A numerical model based on the two-phase flow equations is developed for predicting the intense sediment motions in steady and unsteady flows. As the governing equations are coupled and highly nonlinear, the failure of some earlier attempts in applying two-phase flow models to reproduce the laboratory measurements of flow properties has been found to be largely due to the deficiencies in the numerical schemes rather than the parameterisations of physical processes. In this paper, a numerical scheme for the complete set of two-phase flow equations is presented. It is a two-step implicit-explicit finite difference scheme and particularly designed to resolve flow details in the highly concentrated region at a mobile sand bed where vertical gradients of concentration and flow velocities are very large. Two typical coastal/river flow cases were used to verify the model by comparing the computed sediment motions with experimental data. A detailed evaluation of the convergence characteristics, stability and robustness of the scheme is also provided for these test cases. The initial results indicate that the proposed scheme is stable and sufficiently accurate for predicting the fluid and sediment motions under both steady and unsteady conditions. (C) 2000 Elsevier Science Inc. All rights reserved.