広島湾岸地域の水害 : とくに山津波について A Geographical Analysis of the Landslide (Yamatsunami) in the Coastal Region of Hiroshima Bay
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 According to historical documents, eight times of big Yamatsunami have occurred at twenty-six valleys in this area, and been lost as many as about 2,000 peoples and 4,000 houses completely or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.
地理科学 18(0), 1-12, 1972