Study on setting up of a critical R']value and proposal of a new index
RR as a remarkable rainfall intensity
-Its applicability reflected rainfall history-
Shinji NAKAI, Masahiro KAIBORI, Yasushi SASAKI and Takeo MORIWAKI
Abstract
It has been recognized for a long time that the occurrence of sediment]related
disasters depends on both of long]term rainfall and rainfall intensity for a short
term. R' proposed by the authors is one of the rainfall indices for the warning
and evacuation and can be used for determining the risk of sediment]related disasters
with a single value comprising two kinds of influence by long]term and short]term
rainfalls. Time dependent spatial distribution of R' in an area is easily drawn.
Therefore, it seems to be easily understood for many people. However, since the
vulnerability against sediment movement is not homogeneous at places, a critical
value of R' being a threshold for occurrence of sediment]related disasters should
be varied by the local characteristics. Also, the change of R' during a prospective
severe rainfall can be different depending on a combination of a long]term rainfall
and a short]term one, even if the initial value of R' is the same. So, the authors
firstly considered a critical R']value, and confirmed that it can be expressed
by the R']value of 1/8 probability in each district. Subsequently, they have thought
that a rainfall intensity needing to become very dangerous situation in an hour
should be a new index and proposed a new concept of remarkable rainfall intensity
RR. They have concluded that the new index RR is suitable for helping decision
of warning and evacuation by showing the applicability of RR to recent sediment]related
disasters.