Modeling the evolution of incised streams: I. model formulation and validation of flow and streambed evolution components

A robust computational model for simulating the long-term evolution of incised and restored or rehabilitated stream corridors is presented. The physically based model simulates the three main processes that shape incised streams: hydraulics, sediment transport, and streambed and bank adjustments. A generalized implicit Preissmann scheme is used for the spatial and temporal discretization of the flow governing equations to accommodate large time steps and cross sections spaced at irregular intervals. The solution method introduces several enhancements that increase its robustness, specifically to simulate flashy flows. Transport of cohesive or cohesionless graded bed material is based on a total-load concept, and suspended and bed load transport modes are accounted for through nonequilibrium effects. The model simulates channel width adjustment by hydraulic erosion and gravitational mass failure of heterogeneous bank material. The present paper focuses mainly on the treatment of streamflow hydraulics and evolution of graded streambeds, and reports simulations of published experiments on degrading and aggrading channels with graded bed material. Description and validation of the model's streambank erosion component and the application of the model to incised stream systems are presented elsewhere. DOI: 10.1061/(ASCE)0733-9429(2008)134:6(749) CE Database subject headings: Computer models; Channel morphology; Open channel flow; Bank erosion; Sediment transport; Streams.