Study On Unbalanced And Discontinuous Pebble Transport And Its Impact On Waterway In Guangyangba Reach Of Changing Backwater Area Of Three Gorges

After the 175m water storage operation of the Three Gorges Reservoir,the Chaotianmen-Fuling section was affected by the backwater.During the storage period,the water level was raised,the flow rate was slowed,and the navigation conditions were significantly improved.However,during the low water level period,there are still some obstacles to navigation in local river sections and special periods.The Guangyangba waterway(located in the upper reaches of the Yangtze River with a mileage of 631-641km)is located in the lower reaches of the Tongluoxia.The combination of canyons,relaxation,branching and bending makes the river plane shape very complicated.The Three Gorges variable backwater conditions make this The law of water and sediment movement in the river section changes greatly compared with the natural conditions.The evolution of the beach risk and the obstacle navigation characteristics are difficult to grasp,and the Waterway regulation difficulty is bigger.This paper takes the Guangyangba section of the upper reaches of the Yangtze River as the research object,and collects relevant on-site hydrological and topographic data from the site survey.It mainly analyzes the riverbed evolution of the variable backwaters in the upper reaches of the Yangtze River and establishes the 1:100 Normal physical model of the Guangyangba section,and through the test to specifically study the law of pebble transport in the Guangyangba section,and propose a channel remediation plan for its beach insurance.The main conclusions include the following aspects.(1)According to the analysis of riverbed evolution,the overall river regime in the backwater section is basically stable,and the inter-annual erosion and siltation is relatively balanced.The main change is that the sediment transport period of the Three Gorges is more concentrated after the the three gorges reservoir is formed,and the problem of siltation in the local rivers is further aggravated.(2)Tongluoxia Gorge is a large canyon in the upper reaches of the Yangtze River and the shoal affecting ship navigation during flood period,the water flow is gentle during the middle and low flow.There is no pebble bed output below flood flow.When the flow rate is greater than 25,000m~3/s,the pebbles begin to move the Tongluoxia Gorge,and the sediment transport rate increases with the increase of the flow rate,and the sediment transport rate increases sharply when it exceeds 50,000 m~3/s.(3)After the Three Gorges Reservoir was established,the change of water level caused the change of the pebble bed load transport in the deep trough-shoal section.Compared with before the reservoir,the pebbles output of the Tongluoxia Gorge decreased during the flood period,which greatly reduced the sediment transport capacity and sediment transport time.However,the pebbles in zhizhuqi(the downstream of Tongluoxia Gorge)have a weak ability to transport sediment.During the flood period,Due to the reservoir water storage capacity,the sediment transport capacity of the Tongluoxia Gorge is disappeared,which was beneficial to the siltation of the shoal.While,the storage capacity of the shoal was weakened due to the impoundment during the low flow,and the original scouring period was shortened,which was unfavorable for the scouring.(4)Considering the change of sand and the change of unbalanced sediment transport in the deep trough-shoal after the impoundment of the Three Gorges,the shoal management of the tail of the Three Gorges Reservoir can be changed from the traditional dam sloping water to the multi-level dredging.A multi-level dredging treatment scheme is proposed for the current situation of the Guangyangba-Changyeqi reach and the physical model test shows that the backfill volume of the channel is controllable and the channel scale remains good.