RAINFALL INDEX FOR WARNING AND EVACUATION AGAINST SEDIMENT-RELATED DISASTER: REEXAMINATION OF RAINFALL INDEX R_f, AND PROPOSAL OF R′

摘要

Proper warning and evacuation are essential for mitigating sediment-related disasters during heavy rainfall. For this purpose, a scientific and rational rainfall index should be established in accordance with regional geological and topographical characteristics. A new rainfall index, named R′, is proposed through a revision of the Rainfall Index R_f. The effect of preceding rainfall prior to a disaster-inducing rainfall is taken into account in this newly-proposed Rainfall Index R′ by utilizing the concept of "effective rainfall". Rainfall records which were made during a serious disaster on June 29, 1999 and on September 15, 1999 in Hiroshima district were analyzed. Some other cases were also examined and taken to verify the effectiveness of this newly-proposed rainfall index. As a result of the study, it was found that R′ is superior to R_f for judging the occurrence probability of a sediment-related disaster from on-going rainfall records. It has also been shown from several cases in the past that R′ can identify not only area and time, but also mode and size of a disaster due to slope failures on Masado (decomposed granite soil) slopes which are widely spread around Hiroshima area.