Numerical solution of lateral distribution of suspended sediment in river bends

Background and Objectives: Distribution of lateral velocity and especially suspended sediment concentration in river bends are extremely non-uniform. For better management of hydraulic structures located on river bends, the lateral variation of sediment concentration has high importance. Several two-dimensional and quasi two-dimensional mathematical models have been developed by many researchers which in most of them, the river has been assumed as a straight reach. Also, sediment concentration variations across the river and especially in the case of river bends have been simulated just by complicated two and three dimensional mathematical models. Due to the essence of simplicity for applied studies in flow and sediment transport hydraulics, the simple quasi two-dimensional models have attracted much attention, recently. The aim of this paper is to investigate the quasi-two dimensional modeling of lateral flow velocity and suspended sediment concentration in river bends.
Material and Methods: for lateral distribution of flow velocity and sediment transport at the river bends, the Shiono and Knight method has been numerically solved in this paper using the finite difference method. For calibration of Manning roughness coefficient in this model, the field data of lateral velocity distribution at the one of bends in Karoun river at the vicinity of Ahwas hydrometric station have been used. Furthermore, by selecting three empirical sediment transport equations of Ackers-White, Engelund-Hansen and Yang, the sediment transport module of this model has been calibrated with field data of lateral distribution of sediment concentration at the bend
Results: The results of numerical solution of Shiono and Knight model showed that the among the selected sediment transport equations, the Yang equation has very well agreement with the measured lateral suspended sediment concentration, in comparison to the Ackers-White and Engelund-Hanssen equations. The former two equations predict the sediment transport very larger than the measured value. Due to this fact that the unit stream power is the basis of Yang equation derivation and in this theory, the transport of each sediment particle mainly depends to its flow velocity, hence the Yang sediment transport equation has higher accuracy. With lateral integration of sediment concentration across the river bend, the total load has been computed as 16000 tons per day against the measured value of 26500 tons per day which shows nearly 40 percent error. These errors for Ackers-White and Engelund-Hansen equations are 265 and 325 percent, respectively, which are very high predictions comared with the actual value.
Conclusion: according to the results obtained in this paper, the sediment concentration distribution in any flow discharge and especially in flood conditions may be computed and used for designing lateral intakes on outer bank of the river bend.