【正文】 ameters represent the physical properties of flow and sediment, or the quantities derived from the modeling of flow and sediment transport. In the CCHE1D channel network model, the numerical parameters include putation time step and grid length, and the physical parameters are the Manning’s roughness coefficient, nonequilibrium adaptation length of sediment transport, mixing layer thickness, bed material porosity, etc. Usually, the numerical parameters can be more easily handled than the physical parameters. Some of these physical parameters, such as the Manning’s roughness coefficient and bed material porosity, have been studied by many investigators and may be determined by measurement. However, the nonequilibrium adaptation length and the mixing layer thickness are less understood and must be prescribed empirically. Therefore, the main concern in this paper is to analyze the influence of these two physical parameters on the simulation results. The nonequilibrium adaptation length Ls characterizes the distance for sediment to adjust from a nonequilibrium state to an equilibrium state. Wu, Rodi and Wenka (2000) and Wu and Vieira (2000) reviewed in detail those empirical and semiempirical methods for determining Ls published in the literature, such as Bell and Sutherland’s (1981), Armanini and di Silvio’s (1988), etc. It was found that those methods provide significantly different estimations of Ls. In CCHE1D, the adaptation length for wash load transport is set as infinitely large because the net exchange between wash load and channel bed is usually negligible. The adaptation length for suspended load transport is calculated with Ls=uh/αωs, in which u is the sectionaveraged velocity, h is the flow depth, ωs is the settling vel。 ? Ab / ? t is the total bed deformation rate, defined as ? Ab/ ? t =k=1N? Ab/ ? t。 Ls is the adaptation length of nonequilibrium sediment transport。 Email: wuwm AbstractThe CCHE1D model was designed to simulate longterm flow and sediment transport in channel networks to support the DEC project. It uses either the dynamic wave or the diffusive wave model to pute unsteady flows in channel networks with pound cross sections, taking into account the effects of instream hydraulic structures, such as culverts, weirs, drop structures, and bridge crossings. It simulates nonuniform sediment transport using a nonequilibrium approach, and calculates bank toe erosion and mass failure due to channel incision. The CCHE1D model decouples the flow and sediment transport calculations but couples the calculations of nonuniform sediment transport, bed changes and bed material sorting in order to enhance the numerical stability of the model. In this paper, the sensitivity of CCHE1D to parameters such as the nonequilibrium adaptation length of sediment transport and the mixing layer thickness is evaluated in cases of channel aggradation and degradation in laboratory flumes as well as in a natural channel network. In the case of channel degradation, the simulated scour process is not sensitive to variation in values of the nonequilibrium adaptation length, but the determination of the mixing layer thickness is important to the putations of the equilibrium scour depth and of the bedmaterial size distribution at the armoring layer. The simulated bed profiles in the case of channel aggradation and the calculated sediment yield in the case of natural channel network are insensitive to the prescription of both the nonequilibrium adaptation length and the mixing layer thickness. The CCHE1D model can provide reliable results even when these two parameters are given a wide range of values. Introduction The CCHE1D model was designed to simulate longterm flow and sediment transport in channel networks to support the Demonstration Erosion Control (DEC) project, which is an interagency cooperative effort among the US Army Corps of Engineers (COE), the Natural Resources Conservation Service (NRCS) and the Agricultural Research Service (ARS) of the US Department of Agriculture. The CCHE1D version was based on the unsteady flow model DWAVNET (Diffusion WAVe model for channel NETworks, Langendeon, 1996) and the sediment transport model BEAMS (Bed and Bank Erosion Analysis Model for Streams, Li et al., 1996). It was significantly improved by implementing the dynamic wave model and the nonequilibrium sediment transport model (Wu, Vieira and Wang 2000). The CCHE1D was integrated with the landscape analysis tool TOPAZ (Garbrecht and Martz, 1995) and with the watershed models AGNPS (Bosch et al., 1998) and SWAT (Arnold et al., 1993), through an ArcView GISbased graphical user interface (Vieira and Wu, 2000). The CCHE1D has been successfully tested in various experimental and field cases. Because several parameters in CCHE1D must be prescribed empirically, it is very important to know the response of the model to the uncertainty of these parameters. In this study, the sensitivity of CCHE1D to model parameters such as the nonequilibrium adaptation length of sediment transport and the mixing layer thickness is analyzed in cases of channel aggradation and degradation in laboratory flumes as well as in a natural channel network. Description of the CCHE1D Channel Network Model Hydrodynamic Model. The CCHE1D flow model simulates unsteady flow in channel networks with pound crosssections using either the diffusive wave model or the dynamic wave model. The dynamic wave model solves the full St. Venant equations. The Preissmann implicit,fourpoint, finite difference scheme is used to discretize the governing equations. Linearized iteration schemes for the discretized governing equations are established and solved using a double sweep algorithm. The influence of hydraulic structures such as culverts, measuring flumes, bridge crossings and drop structures has been considered in the CCHE1D model. Stagedischarge relations for hydraulic structures are derive