GRAIN SIZE DISTRIBUTION OF LITTORAL SUSPENDED SAND

Abstract

The purpose of this paper is to describe the grain size distribution of littoral suspended sand. The samples were collected at the offshore area of the Tatado Coast near Shimoda, Izu Peninsula, Central Japan, where bottom material is stirred up to some extent by wave action. The collection work was conducted by using a vacuum system fixed to the sea bottom for drawing up the sea water from the nozzles arranged at various heights (Table 1).
The results of the grain size analyses of the samples are plotted on Fig. 1, in which the ordinate represents the probability density function. Each sample consists of two populations, i.e. sand particle population (coarser than about 3.5_??_) and finer particle population (finer than about 3.5_??_); this is similar to those of wind blown sand and river suspended sand which have been described in the previous paper (Inokuchi, 1980).
In Fig. 1, it seems that the points of the sand particle population for each sample deviate systematically from the broken line which represents theoretical log-normal curve for that population. This tendency of deviation corresponds to that indicated by Bagnold and Barndorff-Nielsen (1980) and suggests that the distribution pattern of the points conforms to the hyperbolic distribution. A pair of solid straight lines is the hyperbolic asymptotes for each sample.
Table 2 shows the values of the slopes of the asymptotes, in which S means value of the finer wing slope and C the coaser wing slope. It is found that for all samples the values of the slopes are confined into a limited range and that for each sample S and C are approximately equivalent. This fact may imply a regularity in suspension of sand particles affected by fluid actions.
Grain size distribution of the four samples from beach sand was also examined. As shown in Fig. 2, each of them consists of sand particle population and their grain size distributions show hyperbolic type. However, the pattern of the size distribution is different from that of suspended sediments as shown in Fig. 1.