Generally, debris flow occurs by heavy rainfall or increment of surface water. But the occurrence condition of debris flow may be influenced by conditions of stream bed deposit, such as shape, gradient and its deformation. In past studies, through the hydraulic model experiment for analyzing debris flow occurrence, experimental flume is steep and equal gradient, huge amount of water must be supplied, and stream bed deposit has been saturated. But in natural torrent, riverbed gradient changes gentle toward downstream, stream bed deposits are not uniform shape, and also deposit sometimes unsaturated. We observed and analyzed the condition and deformation of stream bed deposit at initiation zone of debris flow through the hydraulic model experiment. The experiment was carried out on a experimental flume whose bed slope is 30‹ at the upstream end and 12‹ at the downstream end, supplying a small amount of water nearly infiltration flow at first and rises as surface water level during rainfall. In the steep section as 27‹|30‹ with small amount of surface water, a moving layer has been formed and flow down gradually. This moving layer starts to move when the layer is saturated. Gradient turns small as 21‹|24‹, moving layer start to deposit and form a dune. Upper part of the dune has no surface water but lower part has it and flow down. This dune starts to move or deforms when surface water reaches and flow over the edge of the dune.

Key wordsFdebris flow, initiation zone, deformation of stream bed deposit, moving layer, dune