Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/20/2007
Publication Date: 7/1/2008
Citation: Langendoen, E.J., Alonso, C.V. 2008. Modeling the evolution of incised streams: I. Model formulation and validation of flow and streambed evolution components. Journal of Hydraulic Engineering, 134(6): 749-762. Interpretive Summary: The 1992 National Water Quality Inventory of 642,881 miles of US rivers reported that 44 percent of stream miles inventoried were in a degraded state, mainly caused by excess sediments and nutrients. Sediment problems result from soil erosion from watersheds and streambanks. Today, stream corridors are increasingly recognized as critical ecosystems supporting interdependent uses and values, and interest in restoring stream corridors is expanding rapidly. A particular challenge we face today is the lack of integrated, comprehensive modeling tools to evaluate the long-term effectiveness of proposed corridor rehabilitation designs. Therefore, the U. S. Department of Agriculture-Agricultural Research Service has been developing CONCEPTS (Conservational Channel Evolution and Pollutant Transport System) to study the effects of riparian forests and in-stream restoration measures on stream stability. CONCEPTS is a computer model that simulates flow and sediment transport in open channels such as rivers, and their ensuing evolution. This modeling capability is a critical need for hydraulic engineers, landscape architects, and stream ecologists involved in stream rehabilitation projects. The sediment transport and bed evolution components of CONCEPTS have been validated against published experiments on aggrading and degrading channels.
Technical Abstract: 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 non-equilibrium 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.