Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/13/2012
Publication Date: 7/23/2012
Citation: Zhang, S., Duan, J.G., Strelkoff, T. 2012. A grain scale non-equilibrium sediment transport model for unsteady flow. Journal of Hydraulic Engineering. DOI:10.1061/(ASCE)HY.1943-7900.0000645. Interpretive Summary: The transport of field soil by furrow irrigation is a significant agronomic and environmental problem in areas characterized by susceptible soils, notably the Pacific Northwest. The agronomic and economic consequences stem from the massive transport of topsoil from the upper reaches of an irrigated field to the lower portions, where it can be deposited, as the water slows in the normal irrigation process. In individual instances 65 tons/acre were lost in one hour of irrigation, 22 tons/acre in a 24-hour irrigation, ½ to 12 tons per acre per irrigation in Wyoming. Surveys report 75% of Idaho furrow-irrigated fields lost the entire A horizon in the upper reaches together with a 2 to 4-fold increase in “topsoil” at the lower ends, reducing productivity by 25% over pre-erosion values, and reducing yields by 20 – 50% in areas with lost top soil. Growers have resorted to trucking displaced soil back up to the upper reaches to regain productivity. Environmental consequences stem from the discharge of sediment and absorbed agricultural chemicals in the irrigation tailwater into the environment of receiving water bodies. Modeling by NRCS engineers and other consultants can lead to rational recommendations for design and management of surface irrigation. The submitted research describes a theoretical computer model designed to explore the physics of channel erosion when there are large morphological changes in the channel as well as non-equilibrium conditions for the sediment transport. Analyses of irrigation-induced erosion should lie between the simplicity of traditional models and the exaggerated complexities of the present model.
Technical Abstract: A one dimensional (1-D) finite-volume model was developed for simulating non-equilibrium sediment transport in unsteady flow. The governing equations are the 1-D St. Venant equations for sediment-laden flow and the Exner equation including both bed load and suspended-load transport. The Rouse profile for sediment transport was modified to consider the non-equilibrium transport of suspended sediment. The spatial lag between the instantaneous flow properties (e.g. velocity, bed shear stress) and the rate of bed load transport in unsteady flow is quantified by using an adaptation length, which is derived theoretically by considering the momentum balance of bed load layer. This new method for calculating the adaptation length was verified using data from several experiments, and was shown to be superior to the empirical formulas for a wide range of shear stress. The non-equilibrium model was applied to simulate a series of laboratory dam-break flows over erodible beds, and the simulated results agreed well with the experimental measurements.