A simplified approach to computing flow and bedload along gravel dune-like bedforms

Abstract

River dunes are important bedforms. Problems associated with the development and evolution of dune bedforms include increased flood risks, channel erosion, and damage to fish habitats. The purpose of this paper is to investigate the near-bed flow structure and bedload transport along gavel dune-like bedforms. The velocity field is computed using a relatively simple multi-layer hydrodynamic model, with a parameterization of flow separation in the leeside of dunes. The computation is of high efficiency and avoids uncertainties caused by flow separation. Fractional transport rates for a sediment mixture of sands and gravel are calculated using surface-based techniques. The computed flow velocities and bed shear stresses are in good comparison with acoustic Doppler velocimeter measurements. Bedload transport is shown to increase non-linearly with distance toward the dune crest and reach the maximum at the crest. This implies that dune-length averaged bed shear stress is not suitable for bedload calculations. At low discharges, the bed shear stress is the limiting factor, resulting in insignificant bedload. At high discharges when the bed shear stress exceeds a threshold, the effect of sediment-grain hiding and sediment-size availability are important for bedload calculations. The discharge–transport relationship is highly non-linear. This paper has demonstrated selective transport and potential dune surface coarsening. The simplified modelling approach has a good potential for application to field conditions.