【正文】 p*bk is pbk of the mixing layer when ?Am/ ? t ? ? Ab/?t≤0 , and p*bk is the percentage of the kth size class of bed material in subsurface layer (under mixing layer) when ?Am/ ? t ? ? Ab/?t 0. Eq. (1) is discretized using the Preissmann implicit scheme, with its source term being discretized by the same formulation as that for the righthand term of Eq. (3) in order to satisfy the sediment continuity. Eq. (4) is discretized by a difference scheme that satisfies mass conservation. A coupled method for the calculations of sediment transport, bed change and bed material sorting is established by implicitly treating the pbk in Eq. (2) as pbkn+1and simultaneously solving the set of algebraic equations corresponding to Eqs. (1)(4) by using the direct method proposed by Wu and Li (1992). This coupled method is more stable and can more easily eliminate the occurrence of the puted negative bed material gradation, when pared to the decoupled method, in which the pbk in Eq. (2) is treated explicitly. However, the aforementioned coupling procedure for sediment transport, bed change and bed material sorting putations is still decoupled from the flow calculation. Model Parameters to be Analyzed The parameters in numerical models of flow and sediment transport in rivers can be classified into two groups: numerical parameters and physical parameters. The numerical parameters result from the discretization and solution procedures, while the physical parameters 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。 Q*tk is the potential sediment transport rate, which is determined with SEDTRA module (Garbrecht et al., 1995), Wu, Wang and Jia’s formula (2000), the modified Ackers and White’s 1973 formula (Proffitt and Sutherland, 1983), or the modified Engelund and Hansen’s 1967 formula (with Wu, Wang and Jia’s correction factor, 2000). The bed deformation due to size class k is determined with (3)where p′ is the bed material porosity, which is calculated with the methods of Komura and Simmons (1967), Han et al (1981), or is specified by the user according to available measurement data。 Ls is the adaptation length of nonequilibrium sediment transport。 Qtk is the actual totalload transport rate。 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 Ero