A coupled hydrological and hydrodynamic modelling approach for estimating rainfall thresholds of debris-flow occurrence

Abstract. Rainfall-induced hydrological processes and surface water flow hydrodynamics may play a key role in initiating debris flows. In this study, a new framework based on an integrated hydrological and hydrodynamic model is proposed to estimate the Intensity-Duration (I-D) rainfall thresholds that trigger debris flows. In the new framework, intensity-duration-frequency (IDF) analysis is carried out to generate design rainfall to drive the integrated models to calculate grid-based hydrodynamic indices (i.e. unit-width discharge). The hydrodynamic indices are subsequently compared with hydrodynamic thresholds to indicate the occurrence of debris flows and derive rainfall thresholds through the introduction of a zone threshold. The capability of the new framework in predicting the occurrence of debris flows is verified and confirmed by application to a small catchment in Zhejiang Province, China, where observed hydrological data are available. Compared with the traditional statistical approaches to derive Intensity-Duration (I-D) thresholds, the current physically-based framework can effectively take into account the hydrological processes controlled by meteorological conditions and spatial topographic properties, making it more suitable for application in ungauged catchments where historical debris flow data is lacking.