The CCHE2D model is a two-dimensional depth-averaged, unsteady, flow and sediment transport model. The flow model is based on depth-averaged Navier-Stokes equations. The turbulent shear stresses are modeled using Boussineq’s approximation, and three different turbulence closure schemes are available for the calculation of the turbulent eddy viscosity. The resulting set of equations is solved implicitly using the control volume approach and efficient element method. The numerical technique employed ensures oscillation free and stable solution.
The sediment transport module is used to simulate non-uniform sediment (both non-cohesive and cohesive) using non-equilibrium transport models. Three different non-equilibrium transport approaches are proposed to handle the cases where the sediment transport occurs mainly as bed load, mainly as suspended load, or total load. The equations for this module include transport equations for bed load and suspended load, the bed change equation, and the bed sorting equation. These equations are discretized using efficient element method or exponential difference scheme.
Main Features of the Flow Model
The CCHE2D flow model has the following main features.
• The model strictly enforces the mass conservation within the computational domain through the user of control volume approach. This property is of fundamental importance in achieving reliable and accurate results.
• Wetting and drying of the domain as the nodes are submerged under high flows and exposed during low flows. This feature is particularly important during unsteady flows. The wet and dry nodes are distinguished based on the critical depth specified by the user. During the simulation process any node having flow depth less than the critical depth is considered dry.
• The turbulent eddy viscosity is approximated using three different approaches. The first one is based on the depth average parabolic eddy viscosity model; the second approach employs depth-averaged mixing length model; and the last approach is based on depth-averaged ?-? scheme. The last two approaches are particularly suitable for re-circulation flows and flow around hydraulic structures. The user has the option to simulate a given case with any of the above turbulent closure scheme.
• The user can provide no-slip, total-slip, partial-slip, or log-law boundary condition at the no-flow boundaries. The log-law approach results in an accurate prediction of shear stresses near the hydraulic structures that are important for computing flow and sediment transport in the vicinity of hydraulic structure.
• The model supports both steady and unsteady boundary conditions for flow with multiple inlets and outlets. At any inlet the user can specify specific discharge, total discharge, or discharge hydrograph boundary condition. At an outlet the model accepts open boundary, water surface level, stage-discharge relationship, or stage hydrograph as a boundary condition. In case of open boundary the model uses kinematic wave approximation to assess the water surface level at the outlet. This condition should be applied judiciously and is useful in cases when water surface level at the outlet is not available.
• The model is capable of handling supercritical flow. In addition, mixed flow regime (combination of subcritical and supercritical flow) in a channel reach can be simulated using the CCHE2D model.
Main Features of the Sediment-Transport Model
The main features of the sediment transport module are listed below.
• The sediment model supports both uniform and non-uniform sediment transport. In addition, the transported sediment can be either cohesive or non-cohesive. The user can specify whether a node or sub-region in the computational domain should be treated as erodible or non-erodible. Also, the thickness of maximum erosion and deposition can be specified in the domain.
• Three different non-equilibrium transport approaches are proposed in the model. These approaches correspond to the cases where the sediment transport occurs mainly as bed load, mainly as suspended load, or total load.
• The model can be used to evaluate aggradation, degradation, selective sediment transport, and riverbed armoring processes.
• The roughness of the moveable bed changes due to change in sediment size and change in bed form. Two different approaches are incorporated in the model to compute the roughness of the moveable bed due to changes in sediment size and bed forms. The user can select any of the above option to simulate the test case in hand.
• Based on the non-equilibrium transport model adopted the user has an option to select a sediment transport capacity formula from the list provided. Four different transport capacity formulas are available to the user. These include Wu, Wang, and Jia formula, SEDTRA module, modified Ackers and White formula, and modified Engelund and Hansen formula.
• Secondary flow affects in bends affects the direction of bed shear stresses and mean flow directions that are important for bed load and suspended sediment transport respectively. The sediment module includes the curvature effects for sediment transport in bends.
Model Verification and Applications
The CCHE2D model has been applied to a variety of flow and sedimentation problems in natural channel and laboratory flume. The flow application examples that are solved using the CCHE2D model are provided in the Verification/Validation Document. The CCHE2D Sediment Transport Model Technical Report provides necessary mathematical details of the sediment transport model and includes verification and validation tests. The Technical Report provides mathematical details about the CCHE2D flow module.
Mesh Generator and Graphical User Interface
The CCHE2D model system also includes the CCHE2D-GUI, a graphical user interface, and CCHE_Mesh Generator, a structured mesh generator with interface. The mesh generator and the CCHE2D-GUI provide pre- and post-processing capabilities for the CCHE2D model.
Current Status of CCHE2D Model
The CCHE2D model is available free of charge to the researchers and engineers that sign Beta-Testing Agreement with the NCCHE. Both the mesh generator and the CCHE2D-GUI are developed for the Microsoft Windows system and can run on Windows 95, 98, 2000, and XP.
The CCHE2D model is bundled with the CCHE2D-GUI and can run independently on the client machine. However, the user can also apply for a login and password to run the model on the NCCHE’s server.
New Developments for CCHE2D Model
A new pollutant transport and water quality module is currently under development and will be made available to the beta-testers upon completion.