NUMERICAL INVESTIGATION OF GRAIN SIZE EFFECT ON BED DEFORMATION IN MEANDERING CHANNEL
A special feature of flows in sand-bed channels is the mutual interaction between the flow and the erodible bed through sediment transport. This interaction is responsible for the occurrence of a variety of bed forms starting with ripples up to alternate bars. The form resistance due to these bed forms is dependent on their dimensions as well as on flow and sediment characteristics. The prediction of bed form geometry is an essential component for estimating flow resistance and water levels during floods in rivers. The computer code FAST3D has been developed firstly at the Institute for Hydromechanics, University of Karlsruhe, to calculate flow and sediment transport in open channels. In the code the flow field is calculated by solving the full Reynolds-averaged Navier-Stokes equations with k- turbulence model; the sediment transport module is comprised of semi-empirical models of suspended load and bed load; the bed-load transport is simulated with a non-equilibrium model; the bed deformation is obtained from an overall mass-balance equation for sediment transport. The governing equations are solved numerically with a finite-volume method on an adaptive, non-staggered grid. In order to take into account the influence of grain size distribution of the bed-surface on the evolution of the bed topography and consequently also on the flow field, a sediment transport module has been presently developed at the Institute of Hydraulic and Water Resources Engineering, Technische Universität München, for fractional sediment transport using a multiple layer model. The model has been tested and validated for different flow situations. In all cases, the agreement with measurements is fairly good. This paper presents results of model application to investigate morphological change in a sine-generated meandering channel with sand bed. The numerical results of bed changes were compared with laboratory measurements, which have been carried out by El-Tahawy (2004) for steady flow discharges. A reasonable good agreement between them was obtained. Further, the calibrated model was used to predict the bed deformation due to the change of sediment grain sizes, the results showed that increasing grain size increase the bed form height, decreasing the bed form length, and changing gradually the bed configurations from the alternate bars shape to the ripple shape.