Johannes Albert van Hateren
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Unze van Buuren
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Sebastiaan Martinus Arens
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Ronald Theodorus van Balen
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Maarten Arnoud Prins
Abstract. The way in which sediment is transported (creep,
saltation, suspension), is traditionally interpreted from grain size
distribution characteristics. However, the grain size range associated with
transitions from one transport mode to the other is highly variable because
it depends on the amount of transport energy available. In this study we
present a novel methodology for determination of the sediment transport mode
based on grain size and shape data from dynamic image analysis. The data are
integrated into grain size–shape distributions, and primary components are
determined using endmember modelling. In real-world datasets, primary
components can be interpreted in terms of different transport mechanisms
and/or sediment sources. Accuracy of the method is assessed using artificial
datasets with known primary components that are mixed in known proportions.
The results show that the proposed technique accurately identifies primary
components, with the exception of those primary components that only form
minor contributions to the samples (highly mixed components). The new method is tested on sediment samples from an active aeolian system
in the Dutch coastal dunes. Aeolian transport processes and geomorphology of
these type of systems are well known and can therefore be linked to the
spatial distribution of endmembers to assess the physical significance of
the method's output. The grain size–shape distributions of the aeolian dune
dataset are unmixed into three primary components. The spatial distribution
of these components is constrained by geomorphology and reflects the three
dominant aeolian transport processes known to occur along a beach–dune
transect: bedload on the beach and in notches that were dug by man through
the shore-parallel foredune ridge, modified saltation on the windward and
leeward slope of the intact foredune, and suspension in the vegetated
hinterland. The three transport modes are characterised by distinctly
different trends in grain shape with grain size: with increasing size,
bedload shows a constant grain regularity, modified saltation a minor
decrease in grain regularity, and suspension a strong decrease in grain
regularity. These trends, or in other words, the shape of the grain
size–shape distributions, can be used to determine the transport mode
responsible for an aeolian sediment deposit. Results of the method are
therefore less ambiguous than those of traditional grain size distribution
endmember modelling, especially if multiple transport modes occur or if
primary components overlap in terms of grain size but differ in grain shape.
Identifying sediment transport mechanisms from grain size-shape distributions