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Experimental examination for influence of debris-flow hydrograph on development processes of debris-flow fan
Tsunetaka, Haruka Norifumi, Hotta Sakai, Yuichi Nishighchi, Yuki Hina, Junya
Tsunetaka, Haruka
Norifumi, Hotta
Sakai, Yuichi
Nishighchi, Yuki
Hina, Junya
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2019
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Abstract
In order to assess the influence of different flow hydrographs on fan development processes, we carried out flume tests using a sloped channel (15°, 10 cm wide) with a deposition area (slope decreases from 12° to 3° at a rate of 3° per m). The channel was filled with 0.12 m3 of sediment materials. Debris flows were generated by the entrainment of filled sediment via a steady water flow (0.003 m3/s). We used two types of water supply systems: single surge (60 second duration) and double surge (first surge lasting 50 seconds followed by a second surge with 45 second duration). For the double surge system, there was a 60 second pause in water supply between two surges. Fan formation processes in the deposition area were captured on video, and synchronized interval photography (1 second intervals) using three digital cameras. Time-series changes in fan topography were detected using Structure from Motion photogrammetry (SfM), while flow directions were detected using Particle Image Velocimetry (PIV). The results demonstrate that the flows of single surge cases produced asymmetric fans that inclined to one side due to an increase in the runout distance of the continuing flow. In contrast, the first surge of the double surge cases produced fans that were relatively symmetric. Despite this, the second surge continuously changed flow direction while stopping in the deposition area, and covered the symmetric surge produced by the first surge. Consequently, the final topography of double surge cases was highly variable, despite having the same water supply conditions.
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