Initiation and development of sand dunes in river channels

Jeremy George Venditti
2003
This study investigated the initiation of bedforms from a flat sand bed and the transition between two-dimensional (2D) and three-dimensional (3D) bedforms in stream channels. Experiments were undertaken in which a narrowly graded, 0.5 mm sand was subjected to a 0.155 m deep, non-varying mean flow ranging from 0.30-0.55 m s⁻¹ in a 1 m wide flume. The initial flow conditions over the flat beds, prior to bedform development, were examined using laser Doppler anemometry to ensure that the flow
more » ... e that the flow agrees with standard models of flow and turbulence over hydraulically rough flat beds. Two types of bedform initiation were observed. The first occurs at lower flow strengths and is characterised by the propagation of defects via flow separation processes and local sediment transport to develop bedform fields. The second type of bedform initiation begins with the imprinting of a cross-hatch pattern on the flat sediment bed under general sediment transport which leads to chevron shaped forms that migrate independently of the initial structure. The chevron shapes are organised by a simple fluid instability that occurs at the sediment transport layer-water interface. Predictions from a Kelvin-Helmholtz instability model are nearly equivalent to the observations of bedform lengths in the experiments. The 2D bedforms initiated by the Kelvin-Helmholtz instability developed into dune features that grew exponentially towards equilibrium dimensions. Dune heights and lengths increased with flow strength while their migration rate decreased. There was no obvious transition from small ripples at the beginning of the runs to dunes when the sandwaves are larger. Morphological estimates of sediment transport associated with the dunes and estimates associated with 'sand sheets,' which are superimposed bedforms on the dunes, were identical, indicating that the material moved over the dunes is controlled by the sheets. Bedform phase diagrams suggest that 2D dunes should be formed under the hydraulic and sedimentary conditions observed in the ex [...]
doi:10.14288/1.0091228 fatcat:mjnbkf3ox5b2fhaypogihw5b7i