Characteristics of debris flows recorded in the Shenmu area of central Taiwan between 2004 and 2021

Abstract. The data of debris-flow events between 2004 and 2021 in the Shenmu area Taiwan are presented and discussed in this paper. A total of 20 debris-flow events were observed in 18 years. Debris flows in the Shenmu area usually occurred in the Aiyuzi Stream during the rainy season, May to September, and about once per year between 2009 and 2017. The rainfall thresholds from the observed data are proposed to be Imax⁡ (maximum hourly rainfall), R24 (24 h accumulated rainfall), and Rt (effective accumulated rainfall) of 9, 23, and 67.8 mm, respectively. The rainfall data also imply that the trend curves of intensity–duration (I–D) were different before and after 2009, which is due to the extreme rainfall event of Typhoon Morakot in 2009. The I–D curve obtained from the post-2009 data is proposed as the baseline of the debris-flow I–D relationship in the study area. The extreme rainfall event also influenced the flow speed (average 14.3 m s−1 before 2010 and 4.46 m s−1 after 2010) and the occurrence frequency of debris flows (1.83 times per year before 2009 and 0.75 times after 2009). Recent findings have indicated that the ground surface vibrational signals of debris flows are potentially useful for debris-flow early warning in terms of accumulated energy, and the characteristic frequency of debris flows in the study area was below 40 Hz. The dataset and the rainfall thresholds in this study permit comparison with other monitored catchments and are useful for global debris-flow datasets.