Development of a new rainfall-triggering index of flash flood warning-case study in Yunnan province, China

Flash floods, characterized by rapid, short-duration, and high-velocity flows, are the major causes of property damage and casualties worldwide. Flash flood warning is one of the key measures to prevent flash floods. Relying upon rain gage data and official statistics of flash flood events with casualties, this study proposes a new rainfall triggering index, β, defined as the ratio of accumulated rainfall to intraday rainfall, which effectively divides floods into events triggered by heavy intraday rainfall (0 < β ≤ 5) and those triggered by high cumulative rainfall (β > 5). Then, historical disaster events were used to evaluate the performance of the proposed index. Results reveal that: (1) when 0 < β ≤ 5, the intensity–duration (I–D) curve method is more desirable, and the simulation result has a high correlation coefficient (r = 0.95) with the measured result; (2) when β > 5, the rainfall triggering index (RTI) method is more suitable (r = 0.8); (3) the cumulative critical rainfall using the RTI method ranges from 50 to 400 mm. This paper stretches the thought of flash flood warning method and provides the reference for flood-prone regions.