Debris Flows and Landslides - River Dynamics and Integrated River Management - page 197

of more than 15,000 (Yin, 2008). The zone with a high density of landslides (more than 10% of the
surface area) stretches for 240 km along the Longmenshan fault, on which the Wenchuan Earthquake
occurred, and the width of this zone varies from 25-30 km wide in the southwestern section of the fault
to 3-5 km wide in the northeastern section of the fault.
Several disaster chains have been initiated by the Wenchuan Earthquake, and each of the disaster chains
consists of several episodes (Xu et al., 2010). The disaster chain is defined as a chain of disastrous events;
each event is the main cause of the next event. Four types of disaster chains have been identified. In the first
chain, landslide created a quake lake, which was followed by a flood caused by landslide dam failure and
very intensive fluvial processes. The last episode of the chain is loss of habitats and destruction of aquatic
bio-communities. The Tangjiashan and Huoshigou landslides initiated such a type of disaster chain. The
second chain consisted of a huge landslide burying a drainage system, debris flows, and development of
new drainage system and intensive fluvial processes. The Wenjiagou landslide initiated such a type
disaster chain. The third chain consisted of avalanches, grain erosion (intensive erosion of bare rocks due
to exposure to weathering and temperature change), slope debris flows, and flying stones. Many such
disaster chains occurred on the mountains by the Minjiang River from Yingxiu to Wenchuan. The fourth
chain has only two episodes: the first episode was avalanches during the earthquake at elevations between
100 and 800 m from the riverbed, and the second episode was new avalanches and rock falls due to
increased slope angle in the high mountains (400-1500 m from the riverbed).
1) Chain 1: Landslide—dam failure flood—intensive fluvial process—loss of habitats
Chain 1 consists of four disastrous events: 1) a landslide occurring almost at the same time of the earthquake
and formation of a landslide dam on rivers; 2) landslide-dam failure after a short period of storage of
river water (10-60 days) and subsequent flooding; 3) intensive sediment movement and fluvial processes
in the downstream reaches; and 4) loss of aquatic habitats and dramatic reduction of fish and macro-
invertebrate species.
The most precarious of quake lakes triggered by the Wenchuan Earthquake was the Tangjiashan quake
lake on the Jianjiang River that was formed by a huge landslide from Tangjiashan Mountain. The volume
of the sliding body was about 20.37 million m 3 and the landslide dam was 612 m long (across the river),
803 m wide (along the river), and 82-124 m high. It mainly consisted of quaternary deposits and clastic
rocks. The total storage capacity of the quake lake was about 316 million m 3 . There was a high risk of
dam-break flooding when 200 million m 3 of water had stored in the lake. A spillway was dug on the dam
and then the water began releasing through the spillway. To help the water scour the spillway bed to a
low level, large boulders were removed by explosions. The elevation of the spillway channel on the dam
was eroded down from 740 m to 714 m, and the channel bed was cut wider from about 10 m to 100 m
(Fig. 4.9(a)). The lake water volume reduced from 246 million to 86 million m 3 . Although the peak
discharge rate was 6420 m 3 /s during the course of channel cutting and lake-water draining, no casualties
or damage were caused by the draining flood. The dam draining flood carried a lot of sediment to the
downstream reaches and an intensive fluvial process was initiated. The downstream reach was silted up
by 20 m and intensive sedimentation and erosion occurred in 2008 and 2009. A 20 m thick sedimentation
layer was scoured by floods in 2008 and 2009 and a new channel formed in the river (Fig. 4.9(b)).
The second example of disaster chain 1 was initiated by the Huoshigou landslide. The Huoshigou
Ravine is a second-order tributary of the Minjiang River in Chongzhou, near Chengdu. It was a straight
river with very steep slopes and good riparian vegetation before the earthquake. The Huoshigou landslide
was a high-speed and long-distance landslide, which created an extremely huge air cushion and air waves
with strong and destructive impact during its movement (Zhang, et al, 2008). Hundreds of houses were
buried and 39 people were killed by the landslide.