Debris Flows and Landslides - River Dynamics and Integrated River Management - page 235

and push forward. Debris flow also caused serious bank erosion due to its high velocity and shear force,
and even causes movement of the debris flow channel.
As debris flow carries a huge amount of debris to the mouth of the gully, the solid particles will
deposit because of a sudden reduction of the channel slope. A huge fan-shaped stone-sea appears and
sometimes the deposit dams rivers. The rate of aggradation also is quite high. It is estimated that more
than 1.15×10 8 tons of solid material accumulated and the bed rose 16 m in the mouth area of the Guxiang
Gully in the period 1953-1965.
Figure 4.53 shows the aggradation and degradation of the Jiangjia Ravine in the periods 1957-1985 and
1957-2002 (He et al., 2003). Gully erosion is the main type of erosion, which produces 75-83% of the total
sediment yield from the ravine. The eroded sediment from the upstream reaches was transported in debris
flows and deposited in the downstream reaches of the ravine. The gully bed aggraded by more than 40 m
in the period of 1957-2002. A huge amount of sediment is stored in the downstream reaches and the mouth
of the ravine. The same story has occurred in numerous debris flow gullies. All the mouths of the debris
flow gullies along the Xiaojiang River have been silted up by several tens of meters in the past 50 years.
Fig. 4.53 Aggradation (+) and degradation (-) of the Jiangjia Ravine in the periods 1957-1985 and 1957-2002 as a
function of distance from the ravine mouth (after He et al., 2003)
The Dongchuan Debris Flow Observation and Research Station of the Chinese Academy of Sciences
is on the Jiangjia Ravine. The first observation tower was 30 meters higher than the channel bed in the
1970s. Aggradation of the debris flow channel endangered the observation tower, as shown in Fig. 4.54(a)
and people had to give up the tower and build a new one on much higher ground in the 1990s. The
channel bed silted up and approached to the tower base. In 2009 the observation tower fell into the ravine
due to direct assult of flow at the tower base, as shown in Fig. 4.54(b).
(a) (b)
Fig. 4.54 (a) Aggradation of the debris flow channel endangered the original observation tower on the Jiangjia Ravine
in 2006; and (b) The tower fell into the ravine due to direct assault of flow at the tower base in 2009