Abstract:The conditions of incoming water and sediment changes frequently as the result of earthquake

rainstorm

landslide

debris flow and other disasters.Riverbed clogging and overbank flood caused by these changes can lead to destructive flood disasters.In order to analyze the influences of variable conditions of incoming water and sediment on the channel deformation and flood water-level

the motions of flow-sediment of a typical channel in Longxi River of Min River’s tributary reach was investigated and physically simulated on the basis of the research of topography

the composition of bed material and the characteristics of water-sediment condition.Furthermore

the flood disaster mechanism induced by the dual fuction between flow-sediment and channel responses actions was pointed out.There were two schemes involving pure water and sediment laden flow

and seven test conditions were carried out.Sharmov formula was used to calculate the incipient velocity of sediment compared with mean velocity at across sections.The elevation diagrams of three across-sections was drawn as well as hydrographs along the river.The analysis indicatesd that the flood control safety of Longchi Town could not be affected by pure water.On the contrary

the mean velocity reduces dramatically and the flood inundats the bank when a great deal of sediment flow down with the flood，because the gradient decreased and the river width became wider in this reach.The results highlighted the important role of the sediment supply in mountain rivers and showed that the adjustment of the riverbed transport sediment dynamic of different regions controled the sediment transport distance with the increasing amount of sediment supply

causing local riverbed clogging

leading to an increase of flood level and even overbank flood and flood disaster.