In order to explain the landing posture of artificial reef block on the sea bottom throw down from the barge, we made the experiment to clarify the rolling motion of falling block in the water, and measured the period and amplitude of the rolling motion of several model block bodies in experimental cylindric water tank. The results were as follows : Rolling period and amplitude were related to the parameter (I/v2) obtained by the inertia moment I (g・cm2) and steady falling velocity v (cm/s) of the block bodies. Rolling period was slightlyincreased with growth of I/v2, but rolling amplitude slightly decreased with this. It was assumed that when the large reef block was falling in sea water, the rolling inotion was longer in period and smaller in amplitude than the smaller model block bodies. Furthermore, it was suggested that the rolling motion of the falling block body in the water can be estimated by dynamic analysis theoretically.

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Optical sensors to measure the concentration of suspended solids (SS) were made as a trial using infrared emitting diodes (IRED), a laser diode (LD) and photo-tranSistors (PT). The sensors have been utilized in our experiments and field surveys to study the erosional process of cohesive bottom sediments. Three types of optical sensor are developed and applied to measure SS in laboratory and field. The first two types of sensor are for laboratory use and consisted of IRED and LD, respctively. They realized high emitting power, sharp directivity of emission and high sampling frequency of measurement. The sensors for field surveys built up of IRED could measure vertical profiles of SS with high sampling frequency. These sensors attained our objects of each measurement with good performance. Measurements indicated that turbulences near bottom were cIosely related to erosions of cohesive bottom sediments.

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[in Japanese]

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Growth line analysis of Tapes (Ruditapes) phillipinarum showed that growth was stunted during the summer for individuals living in the higher tidal zone of the Kuzuma area at the Obitsu River mouth,wWhere Zostera beds are well developed. Shells from the Kaneda area exhibited rapid growth in the spring and early summer, and their growth during mid summer was greater than those in the Kuzuma area. Diatom assemblages in the gut of Tapes Phillipinarum were similar to the benthonic diatom assemblages at the same location, suggesting that the diet for Tapes phillipinarum was based mainly on the benthonic diatom fraction. δ13c values for the Tapes Phillipinarum ranged between -13 to -15‰ or -14 to -16‰ as estimated diet values, indicating that they have a mixed diet including benthonic diatoms (-19 to -16‰) , zooplankton (-15 to -17‰) and/or fragments from zostera beds (-11 to -15‰). Amual variations in the biomass, sampled in l m grid units, showed that the benthonic diatom fraction increased during the spring and summer, and that the quantity of benthonic diatoms in the Kaneda area was greater than in the Kuzuma area.

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Fishways are located at each end of the dam across the lower part of the Kitakami River. The fish entrances of each fishway is situated 14m downstream from the foot of the dam, at each end of the dam, and the water flow of each fish entrance reaches to the center of the river through a collection gallery at ebb tide. The vertical slot fishway is 44.6m long with l16 sIots between baffles. The actual velocity of the falling water in a slot varies from l.06m/sec to 2.40m/sec. 732 ayu fish and 90 fish of other species swam up the two fishways at two days of June 12 and 20, 1991. 2,458 chum salmon and 55 fish of other species swam up the two fishways at five days during the period from September 12 to October 23, 1991. It is known from the results that the fishways located at each end of the dam across the lower part of the Kitakami River are navigable for ayu fish, chum salmon, and five other species of fish.

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Fishways are located at each end of the rubber dam across the lower part of the Naruse River. The fish entrance of each fishway is cIose to the foot of the dam. The pool-type Channel of each fishway is 134.5m long and has 39 pools. The size of each pool is 3.0m in width, and 2.5m in length. The difference in hight between two weirs of the chamel is 0.05m, and the actual velocity of the water falling between pools is 0.1m/sec in the lower part of the channel and l.0m/sec in the upper part of the channel. According to a sample conducted at ten-minute intervals, 322 ayu fish and 57 fish of other species swam up the fishway on the north end of the dam, and 98 ayu fish and 35 fish of other species swam up the fishway on the south end of the dam on June 8, 1991. Nine chum salmon and a masou salmon swam up the fishway on the south end of the dam on October 30, 1991. It is known from the results that the fishways which are located at each end of the rubber dam across the lower part of the Naruse River are navigable for ayu fish, chum salmon, and ten other species of fish.

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