Debris Flows and Landslides - River Dynamics and Integrated River Management - page 193

Debris flow occurs often as a consequent mass movement after landslides. Thus, debris flow and
landslide are related in a causal chain of morphological processes. The 1999 Chi-Chi earthquake in
Taiwan, China, induced many landslides in the Tachia catchment. The subsequent rainfall events led to a
significant increase in the debris flow events, and as a result, four years after the earthquake event, the
sediment production rate was still increasing. After the Chi-Chi earthquake, over 20,000 landslides totaling
approximately 113 km 2 had occurred. Consequently, a great deal of loose sediment was produced, which
in turn promoted heavy debris flows during subsequent typhoons and heavy rains. The most outstanding
example was a severe debris flow hazard causing the deaths of more than 240 people during Typhoon
Toraji on July 30, 2001 (Lin et al., 2005).
The Wenchuan earthquake occurred in Sichuan province, China on May 12, 2008 and triggered a huge
scale and several tens of large scale landslides, which yielded a huge amount of sediment. The sediment
deposited on the gully bed and slopes, or even dammed streams creating quake lakes. The loose materials
resulting from the landslides are likely to be removed by flowing water and transported in debris flows. A
large scale landslide occurred on the Huoshigou Gully in Chongzhou County on May 12, 2008, which
has yielded several million tons of sediment in the gully. Several days later a heavy rain triggered a
debris flow, which scoured the landslide deposit and dug a 30-40 m deep channel, as shown in Fig. 4.4(a).
The debris flow carried extremely high sediment concentration into the Anzi River and sediment deposited
on the bed. About 2 km along the channel bed of the river was silted up by 4-9 m, as shown in Fig. 4.4(b).
(a)
(b)
Fig. 4.4 A large scale landslide occurred on the Huoshigou Gully in Chongzhou County, which was triggered by the
Wenchuan Earthquake on May 12, 2008: (a) A flood flow scoured the landslide deposit by 30-40 m and developed into
a debris flow; (b) The debris flow carried sediment into the Anzi River and resulted in 4-9 meters of sedimentation
on the bed (See color figure at the end of this topic)