Riverbed evolution in the Yichang-Changmenxi reach downstream of Three Gorges Reservoir

An index K,which stands for the extent of riverbed evolution,is proposed and used to analyze the riverbed evolution in the reach of Yichang to Changmenxi.The study finds that K is inconsistent with the net scour intensity,while positively correlated to the ratio of width to depth and negatively correlated to the median diameter of bed material when the diameter is less than 0.5mm.Preliminary analysis shows that:it is easy to reach the state of transport saturation for coarse sediments downstream of the Three Gorges Reservoir.And the coarse sediments deposit when transport capacity decreases,causing great local deposition on wide-shallow river channels and the net scour intensity decreases,and riverbed evolution strengthens.When the median diameter is less than 0.5mm,the suspend loads is dominant in the downstream scouring amount,resulting that:the finer the bed material is,the greater the scouring a-mount and the riverbed evolution is.Considering the changes of capacity of sediment transport along the river reach after dam construction,two ways of coarsening the river bed are discussed.