Tatsuki SAWAMOTO

Research on the concept and operation of “Lead time” for flood evacuation

Kazushi SANO

In developing evacuation plans for flood damage, it is very important to consider the time required for residents to evacuate. These times are collectively referred to as “lead times. The term “lead time” does not have a unified definition, and this study attempts to clarify the definition and purpose of “lead time” in relation to disaster prevention and evacuation. We also clarified problems caused by the ambiguity of the term “lead time” and examined the appropriate use of the term. It was found that “lead time” for disaster reduction is used in various definitions, such as the time required for evacuation, an indicator of predictive capability, and a grace period until the occurrence of a disaster. The definitions also differ in information on evacuation, and the meaning may not be correctly conveyed to local governments and residents. Therefore, we proposed a method to use the term “lead time” differently.
Furthermore, in order to clarify how many hours “time required for evacuation, etc.” is set in evacuation plans, “lead time” used for setting water levels in designated river flood forecasts was investigated. The results of interviews with river offices revealed that the lead time is set based on a comprehensive judgment, taking into consideration not only the “time required for evacuation, etc.” but also the frequency of issuing evacuation orders, etc.
As a result of the survey of river administrators nationwide, statistics on the lead time for flood risk levels and evacuation decision levels were obtained, and it was found that the higher the rate of rise of water levels, the shorter the lead time tends to be. The lead time in designated river flood forecasting is defined as “the time required for evacuation, etc.”, but in reality, it may be “a realistic deferment”. For rivers with short lead times, it is necessary to provide information to shorten the “time required for evacuation, etc.”.