Abstract
The initiation of debris flows is commonly attributed either to fluidization as a result of rainfall-induced landslides or to gully erosion induced by concentrated runoffs. A series of flume tests have been performed to show how the initial soil moisture influences the initiation of debris flows. At the start of each experiment, surface runoff was generated over loose granular deposits, triggering debris flows. These experimental debris flows enacted different scenarios according to the small variations among the initial soil moistures. In the loose granular deposits with initial soil moistures ranging from 1 to 5 %, most runoff water could infiltrate and trigger a landslide, which accelerated within 1 s to speed over 1 ms−1 and then transformed into a debris flow. In the same soil deposits with initial moistures >5 or <1 %, the debris flow was initiated by slow gully erosion with episodic events of damming and breaching due to small-scale landslides occurring on the side-slopes of the erosion valley. The slope failures were not triggered by positive pore pressure but by a decrease in suction due to the wetting of the soil. This suction decrease in initially unsaturated slopes explains why the transformation of these slope failures into debris flows are due not only to an increase of pore pressure leading to soil liquefaction, which is one of the expected triggering mechanisms, but also to a loss of the cohesive strength of the soil.
Highlights
It is well known that differing rainfall durations and patterns can affect the occurrence and features of debris flows in channels, but questions are usually raised as to what determines the initiation of scenarios
Can a small difference in the initial conditions cause the runoff to induce sharply differing types of debris flow in the channels? The distinction is important because the occurrence of outburst debris is much more catastrophic and more difficult to predict than the slow sediment transport
The abrupt failure of the soil layer leading to a quick debris flow can be triggered if an abundant amount of runoff water can infiltrate into the soil layer, causing internal erosion within this soil layer
Summary
It is well known that differing rainfall durations and patterns can affect the occurrence and features of debris flows in channels, but questions are usually raised as to what determines the initiation of scenarios. Even if the importance of this effect on critical thresholds has not been widely analyzed, it is clear that the initial moisture content and, the infiltration capacity influences the sequence and intensity of the triggering processes of debris flow.
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