Grain-size distribution patterns of a point bar system in the Usri River, India

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Grain-size distribution patterns in a point bar system of the Usri River, India, were critically analysed in the light of log-normal, log-hyperbolic and log-skew-Laplace distribution models. Sand samples were collected from the cross-bedding foreset of different sizes of bedform; the objectives were to (i) study whether bedform heights have any role in grain-size distribution patterns, (ii) offer a best-fit statistical model, (iii) study the downstream variation of sizesorting in a point bar system, and (iv) study the mechanism of grain sorting.

The results indicate that the bedform heights have no role in grain-size distribution patterns. Quantitatively when the errors in three distribution models were analysed, it was observed that the log-normal distribution is the best-fit statistical model and the next one is the log-skew-Laplace. However, in the upper reaches of the river, log-normal distribution is the best-fit model in the case of large bedforms, whereas in the lower reaches the log-normal model is the best-fit one in the case of small bed forms.

It is also observed that within a point bar, for large and small bedforms, there is a tendency for mean grain size to decrease downstream. Between point bars for large bedforms there is no consistency in decreasing grain size downstream, whereas for small bed forms the decrease of grain size downstream is observed except near the confluence at Palkia.

With distance of transport, the coarser and finer fractions of sediments are gradually chopped off. The coarser fractions are buried below the advancing bedforms on the lee sides and the finer ones are transported further downstream. Thus the finer admixture giving rise to the fining-upward sequence overlies a carpet of coarser materials. This mechanism provides a clue to the process of grain sorting in the fluvial environment.

An interpretation has been offered for the log-normality of the grain-size distribution pattern. During prolonged transportation in a fluvial environment, the larger grain-size fractions are gradually chopped off and buried below the advancing bedforms on their lee sides. On the other hand, the finer fractions are transported further downstream in suspension. Thus the narrow, intermediate size fraction takes active part in the distribution patterns leading to the generation of unimodality and a symmetric distribution pattern downstream, which are the main criteria for log-normality. Similarly, increase of bedform size is the effect of increase of stream power and Froude number leading to the selective segregation of bed materials. Thus the intermediate size fractions take a more active part than the coarser and the finer size fractions in developing log-normality.

Besides the hydrodynamic parameters of the Usri, coarsening of grain size downstream has been attributed to (i) the aggrading nature of the Usri downstream, and (ii) the contribution of coarser materials to the Usri by its tributaries and bank erosion.

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