Definition of Technical Term

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Debris flow is also called a mass flow because the gravel is visible even on the surface of the flow, and it looks like a mass of gravel flowing as a whole. On accumulation layers that have no stickiness or cohesion, the gradient generally needs to be 15° or more for a debris flow to occur.

If the gradient is less than 15°, gravel particles are not capable of spreading throughout all levels of the flow, and instead, the flow becomes separated into two levels; the lower level is similar to a mass flow, and the upper level is similar to a water stream. This type of sediment discharge is called an earthflow. Since the upper level of an earthflow has a flow similar to that of a water stream, and an earthflow is similar to a bed load transport, it is also called a bed-load mass flow.
Furthermore, the less the gradient becomes, the closer the boundary between the 2 levels becomes to the riverbed, thereby forming a sediment discharge like a prototypical bed load.

In the paragraphs above, an earthflow is categorized to be part of the transition process of a debris flow. If that is the case, the measure planning, especially when designing the facility structure, needs to reflect such categorization, and take into consideration the characteristics of the flow and sedimentation of the area in which the facility is being planned.

Sedimentation characteristics of gravel debris flow and earthflows

Categories Characteristics
Debris flow
  • There is/are a boulder(s) in front.
  • Some of the gravel is up to 1.5 meters in diameter.
  • The center rises across the flow.
  • There are signs that indicate that the flow came from upstream, often directly and in a straight manner.
  • The sediment is relatively thick.
  • There are signs that indicate the flow was carried along by a bed load transport.
  • Some of the gravel may be 0.5 to 1.0 meter in diameter, but most are granular.
  • The sediment is wide-spread.

Translated from Journal of Japan Sabo Association, Sabou to Chisui, Vol.161, pp.118, 2004