-dimensional model simulates the nonequilibrium transport of nonuniform total load under unsteady flow conditions in dendritic channel networks with hydraulic structures. The equations of sediment transport, bed changes, and bed-material sorting are solved in a coupling procedure with a direct solution technique, while still decoupled from the flow model. This coupled model for sediment calculation is more stable and less likely to produce negative values for bed-material gradation than the traditional fully decoupled model. The sediment transport capacity is calculated by one of four formulas, which have taken into consideration the hiding and exposure mechanism of nonuniform sediment transport. The fluvial erosion at bank toes and the mass failure of banks are simulated to complement the modeling of bed morphological changes in channels. The tests in several cases show that the present model is capable of predicting sediment transport, bed changes, and bed-material sorting in various situations, with reasonable accuracy and reliability. DOI: 10.1061/(ASCE)0733-9429(2004)130:9(914) CE Database subject headings: Numerical models; Unsteady flow; Sediment transport; Open channels; Hydraulic structures.

A numerical method based on optimal control theories for identifying Manning's roughness coefficients (Manning's n) in modeling of shallow water flows is presented. The coefficients are difficult to be determined especially when the spatial variation is significant, and are usually estimated empirically. The present methodology is applied to determine the optimal values of the spatially distributed parameters, which give least overall discrepancies between simulations and measurements. Through a series of systematic studies to identify the n values in both a hypothetical open channel and a natural stream stretch, several identification procedures based on unconstrained and constrained minimizations are analyzed. It is found that the limited-memory quasi-Newton method has the advantages of higher rate of convergence, numerical stability and computational efficiency. Although the identification of Manning's n is chosen as an example, the identification methods can be applied to numerical simulations of various flow problems. CE Database subject headings: Parameters; Shallow water; Roughness coefficient; Identification; River flow.

This paper presents the development and application of a three-dimensional oil spill model for predicting the movement and fate of an oil slick in the coastal waters of Singapore. In the model, the oil slick is divided into a number of small elements or grids for simulating of the oil processes of spreading, advection, turbulent diffusion, evaporation, dissolution, vertical dispersion, shoreline deposition and adsorption by sediment. This model is capable of predicting the horizontal movement of surface oil slick, the mass balance of oil spill and the oil particle concentration distribution in water body. Satellite images and field observations of oil slicks on the surface in the Singapore Straits, and measurements of the vertical concentration of oil particles in flume are used to validate the newly developed model. Compared with the observations, the numerical results of the oil spill model show good conformity. DOI: 10.1061/(ASCE)0733-9429 (2003) 129:7 (495) CE Database subject headings: Oil spills; Three-dimensional models; Singapore; Coastal environment.

A method for the prediction of a movable bed roughness coefficient is proposed by relating the roughness parameter A to the nondimensional shear stress (tau)'(sub b)(tau)sub c 50) and the Froude number of flow. This relationship is tested with a large number of flume and field data from different sources, and also compared with other prediction methods. The calculated flow depth based on the bed roughness coefficient predicted by the proposed method agrees very well with the measured data.

In this paper a depth-integrated 2D hydrodynamic and sediment transport model, CCHE2D, is presented. It can be used to study steady and unsteady free surface flow, sediment transport, and morphological processes in natural rivers. The efficient element method is applied to discretize the governing equations, and the time marching technique is used for temporal variations. The moving boundaries were treated by locating the wet and dry nodes automatically in the cases of simulating unsteady flows with changing free surface elevation in channels with irregular bed and bank topography. Two eddy viscosity models, a depth-averaged parabolic model and a depth-averaged mixing length model, are used as turbulent closures. Channel morphological changes are computed with considerations of the effects of bed slope and the secondary flow in curved channels. Physical model data have been used to verify this model with satisfactory results. The feasibility studies of simulating morphological formation in meandering channels and flows in natural streams with in-stream structures have been conducted to demonstrate its applicability to hydraulic engineering research/design studies of stream stabilization and ecological quality among other problems.

A depth-integrated two-dimensional hydrodynamic and sediment transport model, CCHE2D, has been developed for analyzing free surface flows for general applications to hydraulic engineering problems. Mixed schemes of finite element method (Efficient Element Method) and finite volume method are adopted for solving momentum and continuity equations. A unique velocity correction method was developed to couple the governing equations. Pressure (free surface) field is discretized on a staggered grid. Two zero equation turbulence closure schemes: a depth averaged parabolic, a depth averaged mixing length eddy viscosity model, and a two-equation k-∈ model for depth-averaged flows are included for different application needs. Algebraic systems of equations are solved implicitly by using SIP method. This model can be used for simulating steady and unsteady near field and natural river flows with relatively shallow water depth. Validation tests for this model show good agreements between the model prediction and data of physical models and field measurements. [ABSTRACT FROM AUTHOR]