Modelling Initiation and Intensity-Duration Thresholds of Extreme Precipitation–Induced Debris Flows: A Case Study of the August 2020 Pettimudi Event

Abstract

The frequency of extreme precipitation events is increasing globally, leading to more frequent catastrophic debris flows. The high velocity and long runout distances of debris flows cause damage to infrastructure and loss of life. Despite ongoing scientific efforts, understanding the debris-flow initiation mechanisms and developing reliable early-warning thresholds remain challenging due to the limited availability of data from extreme historical events. This study develops a numerical modelling framework to analyse the initiation and runout dynamics of an extreme-precipitation-induced debris flow and to derive corresponding Intensity-Duration (ID) thresholds. The 6 August 2020, Pettimudi, Kerala, debris flow is used as a case study. The model correctly reproduces the debris-flow arrival time, peak velocity (~9 m/s), and deposit thickness (7–8 m). The resulting ID thresholds reflect the unique characteristics of extreme events and differ noticeably from regional thresholds derived from historical cases. In addition, the framework developed in this study can help derive ID thresholds for debris flows in the absence of historical data. © 2026 John Wiley & Sons Ltd.