Quantitative estimate of sediment transport processes is important, especially in river-control engineering and water management projects, because it determines the river morphological evolution and affects the estuaries and coastlines morphodynamics. Many predictive mobile-bed one-dimensional models have been developed but, even today, they have not attained an high degree of efficacy being confronted with some difficulties (reliable prediction of bed roughness due to bed forms or/and to the presence of flexible vegetation; reliable interpretation of hydraulic sorting process with non-uniform sediment mixtures; reliable interpretation of water-bed sediment interchanges and of interactions between the bed load and the suspended load in non equilibrium situations). Models can be classified according to the numerical technique used for their solution: uncoupled models, where the equations for the water flow and for sediment are uncoupled during a given time step, or coupled models, where the complete de Saint-Venant equations for water flow and the sediment continuity equation are solved simultaneously. The uncoupled solutions of the governing equations are often based on total sediment load and are unable to simulate the mutual interchanges among suspended load, bed-load and bed material. Coupled models, although consider the transport of non uniform sediments and the sorting process, do not account explicitly for the complex interplay of all factors producing the variation of the alluvial bed roughness in time and in space, which is function both of flow intensity and of sediments distribution on the bed.

In this work a new approach to simulate the erosion processes, the bed levels changes and the entity of sediment transported, taking into account the non-uniformity in sediment size and the interchange between the bed and the stream, is presented. Two options, steady and unsteady, are available. The model's performance is verified through the comparison with laboratory and field data.