| The phenomenon of river confluence exists widely in the natural river network system.When natural rivers intersect,the water flows are mixed with each other,which will form a series of unique water flow structures including separation zones,stagnation zones,and shear layers.Nowadays,the confluence phenomenon of natural rivers has been studied on the basis of open channel confluence,but the confluence phenomenon of rivers under complex topographic conditions still needs to be further studied.The relevant research can help us to further understand the water flow structure and riverbed evolution in the confluence area,and can provide reference for channel planning,flood prevention and disaster prevention in the river confluence area.This paper takes the multi-estuary confluence area of Liuyang River and Laodao River,located in the Changsha hub reservoir area of Xiangjiang River,as the research object.The EFDC mathematical model is used to establish the water flow and sediment model of the confluence,and the research is carried out for the water flow structure,sediment movement and riverbed evolution in the confluence area after the construction of Changsha integrated hub,and the specific research contents and conclusions are as follows.(1)Based on the measured hydrological data of the reservoir area,the hydrodynamic mathematical model of the confluence area under different flow levels was established,and thus the flow structure of the confluence area was analyzed and discussed.The results show that the flow structure of Liuyang River confluence area is relatively simple,and there is no significant change in the flow pattern of the confluence area under different flow levels;in the confluence area of Laodao River,the topographic conditions are more complex,and the flow structure is influenced by various factors such as the incoming flow of main streams and tributaries,central bar and riverbed topography.(2)The water flow characteristics and sediment flushing and siltation dynamics in the confluence area under different confluence ratio conditions were considered against the background of local flooding in the Liuyang River and Laodao River basins.The results show that under different confluence ratios,the confluence area of Liuyang River and Laodao River has obvious water flow zoning phenomenon,and the separation zone and stagnation zone at the confluence,and the maximum flow velocity zone are very obvious.In the confluence area of Liuyang River,under the extreme working condition(confluence ratio 0.5),the influence range of tributary flow will not exceed 1200m downstream of the confluence area.which will not affect the confluence of Laodao River;in the confluence area of Laodao River.the size and range of the separation zone are obviously small due to the influence of the central bar;Liuyang River and Laodao River confluence areas all show different magnitudes of siltation,with key siltation areas including separation areas,stagnant areas,and around the central bar.(3)Based on the 2015-2020 years runoff and sand transport after the construction of the Changsha integrated hub,the sediment flushing and siltation dynamics of the confluence area in the coming year was simulated,and the evolution of the riverbed in the confluence area and its downstream area was analyzed.The results show that during the non-flood period.the local area is affected by topography to change the flushing and siltation.but the overall riverbed remains stable;during the flood period,there is obvious flushing in the main river channel,but in the confluence area.the depth of flushing is significantly smaller than the rest of the area,and the inflow of tributaries has a certain alleviating effect on the erosion of the main river channel of Xiangjiang River. |
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