This study introduces a new index, the unprecedented rainfall index, T_P, for evaluating sediment disaster vulnerability. When we trace back to the past rainfall events, we find a time at which the rainfall magnitude is equal to or greater than the magnitude of the current rainfall. Such time is treated as a candidate for T_P. Because the rainfall magnitude can be evaluated from various aspects, various T_P candidates are derived. Among them, the oldest T_P candidate is defined as T_P. Consequently, in comparison with the rainfall magnitudes during the period from T_P (exclusive) to the start time of the current rainfall event, the current rainfall has the largest magnitude at least in one of the various aspects. The new index was applied to previous sediment disasters, and results indicated that T_P has a potential to be effectively used for a warning system against sediment disasters because it can be directly compared to occurrence times of previous disasters which are famous in each community. Based on the results, this study suggested a novel critical value, T_C, for sediment disaster vulnerability, which is defined as the occurrence time of reduction in resistance against slope failures and debris flows.

Provision of information about sediment disaster by administrative authorities (efforts to inform) and comprehension of the information by residents (efforts to learn) are expected to work concurrently for sediment disaster prevention. According to the Sediment Disaster Prevention Act (Act No. 57 of 2000) and its regulation, mayors should create and disseminate sediment disaster hazard map when sediment disaster prone zones are designated by governors under the Act. The hazard map must provide for risk information including sediment disaster prone zones, dispositions for transmission of disaster information as well as evacuation ways and sites. The hazard map can also provide for other information deemed necessary to ensure alarming and evacuation operation. Sediment disasters caused by recent heavy rains prompted the Japanese government to introduce <5 stages of alert level>. Each stage represents different level of urgency of sediment disaster. Disaster information framework is reorganized so that each information corresponds to one of the 5 stages. It enables residents to realize urgency of sediment disaster and take appropriate evacuation behavior. On the other hand, each resident is expected to make for itself judgements about individual and familial evacuation behavior. These judgements cannot be entirely made collectively because they depend on personal, social and spatial condition of each resident. Improvement of information doesn't mean that individual judgements become unnecessary. Support and facilitation of individual judgements about evacuation behavior is an important challenge of sediment disaster prevention. Residents are encouraged to enhance consciousness and preparedness through participation in formulation of the hazard map. Other community-level initiativess such as community disaster prevention plan shall be used in connection with the hazard map.