It is important to understand the development and decay processes of debris-flows in order to plan effective debris-flow countermeasures. However, few studies have successfully observed the development and decay processes of debris-flows. This study aimed to reveal changes in characteristics of debris-flow surges as they flow down, based on observation using time lapse cameras installed at multiple sites along a debris-flow torrent in the upper Ichinosawa catchment within the Ohya landslide, central Japan. Observation results showed that debris-flow surge volume and flow velocity tended to increase in the section just below their initiation point. In the subsequent section, debris-flow surges tended to maintain their volume and flow velocity while descending. Increases in flow velocity were observed in sections with a fixed bed, the channel bed consists of exposed bedrock with no sediment cover. Debris-flow surge volume and velocity tended to decrease in these sections, in which channel gradient decreases abruptly. These observation results can be explained by the theory of equilibrium concentration, which states that sediment concentration in the flow approaches the equilibrium concentration given from the channel gradient by the erosion and deposition of sediment. At the same time, small debris-flow surges tended to terminate with a short travel distance, which cannot be explained fully by the theory of equilibrium concentration.
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