Abstract
Size characteristics of river bed materials has been studied by many researchers as a key to reveal the fluvial transportation processes, but their chief interests have been concen trated in how the grain size of bed materials be related to tractive force in fluvial transpor tation or to movement of grains in hydraulic conditions forming size characteristics. Be side such fluvial processes, however, we must consider the characteristics of materials them selves supplied to river bed, such as the mode of decomposition of rocks during fluvial trans portation and the grain size of rock-forming minerals of source rocks in the dranage basin. It has not been interpreted how the grain size distributions of river bed materials be related to fluvial processes or to source rock characteristics. For this reason, the authors made clear the grain size distributions of the Yahagi River bed materials and surveyed the lithol logical compositions of gravel and sand (mineral particle) in each size class, and analysed the grain size distributions of rock-forming minerals and weathered materials of granitic rocks which are widely distributed in the dranage basin of the Yahagi. The paper on the hydraulic interpretations of bedforms and size characteristics of this river is in prepara tion by the authors.
Fig. 1 shows the bedforms of this river and the analysed points of them, and Fig. 2 the longitudinal profile of the river. The profile is strongly affected by artificial construc tions such as fresh water dams. Bed materials are analysed in the field on coarser gravels and in laboratory on finer sands by the sieves of 1 /4 phi interval. Total weights of analysed materials are more than 100 kg to obtain precise features of their size characteristics.
Table 1 is the results of analyses (cummulative frequency) and Fig. 3 shows the histograms to represent the size frequency distribution. The histograms of the points No. 1 and 2 have three peaks showing tri-modal distribution and the coarsest peak of No. 1 is larger than No. 2. Finer two peaks lie at about -2_??_-l phi and near 0 phi. The histogram of No. 3 has large peak around -2 phi and becomes inconspicuous in the coarsest peak around -6_??_-5 phi. The peaks around 0 phi are perceptible in No. 1 and 2, but in No. 3. It is very interesting that almost all histograms have two peaks around -2 and 0 phi. The histograms showing uni-modal distribution also represent “shoulder” around -2 phi. This is striking characteristic of the Yahagi River bed materials. In other rivers such as the Kinu . (Inokuchi and Mezaki, 1974) and the Nagara River (Moriyama and Kambe, 1978), the bed materials shows, in general, bi-modal distribution having gravel peak (coarser than -4 phi) and sand peak (finer than 1 phi), and the size range around -2_??_0 phi represents wide “valley” in histogram, showing relative defliciencies of grains.
From the view point that the sediments are composed of some normally distributed component populations (Inokuchi and Mezaki, 1974), the authors separated quantitatively some component populations from the plots on normal probability graph paper. The bed materials of this river are composed of two or five component populations (Table 2 and Fig. 4). Although the coarser A and A'-populations will be discussed in next paper in reference to their hydraulic conditions, it is clear that the coarsest A-populations (in No. 1, 2 and 3) show downstream reduction in grain size and in mixing proportions.
