- 一般社団法人 日本治山治水協会
- 水利科学 (ISSN:00394858)
- vol.56, no.5, pp.11-34, 2012-12-01 (Released:2017-07-12)
The formation of the part of Tokyo East Lowland(below sea level area)and the uncertainties of flood hazards there: Tokyo East Lowland is characterized by the gather of mouths of the Tone,the Arakawa and the Sumida rivers in a narrow area. Therefore, Edogawa city at their down streams always fears flood disasters, which is the low land less than zero mean sea level. The heavy inundation will happen in the drainage area even pumping up water, in the coastal area by high tide and at any local areas with guerrilla rainfall. Tokyo East Lowland including Edogawa city is on the deep as the maximum of 40m alluvium soil layers, which are composed from loose sands and soft clays. The ground water used to be significantly pumped up for the industry and to obtain methane gas. The significant ground settlement by the dewatering made the land to be below the sea level. If the river dykes were collapsed by the earthquake, the 70% of Edogawa city would be flooded. As Japan is also located just on the course of the typhoons,the huger ones than ever might possibly attack Tokyo. Therefore, in this study the worse cases of the Kathleen typhoon in 1947 were assumed and several typhoon patterns were simulated in Kanto area for the uncertainties of flood hazards. Finally, it was found that the design flood at Yattajima of the Tone river might be exceeded in the case of shifting the course of Kathleen in 1947 by only 50km to the northwest. Therefore, the author has concluded that flood prevention works must be conducted for the such worse situations. It is concluded that Tokyo and Edogawa city are always feared by huge flood disasters never experienced and their dykes will be the key of life line for Tokyo and Japan.