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Channel initiation is a major mechanism of landform evolution, and channel heads are the points where channelization occurs. In mountainous or hilly landscapes, the channel initiation often occurs by shallow landslides. Although topographic observation has revealed climatic or lithological controls in thresholds for channel initiation, few studies have reported the migration of channel heads caused by storm events with shallow landslides. The temporal variation in channel-head location before and after storm events was investigated in two granitic mountainous areas of Hiroshima and Hofu cities, western Japan. High-magnitude low-frequency storm events caused shallow landslides and debris flows in 2014 for the Hiroshima area and in 2009 for the Hofu area. Digital elevation models (DEMs) with a resolution of 1 m based on airborne LiDAR (light detection and ranging) are available for both pre- and post-event conditions. The location of pre-event channel heads was morphologically estimated from an interpretation of pre-event DEMs. The location of post-event channel heads was confirmed in the field. The analysis showed a number of channel heads were migrated upstream or initiated in hollows due to shallow landslides during the storm events. While relationship between source-area size and local slope were statistically not significant for pre-event channel heads, post-event channel heads with landslides showed significant inverse area-slope correlations for both Hiroshima and Hofu areas. On the basis of a cyclic model of soil recovery and landsliding in a geomorphic hollow, channel heads with larger size of source areas have higher potential for landslides. This hypothesis is tested in the two areas. For the Hiroshima area, probability of shallow landslides was high in the channel heads with larger pre-event sizes of source area. This agrees with a conceptual model for the migration of channel heads, in which the size of the source area increases with time after the last landslide event. In contrast, the pre-event sizes of source areas were not related to the landslide occurrence for the Hofu area. The result implies that the local conditions control the spatio-temporal pattern of channel-head migration. |
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