<p>Rainfall thresholds for debris flow initiation are a common index for debris flow forecasting across the globe. However, as many debris flow catchments have been managed using mitigation measures such as drainage channels and check dams, the occurrence of debris flow may not necessarily lead to damage to properties or lives. Consequently, a higher false alarm rate may be caused when using the initiation threshold in catchments where mitigation measures have been installed. To overcome this limitation, we have defined and constructed a disastrous rainfall threshold over which debris flows may overflow the drainage channel and cause damage to surrounding residents in a typical debris flow catchment in China. This threshold was obtained by using the SynxFlow debris flow model, which is capable of integrating the process of rainfall-runoff-debris flow dynamics. A series of rainfalls with varying intensities and durations were used to simulate debris flow overflow conditions, and the disastrous I-D threshold (<i>I</i> = <i>8.7D</i>^{<i>−1.26</i>} + <i>30.4</i>) was determined using fitting methods. The disastrous rainfall threshold requires much higher rainfall intensity in order to cause a threat to residential areas compared to the initiation threshold. The disastrous threshold intensity for a one-hour duration rainfall is 39.1&#xa0;mm/h for the study catchment, much larger than the 11.9&#xa0;mm/h initiation threshold. The study also found that differences in rainfall patterns have a significant impact on the magnitude of the disastrous threshold. The threshold for uniform rainfalls is higher than those for other rainfall patterns (delayed, advanced and intermediate). We propose that the differentiation between disastrous threshold and initiation threshold may help improve the accuracy of debris flow forecasting and early warning.</p>