It is aimed in this paper to clear the giomorphological character of Yamatsunami (a type of the landslides) and its influence on the locational and formational pattern of settlements in the Coastal Region of Hiroshima Bay. Yamatsunami may be safely classified into three types according to the place occurred: on cliff, on slope and in valley. In this paper, the case of Yamatsunami in valley will be studied principally. According to historical documents, eight times of big Yamatsunami have occurred at twenty-six valleys in this area, and been lost as many as about 2,000 peoples and 4,000 houses completely or partly destroyed since 1868. Yamatsunami often rushed on straight and rapidly with the river water, mud and sand. Most of mud and sand were accumulated at the mouth of a valley. Since 1904 six Yamatsunami have been caused by the frontal rains except one case by the typhoon. In either case the maximum rainfalls were 50-70mm per hour which were 1.5-2 times as many as the rainfall nueogh to give rise to slope collapse. In this area, the most of frontal rains fell during night. It was the main cause to make damage greater by Yamatsunami which happened as soon as rainfall stopped. Most of the Yamatsunami were observed in the mountain slopes which were composed of granite and they began to flow down at the height of more than 450m above sea-1evel. The collapses of the valley below knickpoint were caused by the landslides in the valley and/or on the slope above knickpoint. It is probable that the collapse is one of the last erosional processes occurred on the steep slopes which had been formed in the previous stage. Some of the Yamatsunami were caused by the break of an irrigation pond. A Katagawa-machi (one-sided settlement along the river) was formed to cope with Yamatsunami. The stone walls (Tsui-ji) were made at the bent of river to drive the stream current to the opposite side. A Iarge quantity of mud and sand accumulated by Yamatsunami was used for reclamation and for raising the ground level. Subsequently, with the increase of population and the development of manufacture, the low marshy land began to be occupied with a number of rented houses (Nagaya). Thus the Katagawa-machi became into sided settlement along the river. After that, the damages of Yamatsunami have concentrated upon these villages which are located in marshy land. Owing to the recent rapid urbanization, as houses concentrate in this area, the damage of Yamatsunami also increases rapidly.
The formation of floodplain can be broadly grouped under two obvious stream environments : channel deposition and overbank deposition. Point bar and channel floor etc. are the landform as the result of the channel deposition. Natural levee and back marsh etc. are as the result of the overbank deposition. Many foreign scholars emphasize that the floodplain mainly consists of the channel deposits and the proportion of overbank deposits is generally very small. There are few studies of this kind in Japan. Then the author selected the lower Kinu River Floodplain as the study area where is situated in the central part of Kanto Plain. He has examined the relationship between the landforms and those deposits in order to discuss the formation of floodplain in Japan. 1. There are natural levees back marshes clevasses, point bars, former river courses and river sand dunes etc. in the floodplain. Particularly the natural levees and the back marshes are developed widely. The point bars are seen much confined along the Kokai River. 2. The author digged the 4m holes with the hand auger at many places in the floodplain, and made the grain size analysis of the sediments. The natural levees are composed of the sand, clayey sand and loam within the depth of I m, and of the clayey silt and clay below that depth. The back marshes are made up of silty clay and clay. The point bars are formed of sand. The crevasses have a depth of I m and composed of sand. In short, in this floodplain the present landforms correspond directly to the deposits within the depth of I m. 3. According to the observation of the deposit faces, the author has found following facts: a. The peat layers are broadly distributed and deposit thickly. b. The peat layers or the back marsh deposits are often covered with the sandy deposits in the natural levees. c. The point bar deposits are confined along the meander sections by the Kokai River. Judging from the facts mentioned above, it is said that the overbank deposition prevails in this floodplain and the overbank deposits occupy the most part of the surface geology. 4. Based on the radiocarbon date of the peat and the age of the unearthed potteries, it is thought that there was a good development of the natural levees around thirteen century in this floodplain.
The Iheya island is located to the northwest of the Okinawa island and belongs to the pre-Tertiary zone of the Ryukyu arc. The island is composed of pre-Tertiary rocks and Quaternary deposits. It. however, lacks the Pleistocene coral reef limestone, although it lies in the coral sea area at present. The present writer summarizes the results of his geomorphological investigation at the island as follows: 1. Gentle erosional slopes which fringe the NE-SW stretching mountain land in the direction of the major geological structure, are considered to have been formed during the early and middle Pleistocene. The slope deposits are weathered deeply and reddish in colour, and form the basements of the coastal terraces. The pre-cyclic valley floors and small erosional basins have remained in the mountain land, especially in the sandstone areas, concordant with the erosional level of gentle slopes. 2. The higher coastal terrace which stands 25-30m above the present sea level was built after, at least, a glacial period successive to the formation of the gentle erosional slopes. The terrace surfaces, in places, are covered with reddish weathered talus and debris related to the gentle erosional slopes. The terrace is considered to have been formed at the high sea level during Mindel-Riss Interglacial. 3. Leaving the glacial period after the Interglacial, the sea level raised again higher and stayed for a comparatively long period during which the 15 m coastal terrace was constructed. This high sea level may be correlated to that of Riss-Wrum Intarglacial. 4. The regressional sea towards the low sea level of Warm Glacial cut the 9m and 6m coastal terraces. The new talus and debris of new gentle erosional surface cover the terrace surfaces. The consolidated and old coastal dune which extends below the present sea level grew between the commencement of the regression and the Postglacial transgression. 5. The coast maintained the sandy beach during Postglacial transgression until the sea level reached its peak, so-called Daly Level at which the off-shore bars blocked the small bays and formed the back marshes with thin mud deposits. 6. The high beach ridges and beach rocks were formed on the bars at Daly Level. The new dune sand was also blown up behind the sandy beach. Judging from the heights of the raised coastal features, Daly Level is considered to have been about 4 m above the present sea level. 7. Main notches and benches are believed to have developed during Rottnest Emergence below Daly Level. 8. Present coral reefs are of fringing or pseudo-barrier reefs, and have grown up since Daly Level in the same manner. Present reefs are not well developed because the steep mountain slopes covered with reddish weathered deposits, in many places, directly compose the basement of the coast.