Warning and evacuation in response to sediment-related disasters

On July 19, 2006, several sediment-related disasters (debris flows and landslides) occurred in Okaya and Suwa, Nagano Prefecture, Japan. In Okaya, neither the municipal government nor the residents had expected such a disaster, and they were unprepared. No evacuation of the population took place prior to the disaster, and two debris flows killed eight people. Another debris flow struck a retirement home. Fortunately, the first-floor residents could be evacuated to the second floor. In contrast, the nearby town of Suwa was evacuated voluntarily before the debris flow struck, and no fatalities occurred. This paper describes the situation at the time of the disaster, the evacuations in Okaya and in Suwa, and their consequences from the point of view of disaster-prevention personnel and retirement-home staff. Factors that should be considered in formulating warning and evacuation systems in response to sediment-related disasters are discussed.