Debris Flows and Landslides - River Dynamics and Integrated River Management - page 204

avalanches occurred almost simultaneously with the earthquake. Landslide dam failure floods occurred
about 10-60 days after the landslides. The Chi-Chi Earthquake in Taiwan, China on Sep. 21, 1999,
caused an unusually high frequency of debris flows and intensive fluvial processes in about 10 years
after the earthquake (Lin et al., 2008). The impacts on ecology are mainly caused by the intensive fluvial
process and last for about a decade. Soil erosion and grain erosion intensified by the earthquake may last
for 100 years (Koi et al., 2008; Lin et al., 2008). New avalanches occurred due to the increased slope and
cracked rocks. The time of occurrence of the avalanches in high elevations is estimated to be within 10
years. Vegetation development may mitigate the risk of avalanches in high elevations.
Fig. 4.16 Side view of the Zongqugou avalanches
Fig. 4.17 Estimated time of occurrence of various disasters: L&A: Landslides and avalanches; Q: Failure of quake
lake dam; D&F: Debris flows and intensive fluvial processes; E: Impacts on ecology; G: Erosion, especially grain
erosion; H: New avalanches at high elevations
In the four disaster chains, the intensity of each disaster episode is lower than that of the previous episode.
There is attenuation along the disaster chain. If the magnitude of the geological disaster is evaluated
using volumes of mass movement the attenuation factor can be defined as the ratio of the volume of mass
movement in one episode of a chain to the volume of the mass movement in the previous episode. For
instance, in the Huoshigou disaster chain, the volume of the landslide was 7.21 million m 3 , but the volume
of all debris flows was 1.25 million m 3 . Thus, its attenuation factor is 0.17.
In the Wenjiagou disaster chain, the total volume of the landslide was 81.6 million m 3 , and mass