Debris Flows and Landslides - River Dynamics and Integrated River Management - page 238

The ratio of drag reduction in pseudo-one-phase debris flow is as high as 60%. The air bubbles play a
role of cushing, which reduces the resistance. It is clearly shown that the concentration of air bubbles
enhances the ratio of drag reduction. About 30% of the drag reduction can be attributed to the effect of
the air cushing.
Fig. 4.58 The relation of the ratio of the drag reduction ( R D ) with the concentration of air bubbles ( C g ) ( C g is the
ratio of the volume of air bubbles to the total volume of the mixture)
4.3.4 slope Debris Flows
4.3.4.1 Slope Debris Flows
Slope debris flow occurs if there are plenty of loose solid materials on the slope. High intensity rainfall
triggers mass movements of the loose solid materials down the slope. These mass movements behave
like debris flows but the distance of movement is, however, much shorter than normal debris flows, and
in general travels only several tens to one hundred meters. Such a mass movement is called a slope debris
flow. In general, slope debris flows occur on steep slopes without apparent drainage area and streams.
This is different from the normal debris flows that flow in gullies with a certain drainage area.
Slope debris flows occur at piled gangue deposit and grain erosion deposit fans, the latter has been
discussed in Chapter 2. Figure 4.59 shows a slope debris flow at a gangue deposit of a gold mine on a
mountain slope in Lixian County, Gansu Province of China. A huge amount of gangue deposit was piled
on the mountain slope Fig. 4.59(a), which had a relatively uniform size distribution with median diameter
of about 20 mm. The slope of the deposit was equal to the repose angle (about 32°). Heavy rainfal caused
slope debris flows (b). The slope debris flow traveled only 60 m and deposited at gengle slopes.
Fig. 4.59 (a) Gangue deposit of a gold mine on a mountain slope in Lixian County, Gansu Province; (b) A slope debris
flow on the gangue deposit which was initiated by heavy rainfall (See color figure at the end of this topic)