Variability in the characteristics of extreme rainfall events triggering debris flows: a case study in the Chenyulan wat
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Variability in the characteristics of extreme rainfall events triggering debris flows: a case study in the Chenyulan watershed, Taiwan Jinn‑Chyi Chen1 · Wen‑Shun Huang2 · Yuan‑Fan Tsai3 Received: 8 January 2019 / Accepted: 17 April 2020 © Springer Nature B.V. 2020
Abstract This work examines the period affected by extreme rainfall that triggers debris flow and the modification of the critical rainfall threshold for debris flows after extreme events in the Chenyulan watershed in central Taiwan. Three rainfall indices, Im (maximum hourly rainfall depth), Rd (maximum 24-h rainfall amount), and RI (= Im × Rd), were used to analyze the variations in rainfall thresholds for triggering debris flow. Five extreme rainfall events with RI > 365 cm2/h that triggered multiple debris flows are discussed. Critical rainfall (the rainfall threshold of Im, Rd, and RI for triggering debris flow) drops significantly at the initial stage after an extreme rainfall event; it then increases to revert to its original value after subsequent rainfall events. The lowered critical rainfall and the required recovery period were associated with the RI driven by an extreme rainfall event. Empirical relationships for modified critical rainfall and recovery period after an extreme rainfall event are also presented. The critical RI and Im (or Rd) could be reduced by 70% and 40%, respectively. The maximum recovery period was approximately three years after an extreme rainfall. The recovery period has an increasing trend with the increasing ratio of landslide area bought by an extreme rainfall. Keywords Debris flow · Critical rainfall · Rainfall index · Recovery period List of symbols Aa Landslide area after an event Ab Landslide area before an event I Region hourly rainfall I1, I2, I3 Hourly rainfall records from the Sun Moon Lake, Yushan, and Alisan meteorological stations, respectively * Jinn‑Chyi Chen [email protected] 1
Department of Hydraulic Engineering, Fujian College of Water Conservancy and Electric Power, Yongan 366000, China
2
Ecological Soil and Water Conservation Research Center, National Cheng Kung University, Tainan 70101, Taiwan
3
Department of Social and Regional Development, National Taipei University of Education, Taipei 10671, Taiwan
13
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Natural Hazards
Im Maximum hourly rainfall during each rainfall event Lr Increasing ratio of landslide area for a rainfall event, Lr = (Aa − Ab)/Ab MW Moment magnitude N Total number of individual debris flows triggered by each rainfall event Rd Maximum 24-h rainfall amount during each rainfall event R2 Coefficient of determination RI Rainfall index, RI = Rd Im rxi Rainfall index ratio, the ratio of critical xi for a rainfall event to the critical xi before TH (the basis of xi) xi Variable of rainfall index representing Im,, Rd, or RI t Period after an extreme rainfall event, the estimated period starting from the time of an extreme event occurring tr Recovery period Abbreviations CCE Chi–Chi earthquake HR Heavy rainstorm in 2006 TH Typhoon
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