Debris Flows - Environmental Geomechanics - page 5

Figure 1.4. Small-scale simulation of a debris flow in the laboratory. The solid and liquid
phases are mixed

mixture takes on the appearance of a “viscous” homogenous fluid flowing down the
bed. Figure 1.4 (slope of 27%) illustrates such a transition and the resulting mass
movement. Most laboratory experiments conducted with water flows on erodible beds
have shown that the bed inclination θ is a key factor in sediment transport dynamics
[KOU 93, LAN 93, RIC 92, SMA 83, TOG 99]. On the whole, it has been observed
that:
- θ< 20%: at sufficiently high water discharges, water flow induces intense bed
load transport near the bed. As a first approximation, the water and solid discharges
(respectively, q w and q s ) are linearly linked: q s ≈ 8 . 2 θ 2 q w (this relationship is an

overly simplified expression of discharge obtained by Smart and Jaeggi [SMA 83] or
Rickenmann [RIC 92, RIC 97]). Three layers can be distinguished: the bed made up
of stationary particles (that can be eroded), the (active) bed layer in which sediment of
all sizes is set in motion (rolling and sliding), and the water layer, where fine particles
are in suspension or in saltation. In two-phase flows of this type the solid concentration
(ratio of solid volume to total volume) does not exceed 30%.
- θ> 20%: at sufficiently high water discharges, bed load transport is unstable. It
changes into a dense single-phase flow. The solid concentration is very high, ranging
from 50 to 90% depending on the size distribution of particles. Such flows simulated
in the laboratory correspond to debris flows in the field.

In the laboratory, the transition from bed load transport to debris flow is reflected
by a discontinuity in the solid concentration. It is suspected that such a discontinuity
still exists in the field, at least in the Alps, but the underlying mechanisms are