Debris Flows and Landslides - River Dynamics and Integrated River Management - page 200

2) Chain 2: Landslide—debris flows—new drainage system—intensive fluvial process
Chain 2 consists of four disastrous events: 1) a huge landslide at the time of the earthquake and burial of
the entire drainage system; 2) debris flows in the first and second flood seasons; 3) development of a new
drainage system in the first few years after the earthquake; and 4) incision of the new channels and
intensive fluvial processes.
An example of disaster chain 2 was initiated by the Wenjiagou landslide during the Wenchuan Earthquake.
The Wenjiagou Ravine is a tributary of the Mianyuan River and is located in Qingping Township, which
was 3.25 km long with a drainage area of 12.31 km 2 . All of the gullies in the drainage area were incised
with depths of more than 100 m and bank slopes of 30-50. The Wenjiagou landslide began from a high
elevation and slid very fast along the ravine to the confluence with the Mianyuan River. The total volume
of landslide deposit was about 81.6 million m 3 (Sichuan Geological Engineering Corporation, 2009). The
landslide buried the Wenjiagou Ravine and its tributaries underneath landslide debris with a thickness of
20-180 m. Thirty-four houses were buried, and more than 80 people were killed by the landslide. The
landslide deposit consists of loose solid materials with different sizes.
The Wenjiagou Ravine was not a debris flow gully and there was no debris flow in the past 100 years
before the earthquake. The landslide provided plenty of loose materials for debris flow. Almost all rainstorms
with an intensity higher than 30 mm/day triggered debris flows in 2008. A large debris flow after a
rainstorm with an intensity of 88 mm/day on Sep. 22-24, 2008 transported 0.9 million m 3 of sediment to
the ravine mouth, burying roads, houses and farmland. The debris flows scoured the landslide deposit
and formed a “V” shaped channel with a depth of 50 m. About 1.7 million m 3 of sediment has been
removed from the landslide deposit by the debris flows, and about 1 million m 3 of sediment has been
carried to the ravine mouth.
A new drainage system was developing in the process of scouring and debris flows. Figure 4.12 shows
the landslide and the new gully that was developed on the landslide deposit. Figure 4.13 shows the
longitudinal profiles of the Wenjiagou Ravine before the earthquake, after the landslide in May 2008, and
after new drainage development in 2009. Very intensive channel bed incision occurred on the new Wenjiagou
Ravine. Because the banks of the new gully were very steep, incision of the gully bed caused collapse of
the banks. Consequently, more debris flows took place. In the meantime, the sediment carried into the
Mianyuan River also caused intensive fluvial processes.
3) Chain 3: Avalanches—grain erosion—slope debris flow
The Wenchuan Earthquake triggered numerous avalanches and rock falls along streams. The avalanches
have left a great area of bare rocks (Wang et al., 2009a). The newly exposed rocks are fragile and easily
broken down under the action of sunshine and temperature change. Intensive grain erosion has been
occurring since the exposure, details of grain erosion are discussed in Chapter 2. The rate of grain erosion
of bare rocks on the Minjiang River was between 3 to 53 cm/yr, with an average of about 17 cm/yr
(Wang et al., 2010a).
Grain erosion caused slope debris flows and generated flying stones, which caused damage to cars and
injured humans. Grain erosion resulted in a lot of loose and uniform particles depositing with high slopes.
Rainfall with an intensity of more than 20 mm/day triggers mass movements of the grains. These mass
movements behave like debris flows but the distance of the movement is, however, much shorter than
normal debris flows, and in general travels only several tens to one hundred meters. This mass movement
is called slope debris flow. It carries a lot of grains into rivers or deposits the grains on highways, causing
blockage of highway transportation or local sedimentation on the riverbed.