Impact of Chi-Chi earthquake on the occurrence of landslides and debris flows: Example from the Chenyulan River watershed, Nantou, Taiwan

The Chenyulan River watershed in central Taiwan was chosen for evaluating the impact of the Chi-Chi earthquake on the occurrence of landslides, and for exploring the initial conditions triggering debris flows. Interpretations of aerial photographs and SPOT images as well as field investigations were used to identify landslide and debris flow occurrences. Precipitation data were then utilized to investigate critical conditions leading to the debris flow. Comparison of five SPOT images taken between June 1996 (before the July 1996 Typhoon Herb) and September 1999 (before the Chi-Chi earthquake) shows that the landslide area increased from 7.1 × 10⁶ to 7.5 × 10⁶ m² during that time. However, by January 2000 (after the Chi-Chi earthquake), the landslide area almost tripled to 20.8 × 10⁶ m². Measurement of images taken in March and August 2001 reveals that the landslide area had further expanded to 24.2 × 10⁶ and 27.5 × 10⁶ m², respectively. Significant differences before and after the earthquake are also noticed in (1) the intensity and amount of precipitation required for triggering debris flow, (2) the size of contributing drainage basin in which debris flows occurred, and (3) the frequency of debris flows. After the Chi-Chi earthquake, maximum hourly rainfall intensity and critical accumulated precipitation necessary to initiate debris flow reduced to as low as 1/3 of the pre-earthquake figures. Prior to the Chi-Chi earthquake, most debris flows occurred in gullies with slopes greater than 10°, and in drainage basins larger than 0.1 km². Conversely, after the earthquake, debris flows were observed even in gullies with effective drainage area smaller than 0.03 km². Furthermore, before the earthquake, the debris flow recurrence time in the study area was greater than 5 years, whereas six debris flow events have been observed in the 2 years since the earthquake.