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Debris flows

Landslides comprising soil, rock and organic material are termed debris slides or debris flows, with a distinction made largely to recognize the form of movement along the event path. Where initial movement occurs as a slide, it often progresses quickly to a flow. Debris flows are a common natural hazard in mountainous terrain.

The path of a debris flow comprises an initiation zone, a transportation zone and a zone of terminal deposition. The initiation zone, where the onset of failure occurs, is typically found within a gully channel or on an open hillslope. In the case of a gully, it may occur at the headwall, on a steep side slope, or within the steep bed of the gully channel. In the case of an open hillslope, it may take the form of shallow sliding on a translational plane of slip, else it may initiate at a discrete point of quasi-liquefied flow. Often, the initial failure volume is small in comparison to the peak magnitude of the resulting event. Travel distance commences at the point of origin of the event.

Downslope movement occurs through a transportation zone. Events that initiate in a gully typically remain within it. Events that initiate on an open slope may remain on the slope, or may enter a gully as a consequence of topographic constraints. Three modes of flow are considered in UBCDFLOW: unconfined flow (UF) on an open slope; confined flow (CF) in a gully channel; and transition flow (TF) deemed to occur immediately upon exiting a gully channel onto an open slope. Movement is rapid and may, on occasion, involve more than one surge of debris. It is accompanied by processes of erosion, entrainment, transport and deposition, resulting in a cumulative flow volume that tends to increase with distance travelled.

Typically entrainment dominates on steeper slopes and deposition on gentler slopes. The zone of terminal deposition is distinguished by the onset of major deposition, typically in response to encountering a relatively gentle slope angle, experiencing a loss of confinement, or a combination of both influences. The cumulative flow volume diminishes with continued runout distance, as debris is deposited and the event comes to a halt. Total travel distance is measured from the point of origin to the end of this terminal deposit, where the event volume diminishes to zero.

Debris flow near a road.

Debris flow activity is a natural hazard, with potential to cause loss of life, to inflict property damage, and to impact the environment. A landslide risk analysis requires the hazard be identified, and the nature of these consequences be established with respect to injury or loss. The probability of occurrence is the hazard. Consequence is governed by the potential for the debris flow event to reach a location of interest, and the likelihood it impacts an element of interest at that time. Travel distance of a debris flow is therefore important to any risk analysis because, given an event that may occur, it describes the potential to reach a location along the expected path of movement.