1960 年 33 巻 3 号 p. 97-104
Flood damages are brought frequently on the land of Japan by heavy rainfall in typhnon season (from middle August to late September) and the rainy season (from early June to middle July), and her communities suffer a large amount of loss by those damages.
In 1958, from Sept. 26 to 27, a large scale typhoon (So called the Kanogawa Typhoon) attacked the eastern Japan (Fig, 1), and the followed heavy rainfall caused violent flood damages on several parts in these regions. The distribution of rainfall at that time was very complicateb (Fig. 2), and an extremely heavy rainfall were concentrated on the middle part of Izu Peninsula, Tokyo metropolitan area and another several regions. The characteristics of disasters, however, much differed in each regions, in accordance with their physio-and socio-characters.
In Tokyo metropolitan area, the extensive alluvial plain develops to the east, and flood damages happened often there by overflowing from large rivers (the Arakawa, the Edogawa and Tamagawa etc.). But rainfall distribution brought by that typhoon was somewhat un usual and the rain did not concentrate on the upper part of these large river area but on the place near Tokyo Bay. So, by overflowing rlom small rivers, flood damages were more severe in and around the low upland and the hill to the west of city than on the extensive alluvial lowland along to these large rivers. The total amount of flooded houses reached to about 460 thousands at that time.
Kanogawa is a small stream which runs in the nouthern part of Izu Paninsula. The disaster occured in the Kanogawa river basin was characterized by the violent inundation, covered almost whole lowland in this river basin, and the severe damage, caused by this inundation: for example, 1, 273 of people were killed and injured, the amount of flowed and destroyed houses exceeded the number of flooded houses and the amount of flowed and buried arable lands are much more than the quantity of flooded (Table 1).
Chiefly from the physical points of view, we tried to make clear the causes which brought the disasters in this river basin. These causes may be briefly summarized as follows;
1) The extremely heavy rainfall concentrated on the upper part of this river basin, and theextremely volumenous runoff took place very rapidly, because of the full saturation of the land surface by foregoing much rainfall and the steep gradient of the river and its branches.
2) Moreover, bridges obstructed the flowing of runoff and temporarily reserved more volumenous water behind them. So, when these bridges were destroyed, the extremely volumenous runoff flowed suddenly downwards.
3) Consequently, the volume of maximum discharge far exceeded the estimated hiqh waten dis charge flood and the bank was broken at everywhere. The bank on undercut slopes of meander course were especially destroyed than others.
4) At the mountain region, many landslides and debris flows occurred, and the debris flows caused some damages on roads and horseradish (Wasabi) fields which are the important fields for the cash products in this region.