Debris Flows and Landslides - River Dynamics and Integrated River Management - page 241

slope of the debris flow deposits in the Jiangjia Ravine was about 5°, and the slope of the slope debris
flow deposit was about 10°.
Fig. 4.63 Deposit slope ȕ of slope debris flows in the Wenchuan Earthquake area and of normal debris flows in the
Jiangjia Ravine as a function of the drainage area
4.3.4.5 Low Water Content in Slope Debris Flows
All water in the slope debris flows was from local rainfall because there was a very small drainage area.
Water and small amounts of clay and silt mixed into a liquid, which plays the role of lubricant among
the coarse particles instead of as a carrier. The concentration of total solid materials in the slope debris
flows was very high. Measured solid concentrations by weight for slope debris flows were 83-91%.
Because surface runoff was not needed for triggering slope debris flows the rainfall intensity to trigger
slope debris flow was much smaller than the rainfall for initiating normal debris flows. The statistics of
the 19 slope debris flow revealed that a rainfall intensity higher than 20 mm/day may trigger a single
slope debris flow; a rainfall intensity higher than 30 mm/day may trigger several slope debris flows
simultaneously; and a rainfall intensity higher than 90 mm/day may trigger a group of slope debris flows
simultaneously. Because the water content in slope debris flows was very low collisions between solid
particles were intense and the velocity was very low. The travel distance of slope debris flow was only
several tens of meters.
4.4 Mitigation of Landslide and Debris Flow Hazards
4.4.1 Mitigation of Landslide Hazards
The Xintan Landslide (Fig. 4.7) was very hazardous but did not cause a great disaster thanks to accurate
prediction. Xintan Town has recorded more than 20 landslides in 1,000 years. A landslide in 1030 was
triggered by an earthquake and dammed the Yangtze River for 20 years. In 1559 a landslide destroyed a
village and killed 300 people. The old sliding body slid again in 1936 and destroyed 20 houses. In 1958 a
sliding body with a volume of 30,000 m 3 moved again and killed 2 people and destroyed a factory. In
1964 a landslide with a volume of 100,000 m 3 occurred. In the period after 1964 a series of small-scale
landslides have occurred in the Xintan area.
The prediction of the landslide on 12 June 1985 was very successful. Figure 4.64 illustrates the Xintan
monitoring system, which has detected creeping of the sliding body since 1982. Movement of the sliding
body intensified in 1983 and at a few observation points the system measured 1-7 m of displacement
from July 1984 to November1985. On 9-11 June 1985, many cracks appeared on the slope, hot air emitted
from the ground and villagers heard abnormal noise from the ground. Scientists predicted a large-scale
landslide 24 hours before the event on 11 June. Because of the accurate prediction all 1,371 people living
in the town were evacuated and great casualties were avoided (HLIG, 1985).