Abstract
A common characteristic of many rivers is the tendency for bed sediments to become finer in downstream direction. This phenomenon, which is generally known as downstream fining, has a strong effect on the morphologic and hydrodynamic behaviour of a river.
The fundamental causes of downstream fining are abrasion (the progressive
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breakdown of particles during their downstream transport) and selective transport (the preferential downstream transport of fine particles), but the effects of these processes are sometimes overshadowed by the external addition of sediment to the river (e.g. at tributary confluences), or by the extraction of sediment from the channel (e.g. by dredging). The intensity of abrasion, selective transport and sediment addition-extraction processes is determined by external controls, such as climate, tectonics and human impact. The downstream fining rate that is observed in a river thus is a function of: (1) abrasion and selective transport, (2) sediment addition-extraction processes, and (3) external controls.
Laboratory and field studies have greatly elucidated the effects of these processes and controls on downstream fining in gravel-bed rivers, but considerably less attention has been paid to downstream fining in sand-bed rivers and to the abrupt transition from a gravel-bed river to a sand-bed river. The general objective of this thesis was to determine the influence of abrasion, selective transport, sediment addition-extraction processes and external controls on downstream fining in sand-bed rivers and gravel-sand transition zones. The focus of the thesis is on large, alluvial, meandering, lowland rivers, such as the downstream reaches of the Ganges, Rhine, Mississippi and Niger. The lower river Rhine served as study case.
To meet the general research objective, different approaches were followed. First, an extensive literature review was done in order to identify all relevant processes and controls. Second, a large data set of sediment transport measurements was analysed to study selective transport processes and an important addition-extraction process: the complex sediment distribution at river bifurcations. Third, geological, historical and modern data were combined to study the external control of human interference on downstream fining. Fourth, a porosity model was developed to determine how the bed structure of a river changes in downstream direction. Fifth, the porosity model was combined with sedimentological data to estimate the effect of bed structure and sediment sorting on the transport selectivity in gravel-sand transition zones. Finally, a numerical downstream fining model was constructed in order to determine the interactions between selective transport processes, addition-extraction processes and external controls on downstream fining.
With respect to sand-bed rivers, it was found that the intrinsic cause of downstream fining is selective transport, which takes place during bed-load transport and suspended load transport, and is strongly enhanced by sorting processes on the river bed. The rate of downstream fining, however, is strongly dependent of addition-extraction processes and external controls. The main addition-extraction processes are: river bifurcations, overbank deposition, channel migration and dredging. The main external controls are: sea level (rise), tectonics, tides, engineering works, sediment mining and navigation.
With respect to gravel-sand transition zones, it was found that their location is often externally forced, but their abruptness must be due to strongly selective transport. The strong degree of selective transport is the result of sediment sorting phenomena (small-scale patchiness, bend sorting, dune sorting) and rapid variations in pore structure.
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