Mechanism study of retrogressive erosion in the lower reach of Dongjiang River

Since 1980s,the intensified sand dredging activities in the lower reach and the delta of Dongjiang River have caused significant changes in river hydrodynamics,which results in retrogressive riverbed deformations.The mechanisms and characteristics of the retrogressive erosion are studied through analysis of the river bathymetric and the hydrological data collected during the different sampling periods.The finding suggests that because of the large-scale intensified sand dredging in the reach and delta:(1) For the same discharge,there is a significant decline trend detected in the corresponding water level time series especially during the dry seasons,the stage-discharge relation of Boluo station exhibits a tendency to right deflection after 1980;(2) a significant reduction in riverbed altitudes has led to an increase in water surface slopes,this is the first-stage retrogressive deformation,which in turn will cause an intensified riverbed erosion and result in the second-stage retrogressive deformation that will stabilize riverbeds in the long run;(3) the changes in river cross-sections are inevitable and divided into three parts,which are deformed reaches(Dongjiang River network),deforming reaches(Shilong to Boluo),and deformation effected reaches(above Boluo),respectively,according to processes of their changes.