Integrated River Management - River Dynamics and Integrated River Management - page 773

2008, buried the Wenjiagou Ravine and its tributaries underneath a thick landslide debris with a thickness
of 20-180 m (read the disasters chains induced by landslides in Chapter 4). Figure 11.59 shows the area
of Wenjiagou landslide deposit, the old Wenjiagou channel before the landslide and the new Wenjiagou
channels developed during debris flows. The landslide deposit consisted of loose solid materials with
different sizes, varying from huge boulders of several meters to clay of less than 0.01 mm in diameter.
The boulders were mostly on the top layer of the landslide. The deposit had a porosity of about 30% and
was liable to be eroded. Because the high porosity of the landslide deposit low intensity of rainfall water
infiltrated into the loose material, and rainfall intensities higher than 30 mm/day triggered debris flows in
2008. A debris flow with a total volume of 0.9 million m 3 was triggered by a high rainfall intensity of 88
mm/day on September 23, 2008. The debris flows incised the landslide deposit and formed a 50 m deep
channel, which was named new Wenjiagou. The debris flows transported solid material to the gully mouth
and buried many farmers' houses. The new Wenjiagou was 1,400 m shorter and about 100 m higher than
the old Wenjiagou, but the new channel was not directly above the old channel and about 150 m to the
right of the old channel.

Mianyuan River

2010 debris
flow fan

2008 debris flow fan

2008-2009 Wenjiagou

2010 Wenjiagou

Landslide

Old wenjiagou
Step-pools

Dam

Sliding direction

Dams

Dam

Before 2008

Tributary

Steep slope

After 2008

Fig. 11.59 Wenjiagou landslide deposit and old and new Wenjiagou channels (See color figure at the end of this topic)

Figure 11.60 shows a cross section of the Wenjiagou valley before and after the landslide in 2008 at a
distance of 1,500 m from the gully mouth. The thickness of the landslide deposit was more than 150 m.
The central part of the deposit was higher than the two sides. Thus, two new channels formed after
several debris flows in the wet season in 2008. The elevation of the landslide deposit on the right hand
side (face from upstream to the downstream) was much lower than the central part and the left hand side.
The main rainfall runoff flowed at the right hand side, and, thus, a deep and large channel formed on the
right hand side, which became the new Wenjiagou channel. The new Wenjiagou had a drainage area of
4.50 km 2 and an average bed gradient of 0.18. It was 50m deep with bank slopes of 40~50°.
Figure 11.61(a) shows the longitudinal bed profiles of the old and new Wenjiagou channels and the
profile of the landslide deposit. Figure 11.61(b) shows the new Wenjiagou channel after the debris flows
in 2008, which was about 50 m deep. The banks had slopes at the repose angle of the loose solid materials.
If a runoff in the new channel incised the channel bed, the loose solid material might slide down from the
banks into the flow and form debris flow. Therefore, the new Wenjiagou gully was not stable. All water
infiltrated into the loose solid bed for small rainfall intensity and debris flows occurred for rainfall intensity
larger than 30 mm/day in 2008.