Debris Flows and Landslides - River Dynamics and Integrated River Management - page 208

Debris flow could also result in the river moving from its original location to a new channel. The
Xiaojiang River was a meandering river. Since the last century the watershed has become a very active
debris flow area. Debris flows have carried a lot of sediment into the river, and, therefore, accelerated the
siltation of the channel. The river then changed from a meandering into a wandering river. Debris flow
also has caused quick sedimentation of reservoirs.

4.1.4.4 Degrading Environment
Debris flow may cut the gully bed by 3-5 m, sometimes by more than 10 m, and carry 0.1-10 million m 3
of solid material to downstream rivers or hill slope fans. The intensive erosion causes instability of the
slope, collapse of the gully banks, and destroys the slope vegetation. The gravel, boulders, and cobbles
carried by debris flow deposit in the gully mouth and cover grassland and farmland there. The mouth is
then changed into a sea of stone. Debris flow transports a lot of sediment into the downstream rivers and
makes the sediment concentration high and the water quality bad.
Debris flow may destroy forests and result in an enlarged diurnal temperature difference and wind
intensification. Debris flow areas become drier in the dry season and suffer from more intense rainstorms
in the wet season. The groundwater table lowers because it is recharged less due to less detention time of
rainfall on the surface and in surface runoff flows. Farmland is destroyed by debris flow and then the
farmers have to plough the slope land speeding up slope erosion. Debris flow may also change the
landscapes. Figure 4.20 shows that a debris flow buried and scoured farmland by the Jiangjia Ravine in
southern China in 2008.

Fig. 4.20 A debris flow buried and scoured farmland by the Jiangjia Ravine in southern China in 2008

4.2 Effects of Landslides on Rivers

4.2.1 Essential Cause of Landslides

4.2.1.1 Riverbed Incision
The continuous rising of the Qinghai-Tibet Plateau has resulted in steep slopes and several active faults
around the margin of the plateau, including the Longmenshan Faults where the Wenchuan Earthquake