DIFFUSION OF SUSPENDED SEDIMENT IN OFFSHORE OF THE KASHIMA BEACH

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The purpose of this paper is to discuss the diffusion of the suspended sediment due to wave action in the offshore of the Kashima beach exposed to the Pacific Ocean. Suspended sediments were captured with bamboo samplers which were set at ten sites located in the range of about 1. 5 km. _??_0.5 km. from the shore. Two types of sampler shown in Fig. 2 were set at each site. The data on amount of the trapped sediment at two sites are shown in Table 1. Since it is commonly stated that amount of sediment trapped by a bamboo sampler is completely proportional to actual sediment concentration in the field, a corresponding sediment concentr-ation (c) can be computed from the trap rate (q) by using the empirical equation (11) for the relation of sediment concentration to the trap rate which was obtained by the authors in the Kashima area. Thus, sediment diffusion coefficient (ES) in the field can be calculated by using eq. (3), and their vertical distributions are as shown in Fig. 5. On the other hand, Homma, Plorikawa, and Kashima assumed, for the case of oscillatory flow, the coefficient Em1 (a mo-mentum diffusion coefficient) as eq. (6) in which K is a constant. Fig. 5 displays also the solutions of equation (6) for four values of K indicated by solid lines. As the distributions of ES are somewhat irregular in this figure, it may be felt that they should be examined more closely. In order to examine this irregularity, the size analysis of the trapped sediments and the inspections of the wave condition during the bamboo sampling period were carried out. These examinations made it clear that the vertical gradients of the trap rate of sediment (also sediment concentration) show the distinguishable difference between the cases of coarser and of finer fractions as shown in Fig. 7, and the wave condition during the bamboo sampling period was not steady but consisted of both weaker condition and vigorous condition (Fig. 6) The authors, then, assumed that the finer fraction of the trapped sediments has been caught during the period of the weaker wave condition, while the coarser one during the period of the vigorous wave condition, and based on this assumption the values of e on each fraction were calculated separately. The verticals distributions of suchh values of cs are shown in Figs. 8 and 9. Making a comparison between Figs. 5 and 8 it is clear that the distribution of ES in Fig. 5 is governed by the finer fraction's ES at higher levels (ky>0.08), while in the layer near the bottom (ky<0.05) by the coarser fraction's ES. In Figs. 8 and 9, the distributions of Em1 and E'm2 under the conditions of various values of K and are also shown respectively, in which α is a correction factor, and ES plotted in Fig. 9 fits better with E'm2of eq. (8) than that of eq. (6) in Fig. 8. Therefore, the best fit values of kα in eq. (10) were calculated because k is not constant in sediment laden flow, and were obtained as follows, ka equals to 0.2 in the case of coarser sediment and vigorous wave condition of sample 1-3, kα=0.06: finer sediment and weaker wave, the sample 1-3, kα =0.06: coarser sediment and vigorous wave, the sample 8-2, kα=0.028: finer sediment and weaker wave, the sample 8-2. The distributions of the sediment concentration can be obtained by calculations using these values of kα and eq. (9), with which measured one compared in Fig. 11.
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DIFFUSION OF SUSPENDED SEDIMENT IN OFFSHORE OF THE KASHIMA BEACH

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