Abstract
Abstract. Debris flows usually occur in steep mountain channels and can be extremely hazardous as a result of their destructive power, long travel distance, and high velocity. However, their characteristics in the initiation zones, which could possibly be affected by temporal changes in the accumulation conditions of the storage (i.e., channel gradient and volume of storage) associated with sediment supply from hillslopes and the evacuation of sediment by debris flows, are poorly understood. Thus, we studied the relationship between the flow characteristics and the accumulation conditions of the storage in an initiation zone of debris flow at the Ohya landslide body in Japan using a variety of methods, including a physical analysis, a periodical terrestrial laser scanning (TLS) survey, and field monitoring. Our study clarified that both partly and fully saturated debris flows are important hydrogeomorphic processes in the initiation zones of debris flow because of the steep terrain. The predominant type of flow varied temporally and was affected by the volume of storage and rainfall patterns. Fully saturated flow dominated when the total volume of storage was < 10 000 m3, while partly saturated flow dominated when the total volume of the storage was > 15 000 m3. Debris flows form channel topography which reflects the predominant flow types during debris-flow events. Partly saturated debris flow tended to form steeper channel sections (22.2–37.3°), while fully saturated debris flow tended to form gentler channel sections ( < 22.2°). Such relationship between the flow type and the channel gradient could be explained by a simple analysis of the static force at the bottom of the sediment mass.
Highlights
Debris flows often occur in steep mountain channels and can be extremely hazardous as a result of their destructive power, long travel distance, and high velocity (Lin et al, 2002; Cui et al, 2011)
Photographs of one debris flow taken by Time-lapse cameras (TLCs) with an interval of 1 s was analyzed (Table 2)
Many sections in the upper Ichinosawa catchment are greater than 22.2◦ (Fig. 10), which is the theoretical channel gradient needed for occurrence of a partly saturated debris flow (α2) in the Ichinosawa catchment
Summary
Debris flows often occur in steep mountain channels and can be extremely hazardous as a result of their destructive power, long travel distance, and high velocity (Lin et al, 2002; Cui et al, 2011). To lower the hazard of debris flow, field monitoring has been conducted in many torrents in the world including in Switzerland (McArdell et al, 2007; Berger et al, 2011b), Italy (Arattano, 1999; March et al, 2002; Arattano et al, 2012), the United States (McCoy et al, 2010; Kean et al, 2013), and China (Zhang, 1993; Hu et al, 2011). In situ classification of debris-flow type is mainly based on moni-
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