Mechanism of Sudden Channel Change of Mountainous Rivers due to Hydraulic Jump in Channel Bifurcations.

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The purpose of this study is to make clear the origin of channel blocking phenomena sometimes observed in a bifurcated channel in mountain rivers. The followings were found through field observations, model experiments and theoretical analysis for origin of channel blocking :1.Three kinds of bifurcation patterns of channels are found in mountain rivers : Those are inner, outer and straight branching. Large scale of blocking takes place in outer branching channel.2.A large amount of sediment deposition to block the entrance of one bifurcated channel are brought on by a hydraulic jump which takes place in the front of bifurcation wall (i.e. island). This was verified through the experiments.3.The momentum equations for the flowing direction and for the normal direction to it, applied to two control volume sections enclosing the bifurcation of channel, have four kinds of solutions with respect to the divided discharge ratio and water depths at the two entrances of bifurcated channel : They are corresponding to any state that two channels are in supercritical flow, or one is in supercritical and the other is in ordinal flow, or both are in ordinal flow.4.It was confirmed in the experimental channel that the divided discharge ratio shifts from one solution to others when total discharge was reduced in steps ; the channel blocking was advanced with the change of discharge.5.From the above, it will be concluded that origin of a blocking of bifurcated channel is a large amount of sediment deposition in a hydraulic jump which takes place near the bifurcation of channel and the sudden discharge change due to the shift of flowing state.