Journal of the Japan Society of Erosion Control Engineering, Vol.60,No.2,2007
Case study on the flash floods occurred in Japan in 2006
Jun'ichi KURIHARA, Wataru SAKURAI, Nagazumi TAKEZAWA, Satoshi TAGATA, Ryuji
SUZUKI and Nobuyuki MORI
Abstract
Flash floods occurred in Japan in 2006. The definition of gflash 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.
Key wordsFflash flood, natural dam, rainfall intensity, hydraulic jump
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