ROUNDNESS OF PEBBLES IN THE COURSE OF RIVERS IN JAPAN

Abstract

The roundness of pebbles has been measured for the purpose of finding the effects by abrasion and transportation in streams. In the lower course of ten rivers : the Abe, Oi, Kiso, Nagara, Ibi, Sho, Tedori Rivers in the Chubu District, and the Inugami, Hino Rivers in the Kinki District and Monobe River in the Shikoku District, two or three stations were chosen for sampling the pebble population, where we have measured the size of sediment and roundness of pebbles 32-64mm each of some kinds of rock types. The results obtained for these values are given in figures 3 to 13 shown by mean size, mean roundness, and ratio of roundness in each rock type with distance of transport. From these figures we considered the cause of abrasive action accompanying transportation. Their results are as follows: (1) On the relationship between roundness of pebbles and distance of transport in the same rivers, it is clear that the roundness differs with rock types. For example, it is clearly shown that the roundness of limestone is to increase rapidly downstream, but that of chert little changes is the roundness. The analysis of roundness diagram which shows the relation between ratio of roundness and distance of transport explains the abrading processes. Hard rock, such as chert, quartz-porphyry, liparite or graywacke, is worn mainly by chipping and grinding. On the other hand, soft rock such as granite, slate and shale, brings about the exfoliation, and is worn more by splitting and crushing than hard rock. (2) This study of the roundness of pebbles, by means of using graywacke which is found in many rivers, shows that the steeper the gradient of the river floor is, the more the roundness of pebbles is. The cause seems to be a rapid flow, variation from streaming to shooting in a stream form or increase in the intensity of turbulence on the steeper gradient of a river. But, on the Oi River the mean roundness shows no change downstream in spite of the steep gradient of the river floor. This may be that turbulence weakens the attrition of pebbles, as the river is wide downstream. (3) The ratio of the loss in the weight of pebbles varies with the river property, distance of trans-portation, and rock type. In the estimation by roundness, it seems that the loss in the weight of pebbles is about 7 per cent as far as the mouth of a canyon and 9 per cent as far as the mouth of a river in Japan. Moreover, on the basis of the results obtained from the estimation, selective transport accounts for 93 per cent of the size decrease and 2 per cent is accounted for by abrasion in the lower course of the river in Japan. (4) The ratio of gravel to sand in sediment has not so much effect on the attrition of pebbles as gradient of the river floor. For one thing, it seems that the ratio of sand in sediment in the river in which the roundness is measured, is about 10 per cent in difference. But, the former studies are made in the river where there is found no sand or much sand. Therefore, the relation stated above is only found in an extremly different ratio of sand. (5) The order of resistance for abrasion obtained from roundness is as follows: chert is heighly resistant, liparite, and quartz-porphyry are resistant, graywacke is moderate, granite, shale and slate are less resistant, and limestome is small. This order coincides to that of Kuenen's one.