Debris Flows and Landslides - River Dynamics and Integrated River Management - page 202

4) Chain 4: Avalanches—new avalanches
Chain 4 consists of only two disastrous events: avalanches during the earthquake, which mainly occur at
an elevation near the riverbed; and new avalanches at higher elevations. More than 90% of the avalanches
directly triggered by the Wenchuan Earthquake occurred in an elevation range of 100-800 m above the
riverbed. The avalanches increased the slope of the upper part of the mountains.
Many avalanches were triggered by the Wenchuan Earthquake on the Minjiang and Yuzixi rivers,
where the lithology was mainly granite. The Yuzixi River is a tributary of the Minjiang River, which
flows into the Minjiang River at Yingxiu. The density of avalanches was so high along the two rivers that
about 50% of the valley slopes (0-500 m from the riverbed) were covered by avalanches. Figure 4.14(a)
shows the area percentage of avalanches on the valley slopes along the river from the mouth at Yingxiu
to Qicenglou. The percentage was measured with laser range meters. Figure 4.14(b) shows the riverbed
profile before the earthquake, which was obtained from DEM data measured in the 1960s; the riverbed
after the avalanches in 2008, which was measured using GPS receivers; and elevations of avalanches,
which were measured by using laser range meters. The avalanches created many small quake lakes and
enhanced the bed elevation by several tens of meters at different places. The avalanches mostly occurred
at elevations between 100 and 600 m from the riverbed.
Fig. 4.14 (a) Area percentage of avalanches on the valley slopes along the Yuzixi River; (b) Riverbed profile before
the earthquake, after the avalanches, and the elevation of the avalanches
On the Minjiang River, the upper part of the mountains (400-1500 m from the riverbed) lost the
support of the lower part due to the avalanches below. The slopes of the upper mountains increased or
even some parts became suspended. Moreover, the earthquake caused the rocks to be cracked and broken.
In the following years, new avalanches occurred when rainstorms or tremors occurred. In 2009, such
avalanches occurred at the Chediguan Bridge on the Minjiang River and Zongqugou Ravine, a tributary
of the Minjiang River.