Downstream fining in a megaclast - dominated fluvial system: the Sabeto River of western Viti Levu, Fiji

Gale, Stephen J. and Ibrahim, Z.Z. and Lal, Jashika and Sicinilawa, Ulamila (2019) Downstream fining in a megaclast - dominated fluvial system: the Sabeto River of western Viti Levu, Fiji. Geomorphology, 330 . pp. 151-162. ISSN 0169-555X

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Abstract

Megaclast-dominated fluvial systems have been largely ignored by researchers. Yet megaclasts exert an important
control on channel geomorphology, flow behaviour and sediment transport, and such systems are probably
far more common than this neglect would suggest. Despite the explicit identification of megaclasts as the
products of cataclysmic flow, observation of modern fluvial systems confirms that megaclasts can be entrained
by flows that lie within the normal flow regime.
The pattern of fining along the megaclast-dominated Sabeto River of western Viti Levu, Fiji reveals a strong
negative exponential relationship between particle size and distance downstream, and a rate of downstream
fining comparable with that of other gravel-bed rivers. The downstream fining pattern is straightforward and
step-like discontinuities in the fining gradient are absent. This is suggestive of a sedimentary system only minimally
disrupted by the supply of sediment from tributaries or the reworking of material from sources such as
hillslopes and alluvial terraces. There is evidence, however, of anomalously coarse sub-populations at two
sites. These coarser components must pre-date the other deposits at the sites.We speculate that they represent
the products of earlier and higher-magnitude events. The bed sediments may thus be composite features, made
up of individual components each transported by a specific event of particular magnitude.
Our work confirms the relationship between the rate of downstream fining and particle size. This may arise because
the distance overwhich individual particles are transported is likely to be size dependent. It is also possible
that abrasion may be more effective in the case of larger particles than smaller ones, with the result that the
coarser fraction of the river's bed sediment fines downstream more rapidly than the finer fraction.
There is a long-standing debate about the relative roles of abrasion and sorting in downstream fining. Our data
reveal little evidence for a downstream increase in the roundness of the river's gravels, implying that abrasion
is likely to play a minor role in the fining process in this system.

Item Type:	Journal Article
Subjects:	G Geography. Anthropology. Recreation > GE Environmental Sciences Q Science > QE Geology
Divisions:	Faculty of Science, Technology and Environment (FSTE) > School of Geography, Earth Science and Environment
Depositing User:	Naohiro Nakamura
Date Deposited:	11 Mar 2019 23:41
Last Modified:	11 Mar 2019 23:41
URI:	https://repository.usp.ac.fj/id/eprint/11382

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