DISTINCTION OF ELEMENTARY SAND POPULATIONS IN THE GIRONDE ESTUARY (FRANCE) BY R-MODE FACTOR ANALYSIS OF GRAIN-SIZE DATA

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ABSTRACT

An R-mode factor analysis was made on the grain-size classes of the sediments in the Gironde estuary in order to group the sieve sizes into dynamically significant factors. It was found that three main groups of sand populations exist within the distributional province investigated. Comparison with previous works suggests that each group reflects a specific transport mechanism. The main group boundaries are the same as those observed on the log-probability cumulative size curves of the sediments. The three main groups consist of the following sizes: finer than 3.0 Φ, 3.0 Φ-0.6 Φ, and coarser than 0.6 Φ. The three groups are thought to represent respectively “pure” (or uniform) suspension transport, mixed suspension bed-load, and “pure” bed-load (surface creep) transport. Furthermore, a subdivision exists in the mixed-suspension-bed-load group: the sediments between 3.0 Φ and 2.0 Φ seem to represent a “graded suspension” transport while the sizes between 2.0 Φ and 0.6 Φ are thought to represent “saltation” (graded suspension plus bed-load). All the samples containing more than 5% of the surface creep population plot either as “graded-suspension plus rolling” or as “rolling” in the CM pattern of the sediments.

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