Distribution of hazard types in a drainage basin and its relation to geomorphological setting

Abstract

Hazards along a river course in a drainage basin are characterized by three types of events: debris flow, turbidity flow and flood. Each has its own channel segment with different sediment transport processes. The sediment transport process is controlled by transportation force: tractive force and slope-direction component of sediment weight, both of which are significantly affected by channel slope. Along large rivers in Japan the boundary between the upstream turbidity flow segment and the downstream flood segment is located at the position with a channel slope of about 1/1000. Along steep, small rivers the boundary between the upstream debris flow segment and the downstream turbidity flow segment is located at the position with a channel slope of about 80/1000. The channel slope depends on the shapes of longitudinal profiles of rivers. The longitudinal profiles of Japanese rivers, main rivers and tributaries, can be described by an exponential, power or linear function. The transportation force of the rivers fitted with exponential functions markedly decreases downstream due to the large curvature of the profiles, causing noticeable sediment deposition in the middle courses. The transportation force of the rivers fitted with power or linear functions maintains its strength through the whole river courses due to the small curvature, causing sediment transportation down to the lower courses. The function types are strongly affected by relief in the drainage basins. The rivers flowing in small relief areas are fitted with exponential functions and those flowing in large relief areas are fitted with power or linear functions. Thus, the distribution of hazard types along a river course in a drainage basin is controlled by the distribution of relief in the drainage basin.