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 R_R. They have concluded that the new index R_R is suitable for helping decision of warning and evacuation by showing the applicability of R_R to recent sediment-related disasters.

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Existing risk assessment systems for sediment-related disasters, such as“Sediment Disaster Warning”, usually evaluate risk from precipitation data and do not incorporate premonitory phenomena. Premonitory phenomena have three uncertainties:1)when a premonitory phenomenon occurs, it is uncertain whether it will be seen;2)if a phenomenon is assumed to be a premonitory phenomenon, it is uncertain whether a sediment-related disaster will really occur;and 3)when no premonitory phenomenon is reported, it is uncertain whether the inhabitants actually saw the phenomenon. In this study, the authors built a risk assessment system for sediment-related disasters that utilizes data on premonitory phenomena and precipitation. The system considers the uncertainty of premonitory phenomena. It uses precipitation data as the main factor for evaluating the degree of risk of a sediment-related disaster, and the information on premonitory phenomena is used as a sub-factor. The system evaluates the risk of a sediment-related disaster for each evacuation unit, when ongoing sediment disaster warning systems indicate a degree of risk for each local authority. Two cases were evaluated using the system:the case of Typhoon MEGI at Niihama City in 2004 and the case of Typhoon Nabi at Tarumizu City in 2005. The following results were obtained:1)the system can specify high-risk areas in a local authority;and 2)by using information on premonitory phenomena, high-risk areas were specified 2 hours before the sediment-related disasters occurred.

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Early warning is important to prevent sediment-related disasters. However, reality is that it is difficult for local authorities to issue evacuation order and also difficult for residents to evacuate even if it would be issued. Therefore, it is critical to provide easily comprehensive information which can contribute to immediate judgment of local authorities and self-imposed evacuation by local residents. In this study, we prepared a portal site for early warning against sediment-related disaster through Internet and cellular phone data communication system (i-mode). For one year, we kept providing the information through the portal site to the selected local government officers, members of volunteer fire service, and residents all who have a leading role at the time of disaster as a demonstration test. Information on the risk of sediment-related disaster, radar rainfall data, and ground-based rainfall data are all provided in real-time. If the amount and intensity of rainfall exceed a previously designated value, alert mails are sent. During and after the test, we had questionnaires on the information provided to the examinees. As a result, the following is clarified. 1) Throughout the test, there was no serious rainstorm which is large enough to trigger landslide. However, we confirmed that the information provided sometimes triggered the examinees to make actions such as a patrol around their town. 2) The imode system was utilized much more frequently than internet, 3) The number of accesses to the radar-rainfall data was largest in all the provided contents. 4) The wider view of the information on the risk of sediment-related disaster was preferred. 5) The timing of the alert mail is important.

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In recent years, a number of disasters in Japan have affected the elderly. A disaster warning and evacuation system for vulnerable people is important. We studied a case in which elderly people took shelter from a sediment-related disaster in Okoku-en, in the city of Nanjyo (Sasiki town, Simajiri-gun), in Okinawa, on 17 June 2005, and studied the problems that emerged with respect to a warning and evacuation system in a home for the elderly. We found that it is hazardous trying to shelter elderly persons while it is raining, and that prolonged use of a shelter requires cooperation with other institutions in that the administrative body must play a central role, and prolonged use of a shelter is expensive. Therefore, we propose that disaster-planning and welfare organizations should design shelters for institutions housing the elderly for use during sediment-related disasters, thus reducing the expenses associated with other forms of necessary emergency shelter.

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On July 19, 2006, several sediment-related disasters (debris flows and landslides) occurred in Okaya and Suwa, in Nagano Prefecture, Japan. In Okaya, no evavuation took place before the disaster, and two debris flows killed eight people. Nearby Suwa was evacuated voluntarily, before the debris flow struck. In Okaya, a debris flow struck a retirement home. The first floor residents were evacuated to the second floor. Neither the Okaya City Government nor the residents had thought beforehand that a debris flow disaster could occur and, therefore, had not prepared for it. The law related to promoting measures for sediment-related disaster prevention requires cities in areas specified by prefectures to develop a warning and evacuation system for sediment-related disasters. In Okaya, no sediment-related disaster warning areas had been designated and the city was not prepared. By contrast, Suwa was prepared and avoided the loss of lives. This paper points to the importance of information sharing, between city officials and residents, concerning warning of, and evacuation plans for, sediment-related disasters based on what we have learned from the disasters that occured in Okaya and Suwa ; it also introduces the utility of the Sabo Data Management Center (Sabo D-MaC) and other systems as a means of information sharing.

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