Downstream fining in large sand-bed rivers
Introduction
Rivers commonly show a downstream fining of bed material (Morris and Williams, 1999). Although downstream fining is more obvious in gravel-bed rivers, it also occurs in sand-bed rivers (Fig. 1). The underlying processes, however, are not necessarily the same due to the greater sediment mobility in sand-bed rivers, the narrower size distribution of the bed sediment and the different geographical setting: sand-bed rivers predominating in lowland areas and gravel-bed rivers predominating in upland areas.
Both geomorphologists and engineers have studied the fining phenomenon extensively, but nearly all downstream fining studies have focused on upland gravel-bed rivers. Downstream fining in sand-bed rivers deserves more attention, because a downstream change in grain size from very coarse sand to silt changes the dominant mode of sediment transport, the bedform dimensions and also the grain size of overbank deposits.
In this paper, the processes that affect downstream fining in gravel-bed rivers are reviewed, and it is evaluated to what extent they are relevant for sand-bed rivers. Furthermore, several ‘new’ downstream fining processes are discussed. This will end up in a conceptual model of downstream fining in sand-bed rivers, which may help to provide directions for future research. The focus is on large, alluvial, sand-bed rivers such as the downstream reaches of the Mississippi, the Rhine, the Ganges and the Niger (Fig. 1).
In this paper, downstream fining processes are divided into three categories: abrasion, selective transport, and sediment addition–extraction. Abrasion processes cause a progressive breakdown of particles during their downstream transport, while selective transport processes cause a preferential downstream transport of fine particles. Sediment addition and extraction processes, finally, may obscure the effects of abrasion and selective transport, for instance at river confluences.
Section snippets
Abrasion
Since the work of Leonardo da Vinci in the early 16th century (Richter, 1939, pp. 165–166) until the middle of the 20th century, abrasion was regarded to be the primary cause of downstream fining. This induced a large number of abrasion experiments, demonstrating that abrasion rates do not only depend on grain size as was assumed before (e.g. Sternberg, 1875), but also on lithology, grain velocity, grain roundness, grain shape, amount of weathering, presence of other-sized grains and amount of
Selective transport
Daubrée (1879, op. cit. Krumbein, 1941) already recognised that downstream fining is not only caused by abrasion, but also by selective transport: the preferential downstream transport of fine particles. Selective transport mechanisms are different for low flow periods (threshold-of-motion conditions) and high flow periods (intermediate and high transport stages) and both are discussed below for conditions of unsorted beds. Afterward, the effects of local sorting processes on the selectivity of
Sediment addition and extraction
The introduction of sediment of different origin into a river can obscure the effects of selective transport and abrasion. The same counts for the size-selective extraction of sediment from the main channel. In gravel-bed rivers the main sediment addition and extraction processes are: tributary confluences, non-alluvial sediment sources and anthropogenetic dumping and dredging of sediment. In the following sections these processes and their relevance for large sand-bed rivers are described
Synthesis and discussion
After having discussed a range of processes that affect downstream fining in large sand-bed rivers, three questions remain to be solved: (1) Which processes are dominant? (2) How do these processes lead to downstream fining? (3) Can existing numerical models simulate these processes?
Conclusions
The literature discussed in the preceding chapters is summarised in the following conceptual model of downstream fining in large sand-bed rivers.
The influence of abrasion on downstream fining in large sand-bed rivers is small: estimated abrasion rates typically are a factor 10 to 100 less than observed downstream fining rates (but abrasion rates may be slightly underestimated due to methodological shortcomings). Only in the upstream part of large sand-bed rivers (near the gravel–sand
Acknowledgements
Thanks are due to Maarten Kleinhans, Janrik van den Berg, Ward Koster, Leo van Rijn and Hanneke Schuurmans (Utrecht University) for their critical reviews which greatly helped improve the manuscript. The suggestions and helpful comments of Rob Ferguson (Durham University) and one anonymous reviewer on an earlier draft of this manuscript are deeply appreciated. The study was funded by the Dutch Ministry for Public Works and Water Management (Rijkswaterstaat).
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