2007 Volume 60 Issue 2 Pages 39-44
Flash floods occurred in Japan in 2006. The definition of “flash flood” is different in the world. Here in this report, it is defined as a flood which contain few sediment and with an abrupt rise of the water level. The authors made field surveys at the 2 sites where the flash floods occurred in 2006. As a result, we found different processes of the generation of the flash flood for each site. In one site, we found some evidences of the formation of small natural dams and some existing step pools in the upper stream. According to the eyewitness, flash floods occurred several times. It is considered that boulders and woody-debris transported by the water flow might have caused several dam-up-dam-break processes in the upper stream and caused flash floods observed several times in the downstream. On the other hand, in the other site, neither evidence of the formation of natural dams nor that of sediment transport have been found. The nature of the watersheds in the upper stream shows some properties favorable to generate flash floods, that is, the upper basin shows typical caldera morphology and rock surface appears distinctively. In addition, it is considered that there would have been localized strong rainstorm in the upper stream, which might have caused the occurrence of the flash floods.