ANALYSES OF THE SIZE, ROUNDNESS, AND FLATNESS OF BEACH PEBBLES

Abstract

In this study the size, roundness, and flatness of beach pebbles on the beaches of Ukishima, Miho, Kochi, and Kumanoura on the Pacific coast of Japan are examined. These coasts afford ideal opportunities for a study on the processes of transportation and deposition of beach sediments, and, except for the case of Kumanoura, large-scale charts and various other useful data such as of wave period, wave height, and wind velocity, all of which may contribute to the solution of these problems concerning beach pebbles, are to be found easily. Conclusions drawn from this study are as follows:(1) The grain sizes of beach sediments are similar to those of stream sediments which are supplied to beaches. Ratio of gravels in beach sediments has a relation neither to that in the stream sediments, nor to the intensity of wave action, but is related to both beach slope and offshore bars.(2) As seen in Figs. 2, 6, and 10, several zones of grain size are distributed on a beach where the grain size of beach sediments gradually decreases from upper (inland) to lower part (seaside). Between the zone of sediment and the zone of swash exists a close correlation. Generally, these zones show a complicated feature on the foreshore and the lower backshore, but a simple feature on the upper backshore. This fact can quite simply be explained in terms of the number of waves reaching the shoreline.(3) Distance of transportation of the same grain size along the shore on each beach is mutually alike. It seems that the properties of wave resemble each other. From this phenomenon it may be possible to say that the gravels with a diameter of 256mm or less are transported to a distance on the Pacific coast of Japan. In general, the pebble size varies in accordance with the difference of pebble quantity, topography, and beach slope.(4) Even such hard rocks as chert become considerably rounded by abrasion. Change of roundness from the river mouth to a further point on the beach is not abrupt but rather gradual. If the sediments move on the bedrock or are deposited at the same place for a long time, roundness of pebbles increases rapidly.(5) A proportional relationship between roundness and flatness also exists, i. e., the rounder the pebble becomes, the flatter it is. But in case of smaller grain sizes, the relationship is obscure. It is because that abrasion process is different between larger pebbles and smaller ones or granules. It is probably ascribed to the fact that the larger ones move only occasionally, so that their exposed surfaces are worn off by the wash of finer materials, but finer pebbles and granules are worn off quite uniformly by rolling, resulting in a spherical shape.(6) A study of 32-16 mm graywacke pebbles shows that, if the average roundness is more than 0.60 and the average flatness is more than 0.40, these pebbles can be considered as beach pebbles.