Debris Flows and Landslides - River Dynamics and Integrated River Management - page 255

where ' u 0 is the initial perturbation in velocity, and

is the Froude number. Equation
(4.A23) proves that in a high velocity flow of low yield stress, as long as Fr < 1, the perturbation in velocity
' u 0 always decreases, hence, the flow is stable.
Equations (4.A19) and (4.A23) give the results for the two extreme cases. In the general case, both
terms on the right hand side of Eq. (4.A13) should be taken into account. Then

Fru /( ) h

ª º

d 1 W K

() ' u

B

' 1 Fr ) u (4.A24)

« »

d t 2 U h G

gh

« »

¬ ¼

m

If the fluid has a large yield stress and the Froude number is larger than one, the flow will be very
unstable and quickly develops into roll waves. Many pseudo-one-phase debris flows in the Jiangjia Ravine
are examples of such flows (Kang, 1985a). If the Froude number is smaller than one but the rigidity
coefficient K is small and the yield stress large, the flow also is unstable and develops into wave flow.
Some unstable hyperconcentrated flows in the tributaries of the Yellow River are examples of this type of
flow. Whereas if the yield stress is small and K is large, a non-Newtonian flow is stable at low Froude
number, e.g., flows of lava are stable because magma has an extremely high rigidity coefficient.

Review Questions

1. Where do landslides or debris flows occur in China? What disasters can debris flows and landslides
cause?
2. What is the essential cause of landslides? Why?
3. What disaster chains have been initiated by landslides and avalanches?
4. Under what conditions should a landslide dam be removed?
5. Under what conditions should a landslide dam be preserved? Why?
6. List the main phenomena of pseudo-one-phase debris flows and explain simply the mechanisms of
the phenomena.
7. List the main phenomena of two-phase debris flows and explain simply the mechanisms of the
phenomena.
8. What are the main strategies to control landslides and debris flows?
9. Why do large particles concentrate in the debris flow head even though small particles have higher
instantaneous velocities than large particles?
10. Why do viscous debris flow develop into roll waves?
11. Why does drag reduction occur in viscous debris flows?

References

Bagnold R.A., 1954. Experiments on a gravity free dispersion of large solid spheres in a Newtonian fluid under shear.
Proceedings of the Royal Societyof London, Series A, Mathematical and Physical Sciences, 225(1160), 49-63
Bagnold R.A., 1956. The flow of cohesionless grains in fluids. Philosophical Transactions of the Royal Society of
London, Series A, 249 (964), 235-297
Becker J.S., Johnston D.M., Paton D., Hancox G.T., Davies T.R., McSaveney M.J. and Manville V.R., 2007. Response
to landslide dam failure emergencies: Issues resulting from the October 1999 Mount Adams landslide and dam-break
flood in the Poerua River. Westland, New Zealand. Natural Hazards Review, 8(2), 35-42
Browning J.M., 1973. Catastrophic rock slide in Mount Huascaran North-Central Peru, May 31, 1970. The American
Association of Petroleum Geologists Bull, 57(7), 1335-1341
Chanson H., 1994. Drag reduction in open channel flow by aeration and suspended load. Journal of Hydraulic
Research. 32(1), 87-101