Channel Network Model

FLOW AND SEDIMENT TRANSPORT MODELING

Introduction

The CCHE1D model is a powerful tool in the computation of flow and sediment transport in channel networks. Its watershed-based approach facilitates the integration of the channel model with rainfall-runoff and upland erosion models. CCHE1DFL computes unsteady channel flows considering the effects of hydraulic structures. The sediment transport module CCHE1DST can predict channel morphological changes and sediment yield in long-term, continuous simulations.
 

Unsteady Free-Surface Flow Modeling

The CCHE1D channel network model computes unsteady flows using either the Dynamic or Diffusion Wave approaches. The implementation of the full St. Venant equations (Dynamic Wave) enhances the accuracy of the model in situations where dynamic effects cannot be neglected, and permits the model to be applied to a wider range of flow conditions. The diffusion wave model, as well as the new hybrid diffusion/dynamic model implementation, enhances the numerical stability of the model. CCHE1DFL differentiates the flows in the main channel and flood plains of a compound channel by dividing the entire cross section into three subsections: left flood plain, main channel and right flood plain. For version 3.0, CCHE1D supports arbitrary cross section shapes, with varying roughness within each subsection.

 

In-Stream Hydraulic Structures 

CCHE1D contains special procedures for the computation of flow across hydraulic structures. Supported structure types include culverts, low and high-drop structures, bridges, and measuring flumes. CCHE1D's graphical interface provides all the tools necessary for the specification of the location, type, and geometric data for in-stream structures. Usually, in-stream structures determine the local flow hydraulics, therefore affecting the erosion and sedimentation processes in their neighborhood. Some of these structures are built as erosion control devices; therefore, it is important that the model simulates their effect on sediment transport with reasonable accuracy.

 

Sediment Transport and Channel Morphology Modeling

CCHE1DST calculates non-equilibrium; non-uniform sediment transport in channel networks under unsteady flow conditions. It simulates bed aggradation and degradation processes and the resulting morphologic changes. The model computes variations of the bed material gradation by dividing the bed into several layers, simulating hydraulic sorting and armoring. The model also simulates channel-widening processes through bank erosion and stability analysis algorithms.

CCHE1DST computes sediment transport, bed changes, and bed material gradation using an advanced coupling procedure (Wu et al., 2000). The coupled method enhances the stability of the numerical solution and eliminates the occurrence of numerical oscillation and negative bed-material gradation. However, the sediment computations are still decoupled from the flow calculations.
 

Empirical Formulas

The model provides several well-known equations for the determination of transport capacity, and a series of options for the computation of auxiliary parameters such as bed material porosity, mixing layer thickness, non-equilibrium adaptation length, wash load size range, movable bed roughness coefficient, etc. The implementation of these multiple options enables the model be widely used, allowing the user to choose the most appropriate formulas for different real-life problems.

Sediment transport capacity can be calculated by four formulas:

· * SEDTRA module

· * Wu et al's formula

· * Modified Ackers and White's formula

· * Modified Engelund and Hansen's formula

 

Bank Erosion and Stability Analysis

Bank erosion and channel widening can significantly affect the sediment balance of a channel system. These processes must be modeled for the prediction of sediment yield and channel morphological evolution. Bed degradation and lateral erosion at bank toes may cause river banks to become unstable. CCHE1D simulates toe erosion using an empirical relationship. A bank stability algorithm computes a safety factor defined as the ratio of the resistance and driving forces for the bank failure. Once the stability criterion is exceeded, a mass failure event occurs. The failed bank materials deposit first on the bed near the bank toe (or are saved to a virtual tank), and then are eroded away by bank toe erosion.

 

Model Tests and Applications

The CCHE1D channel model has been tested and validated for a series of laboratory experiments and field measurements. These test cases showed that the CCHE1D model could predict well the sediment yield and channel bed evolution due to scouring and deposition processes with both uniform and non-uniform sediments. The CCHE1D 3.0 Technical Manual describes in detail some of the laboratory and field test cases used for the validation of the channel model, as well as the application of the model in the study of long-term sedimentation processes in the Goodwin Creek watershed.