Debris Flows and Landslides - River Dynamics and Integrated River Management - page 192

drainage is primarily by subsurface flow with little or no overland flow. During major storm periods, high
soil moisture levels and local areas of saturation are produced on slopes, greatly increasing the unstable
character of the surface materials. Under these conditions, landslide is a dominant process of soil erosion
and sediment transport. In the early 1960's, prior to large scale harvest activities, 1,374 failures occurred
at forested sites scattered over the Tongass National Forest south. Most failures occurred in unique
topographic situations and appeared to be directly linked to initiation by temporary water table development
during high intensity storms (Swanston, 1969). Of the natural (undisturbed) failures identified in this
early survey, 87% were of the debris avalanche/debris flow type (Varnes, 1978).
In 1960 and 1961 the forest experienced large-scale logging. An aerial photographic survey of landslides
occurring between 1963 and 1983 in forested terrain in the Tongass National Forest provided region-wide
data on landslide type, frequency, distribution, and general relations to harvest activities. This survey
involved the location, typing, and terrain characterization of all post-1962 landslides greater then 77 m 3
(100 yd 3 ) in initial failure volume (Varnes, 1978). The post-logging landslide activity showed rate 4 times
the natural rate following the first clear-cut harvesting on a major scale in southeastern Alaska.
4.1.2 Debris Flows
Debris flow is a distinct type of mass movement commonly triggered by intense rainfall and/or melting
snow on steep hill slopes. It differs from landslide in its “flowing” feature. Flow means relative movement
in numerous layers of the medium but a slide occurs only along one or several interfaces or beds.
Although great efforts have been made in the study of the mechanism of motion, considerable ambiguity
persists concerning the initiation and motion of debris flows. Debris flow is turbulent because of its high
flow velocity in some circumstances and is laminar due to its high viscosity in other cases. Debris flow is
extremely unsteady, which is initiated on steep slopes, flows down gullies, and deposits at the mouth of
debris flow gullies.
Debris flow is a widely distributed and frequently occurring hazard in mountainous regions of China.
Chinese people call the phenomenon "dragon", because of its power and irresistible nature. Debris flow
is very disastrous because it carries an extremely high concentration of sediment load and big rocks.
Mechanical properties of debris flow depend highly on the concentration and composition of the solid
material. Debris flows are classified into mudflow, mud-rock flows and water-rock flow according to the
composition of the solid materials. The mud-rock flow, consists of mud and stones. The size of the solid
particles ranges from clay finer than 0.001 mm to boulders of several or tens of meters in diameter. Clay
particles make up a certain proportion of the total amount of sediment, generally about 3-5%. The flocculent
structure of clay particles has a great effect on the dynamic properties and transport capacity, and causes
the flow to experience some unique phenomena. Mudflow often takes place on the Loess Plateau in
China. Water-rock flow mainly occurs in marble, dolomite, limestone, and conglomerate rock areas or
partly in granite mountain areas. The solid material in water-rock flow is mainly composed of coarse
sand, gravel, and boulders. Debris flow can also be classified into glacial debris flow and rainfall debris
flow according to their genesis.
It is more important to classify debris flow according to its dynamic characteristics. So that debris
flows are classified into pseudo-one phase debris flows and two-phase debris flows (Wang et al., 1999).
Pseudo-one phase debris flows are further classified into viscous debris flows, subviscous debris flows,
and low viscous debris flows. Pseudo-one phase debris flow is non-Newtonian, has a large yield stress,
and exhibits laminar flow and intermittent features in many cases. In two-phase debris flows the solid phase
consists of gravel and boulders and the liquid phase consists of water with clay and silt in suspension,
sometimes also sand and fine gravel are in suspension. There is obvious relative movement between the
solid phase and the liquid phase. The two types of debris flows are discussed in detail later.