Estimating the natural disaster ınter-event time defition (NIETD) to define compound natural disasters in South Korea

Abstract Previous study on natural disaster events has been conducted under the assumption that each event is independent of others, which underestimates the risk of natural disasters by ignoring interactions between events. To solve this problem, the concept of compound natural disaster (CND) which is the combination of events has been proposed; however, there is no quantitative method for defining CND. The aims of this study are to examine the estimation methods for IETD (Inter-Event Time Definition), which separates continuous rainfalls into independent rainfall events, and to define NIETD (Natural disaster Inter-Event Time Definition), which is a criterion for determining the independence of natural disasters. This study used the method of average annual number of events for estimating NIETD. Two natural disasters can be defined as CND if the duration between them is less than the NIETD. We estimated the NIETD as 8 days using natural disasters that occurred in South Korea and identified a total of 89 CNDs of 14 different types such as consecutive rainfall events. The largest number of CNDs was caused by the combination of rainfall and typhoon, which also resulted in the most damage. To examine the randomness of event occurrences, we applied a bootstrapping approach and found that there is no evidence of randomness. The frequency analysis showed that CNDs consisting of rainfall and typhoon (7.6years), and consecutive rainfalls (9.4years) had overwhelmingly more frequent occurrences than other types. The CND definition and concept proposed in this study could be useful in the research on CND.