Study On The Shaping Process At The Doulizi Pebble Beach After Sand Mining In The Yangtze River

Doulizi beach is the well known dry rapids at the upper Yangtze river.Because the local infrastructure investment has increased significantly,the increasing demand of sand has caused the increasing of the mining scale along the beach.Long distance and wide range of river sand mining,irregular pit and the changing characters of the bed load transport have increased the uncertainty of channel grade promotion and affects the advantage of shipping in mountain rivers,thus the river bed reconstruction after sand extraction should be researched urgently.Based on the analysis of the prototype data,a combination method of fixed and movable bed physical model experiment and the 2D water sediment dynamics simulation based on the orthogonal body fitted coordinate system was selected.Bed load movement was observed and analyzed in Doulizi beach.The scree bed load transport characteristics in the whole beach section under different boundary conditions and the changes of water flow and sediment transport indexes before and after sand mining were simulated,respectively.The recycling process focusing on the sand reclamation area was predicted,and the results are as follows:Based on the experiment of bed sediment transport,the distribution of pebble transport zone under typical flow rate is obtained.According to the distribution obtained,bed moving experiment of typical scree is developed.Through the analysis of Doulizi beach section of bed load movement observation,the scree transport speed is obtained.The velocity formula of the pebble bed load movement is corrected based on the experimental results,which improved the calculation accuracy.Based on the turbulent structure of water flow,a mathematical model calculation method of bed load transport route is proposed.A 2D mathematical model of water and sediment is established,and the water level,velocity,diversion ratio,river bed deformation,and scree bed load transport route and so on are calculated.Compared with the existing actual measured data,the coincidence degree is high,which proves that this model has the ability of simulating the water and sediment characteristics of the river,thus this model can be used as a method of quantitative study on the reconstruction of Doulizi beach section after sand mining.By using the established mathematical model of 2D water sediment and bed load transport route simulation,water flow and sediment characteristics are simulated at Doulizi beach section under different flow.The sediment transport path of scree in both flood and dry season are compared and analyzed.And erosion and deposition are predicted in this reach.On this basis,four typical flood flow: 22000m3/s,37100 m3/s,44100m3/s,54500 m3/s are selected.The water level,typical section velocity and the changing of sediment discharge rate factor are studied at the Doulizi sand section before and after sand mining.And the mining district recycling process is simulated through the mathematical model,the results show that the recovery process in the sand mining area is slow comparatively,and that it is difficult to restore the original riverbed.But overall,the existence of the sand mining area has rare influence on navigation of the Doulizi beach section.