The sedimentology and dynamics of mega-dunes, Jamuna River, Bangladesh
This thesis quantifies the occurrence, morphology and flow dynamics of dune bcdforms in the main rivers of Bangladesh: the Jamuna, Meghna and Ganges. Field work was conducted as part of the River Survey Project (Bangladesh Flood Action Plan 24) between November 1993 and March 1996, hence encompassing two flood hydrographs. River surveying was accomplished using a differential global positioning system to obtain vessel position and high resolution echo sounders and side scan sonar to monitor bed morphology. Flow, turbulence and suspended sediment transport were investigated using an Acoustic Doppler Current Profiler (ADCP) which measured both the instantaneous three-dimensional velocity structure and the relative suspended sediment concentration for the entire flow column beneath the survey vessel. Dunes are ubiquitous in the Jamuna and range in height from 0.15 to over 6m and in wavelength from 3 to 300 m. Both dune height and wavelength alter systematically with flow stage, increasing by factors of 6 and 8 respectively between low and high flow conditions. Lee side angle, which effects flow separation downstream of the dune crest and hence turbulence production and flow resistance, ranges from 2° to 58° and steepens with flow stage. There is little evidence of dune flattening with increasing flow stage, indicating that flow conditions do not reach the transition to upper-stage plane beds. An important corollary of this is that dunes are the dominant bedform at all flow stages and hence will dominate the preserved sediments of such alluvial channels. Existing models of dune morphology and occurrence are poor predictors at high flow stage in the Jamuna, most likely because of the low Froude numbers associated with such deep channels. Mean flow and turbulence fields, obtained from at-a-point profiling during different flow stages, have been examined for dunes with steep (>20°) and low (<10°) lee side angles. No flow separation was detected over dunes with low lee face angles. Over dunes with steep lee face angles, mean negative downstream velocities do not occur downstream of dune crests, although intermittent flow separation (flow reversals) has been measured at both low and high flow stages, with instantaneous velocity reversals reaching -60% of the depth averaged mean. The free shear layer of the flow separation zone is temporally and spatially unstable, the growth and decay of high velocity gradients acting across the shear zone controlling both eddy shedding of quadrant-2 type 'ejections' and also movement of the shear layer within the water column. These 'ejections' from the shear layer are associated with substantially higher suspended sediment concentrations than the background ambient value and are therefore responsible for increasing suspended sediment transport over dunes. Near the flow reattachment zone, quadrant-4 type 'sweep' events are most important in terms of event occurrence, contribution to Reynolds stress and suspended sediment transport and hence are responsible for scour in the dune trough. These results show that dune stability is controlled by the spatial variation in sediment transport, which in turn is dependent on the turbulence structure that is a consequence of flow expansion and separation downstream of the dune crest.