Sediment transport on transversely sloping beds in a carousel

Abstract

The transverse slope effect is one of the most important parameters in the modelling of fluvial morphology. It determines to a large extent the properties and patterns of channels and bars, bifurcation stability, and grain sorting. Quantification of the transverse slope effect has been realized by mathematical modelling in combination with bed levelling experiments in straight and curved flumes. Though an inverse dependence on the square root of Shields’ number is generally accepted, current transverse slope predictors contain multiple calibration factors due to a lack of experimental data and limitations in chosen transport conditions. In this thesis experiments are conducted in a carousel with of which the lid and floor can be rotated independently, which allows the generation of a wide range of secondary flow intensities. This ability leads to the possibility of conducting experiments under a much wider range of morphological conditions compared to regular flumes, as different bend radii and straight channels can be simulated. The presence of helical flow allows the self-formation of a transverse slope. Over 100 experiments were conducted using 1mm or 4mm grains. The resulting bed slope effect was derived from bed elevation measurements in combination with 1D calculation of helical flow. The results show a large dependence of the transverse slope effect on sediment transport mode. Transition from rolling sediment to saltation led to an increase in magnitude and a lower sensitivity to Shields’ number. Similarity of the transverse slope effect between grain sizes was found by relating the slope effect to the ratio between Shields’ number and the critical Shields number of sediment transport initiation. Because of the effect of bed forms on local helical flow intensity, and the resulting nonlinear effects on cross channel sediment transport and local transverse slope steepness, a 3D flow model should be used in further research on the bed slope effect in carousels.