Processes and characteristics of recent channel adjustment in the Lower Yellow River

Continuous channel degradation has occurred recently in the Lower Yellow River(LYR)due to water impoundment of the Xiaolangdi reservoir. In this paper,the observed hydrological data from 1999 to 2005 are used to analyze the processes and characteristics of the recent channel adjustment in the LYR. The main research results can be described as follows. The scour amount in the braided reach makes up 77% of the total one in the LYR,and the scour magnitude per unit channel length in the braided reach upstream of Gaocun is relatively large,while the value in the reach between Gaocun and Sunkou is lowest among all the lower reaches. The cross-sectional profiles adjustment in the lower reaches is characterized by the channel narrowing Since 1855 the longitudinal main channel profiles in the LYR are concave-shape curves,and concavity values of these profiles increased slightly with the recent channel degradation. The migration amplitude of the thalweg in the braided reach is greater than that in the meandering reach. The bankfull discharges in the lower reaches increases due to the scouring in the main channel Due to the obvious armoring of bed material,the longitudinal coefficients of channel stability increases slightly in the different reaches. The lateral channel stability almost does not change in the braided reach,and increases slightly in the transitional or meandering reach.