2008 Volume 60 Issue 6 Pages 4-10
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 sedimentrelated 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 118 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.
