Debris Flows and Landslides - River Dynamics and Integrated River Management - page 203

The highway from Dujiangyan to Wenchuan was seriously damaged by the earthquake and was
reconstructed and reopened before the one-year anniversary of the earthquake. The Chediguan Bridge is
a key bridge crossing the Minjiang River. The bridge was broken by new avalanches from the right bank
of the Minjiang River (N31°18'05.9”, E103°27'59.1”) at 4:40 in the morning of July 25, 2009 (Beijing
Time). In fact, this event was the second episode of the disaster chain. The Wenchuan Earthquake caused
many avalanches on the slopes with elevations of 100-400 m from the riverbed at this place. Consequently,
rocks at higher elevations became unstable. Consecutive rock falls occurred from elevations of about 400
m in the afternoon of July 24, 2009, as shown in Fig. 4.15(a). In the morning of July 25, 2009, an avalanche
occurred from elevations of 500-600 m. A huge stone of more than 200 tons fell down and broke the
third pier of the Chediguan Bridge, causing more than 60 m of the bridge to collapse. Five trucks and a
van dropped into the Minjiang River, six people were killed and 12 injured. Transportation was cut off
for two weeks (http://bbs.3608.com/showtopic-247080-1.html).
Figure 4.15(b) shows a side view the cross-section of the avalanches. The banks are so steep because
the riverbed has been incising for centuries due to the fluvial process of the minjiang River. The slope angle
of the right bank is 40°-50°, which is higher than any repose angle of solid materials. The avalanches
during the earthquake increased the slope in the upper part of the bank. Thus, new avalanches occurred
and caused the tragedy.
Fig. 4.15 Small scale avalanches and rock falls at Chediguan on July 24, 2009; (b) Side view of the Chediguan
avalanche (on July 25, 2009)
A similar event occurred at the Zongqugou Ravine, a deeply incised tributary of the Minjiang River near
Maoxian. The lithology was mainly composed of limestone which was easily cracked by the earthquake.
The bank slopes, especially the left bank, were also in the range of 40-50° before the earthquake (as shown
in Fig. 4.16). The Wenchuan Earthquake caused avalanches and rock falls from the elevations of 400~800 m.
Huge stones with diameters of 1-15 m dropped into the Zongqugou Ravine and formed a quake lake with
a capacity of about 200,000 m 3 . In March 2009, an avalanche occurred at high elevations (>800 m) and
several huge stones larger than 10 m of diameter fell down and hit against on the quake lake dam. The
dam was partly broken and a small dam failure flood occurred, which caused no casualties.
5) Attenuation factor
In the four disaster chains, the time of occurrence of each disaster episode is longer than that of the
previous episode. Figure 4.17 shows the estimated time of occurrence of various disasters. Landslides and