A Review of Japanese Debris Flow Research

The phenomenon of debris flow has been recognized since ancient times in Japan, where debris flow disasters have caused damage and suffering. In steep mountainous areas of Japan, residents have given evocative names to debris flows to warn future generations of their danger. Such names include “Ja-nuke” (the runoff of the king snake), “yama-tsunami” (mountain tsunami), and “yama-shio” (mountain tide). However, because debris flows usually originate on remote high mountain slopes or steep ephemeral gullies and arise during adverse weather conditions, the characteristics and mechanisms of debris flows had long remained poorly understood and the phenomenon was even considered a “phantasmal disaster. ” Scientific investigations of debris flow began in the 1950s and included qualitative discussions of the definition of debris flow. Nomitsu and Seno [1959], Tani [1968], and Murano [1968] defined debris flow as the gravitational motion of a porridge-like mixture of sediment and water, in which the volume of sediment is much larger than the volume of water. Koide [1955] classified debris flow into “yama-tsunami” and “doseki-ryu” types. He defined a yama-tsunami as the pushing ahead of a cohesive earth mass produced by a landslide along a valley and a doseki-ryu as the pushing ahead of sediment accumulated on a gully bed. Thus, a doseki-ryu had no direct relationship to a landslide. Kaki [1954] defined a yama-tsunami as a forward-moving mixture composed of 70% soil and 30% water and a doseki-ryu as a flow composed of 30% soil and 70% water with relatively thin depth. The term doseki-ryu is now widely used in Japanese to refer to debris flow; “doseki” means a mixture of soil and stone, and “ryu” means flow. Tani [1968] classified three types of debris flow initiation: 1) transformation of a landslide block into debris flow, 2) collapse of a landslide dam, and 3) sediment entrainment into the surface water flow on a gully bed. He also compiled several observed characteristics of debris flow: 1) The lateral surface profile of flow is upward convex, with the front part swelling longitudinally. 2) The flow is dominated by soil and stone rather than water and resembles a moving hill composed of stone, sand, mud, and wood that produces a smoke-like cloud of debris and a bad smell. 3) The sound is like that of thunder or a formation of airplanes. Yano and Daido [1965] referred to debris flow as “mud flow. ” They defined it as a flow of muddy clay and modeled its motion by applying concepts of pseudoplastic fluid and Bingham fluid. Full investigations were not undertaken until the 1970s. By that time, improvements of major rivers had lessened the risk of large-scale flooding disasters. However, management of minor rivers lagged behind that of major rivers, and the use of slope lands for human activities increased vulnerability to sediment hazards. Before 1964, among human victims of total water-related hazards, 32% were affected by sediment events. Since 1965, this percentage has increased to approximately 50% [Takahashi, 2009]. This situation provides strong motivation for research on debris flow and other sediment hazards.