Debris Flows and Landslides - River Dynamics and Integrated River Management - page 233

The extremely high superelevation of debris flow is disastrous and causes casualties very often because
people pay less attention to the risk of debris flow if their houses are located on high slopes. Figure 4.50
shows a building on a slope of the Qiongshan Gully in the upper Dadu River basin, which was destroyed
by a debris flow that occurred at about 11 pm July 15, 2003. The head of the debris flow climbed on the
slope of more than 15 m smashing the building and killed 51 people, when they had a party in the building
(Chen et al., 2004).
Fig. 4.50 A debris flow in the Qiongshan Gully in the upper Dadu River basin climbed on a slope of more than 15 m
and smashed a building killing 51 people in 2003
4.3.3.6 Bimodal Grain Size Distribution
Many debris flows have a bimodal grain size distribution. Figure 4.51(a) shows the measured granulometric
curves of debris flow in the Liuwan Gully, northwest China. In the figure p is the frequency density of grains
of diameter D . Each curve in the figure has two peak values in the frequency density. The first peak is
located at D = 0.01- 0.1 mm and the second one is located at D = 2 - 40 mm.
Fig. 4.51 (a) Bimodal grain size distribution of debris flow in the Liuwan Gully; (b) Grain size distributions of
torrential flood, sub-viscous debris flow and viscous debris in the Jiangjia Ravine (after Kang, 1985b)