Response of debris flow occurrence to daily rainfall pattern and critical rainfall condition in the Anning River–Zemu River Fault Zone, SW China

Rainfall is one of the most critical and dynamic factors affecting debris flow occurrence. However, the complexity and variety of the rainfall process mean it is usually difficult to determine the precise impact of rainfall on debris flow occurrence. This study investigated debris flows in the Anning River–Zemu River Fault Zone. This area underwent the most intense Late Quaternary tectonic deformation of southwestern China and it features hundreds of debris flow gullies. Based on the contributions of antecedent and daily rainfall to 82 debris flow events, the characteristics of daily rainfall patterns and the response of debris flow occurrence were analyzed. Moreover, the daily rainfall conditions necessary for debris flow initiation under various rainfall patterns were established. The conclusions derived from this study were as follows. (1) Four rainfall patterns were classified: an intraday-rainfall-controlled pattern (pattern A), intraday-rainfall-dominated pattern (pattern B), intraday-antecedent-rainfall-balanced pattern (pattern C), and antecedent-rainfall-dominated pattern (pattern D). (2) Debris flow occurrence varied greatly with rainfall pattern. Patterns B and C each accounted for over 30% of debris flows, pattern D accounted for nearly 22%, while pattern A accounted for approximately 16%. (3) The critical intraday rainfall condition for triggering debris flows also varied with rainfall pattern. The critical daily rainfall condition with an occurrence probability of > 50% was > 23 mm. The results of this study have significance regarding the mechanism of debris flow initiation and disaster prediction and warning.