Flow over fluvial bedforms

• Main Author: Clunie, Thomas MacDougal
• Other Authors: Coleman, Stephen
• Published: ResearchSpace@Auckland 2012
• Online Access: http://hdl.handle.net/2292/13073

The presence of bedforms (ripples or dunes) in fluvial flows creates complex flow fields, altering local velocities, flow resistance and sediment transport. The characteristics of these flows have previously been studied with the use of fixed beds, due to the difficulties in measuring over naturally variable and mobile beds, and interpreting the results of such measurements. This thesis presents the results of an investigation into flows over naturally-mobile fluvial bedforms in laboratory flumes, with the results interpreted in the context of the double averaging framework. To measure flows over developing beds (from planar sand beds to a bedform configuration in equilibrium with the flow), a novel 'flying-probe' system was devised and used, with acoustic Doppler velocimeter (ADV) and depth-sounding probes mounted on a carriage that regularly traversed lengths of multiple bedforms. To measure flows over equilibrium configuration beds, the arrays of probes were held stationary above migrating bedforms, sampling throughout the passage of multiple bedforms. Measurements of velocity fields over fixed two-dimensional dune beds were used to confirm the validity of the flying-probe measurement strategy and to explore data analysis techniques such as double averaging. The results obtained provide a dataset for comparison with previous studies of flow over fixed bedforms and with the subsequent measurements of flow over mobile-bed bedforms. In flows over the fixed bed, spatial fields of turbulent kinetic energy, velocity skewness, velocity kurtosis, and Reynolds stress all reveal a region of heightened turbulence along the shear layer and wake zone in the lee of the dunes, in line with existing literature. Double-averaged vertical profiles of these characteristics show similar patterns to previous research over planar beds. The normalised, averaged flow field is shown to be similar for the present flows of different relative bed roughness. The novel velocity measurements over developing mobile beds (from plane-bed to ripples, dunes, and ripples transitioning to dunes) provide insight into the changing flow fields as beds develop. The flow parameters investigated, including double-averaged velocity, variance, skewness and kurtosis of velocity fluctuations, and fluid stresses, generally reached equilibrium values more quickly than bed parameters. Normalised double-averaged velocity was seen to decrease with bedform growth, which is consistent with expectations based on conventional rough-wall velocity distributions. Variances of velocity fluctuations were seen to increase during bed development, while normalised variances, termed fluctuation intensities', appeared to reduce during bed development, increasing again at later stages of development. The spatial variations in flow parameters measured over migrating equilibrium-sized dune beds match expectations arising from the fixed-bed experiments. They also match the conventional models of flow over dunes, with heightened turbulence (e.g. turbulence intensities, Reynolds stress, form-induced stress, turbulent length scales and energy dissipation rates) measured in the separation cell, the shear layer and the wake zone that form behind dune crests. Profiles of double-averaged velocity, turbulence intensity, velocity skewness and kurtosis, and fluid stresses are consistent with the results for fixed bedforms. Turbulent length scales were found to increase with distance above the bed, while energy dissipation rates were found to increase toward the bed. The present study reveals that flows over naturally-mobile fluvial bedforms can be explained by reasoned extension of previous studies of rough-bed flows and flows over fixed bed dunes.