Study On Channel Stability And Waterway Carrying Capacity In The Middle And Lower Sandy Reaches Of The Yangtze River

The joint operation of the reservoir group dominated by the Three Gorges Project(TGP)has changed the flow and sediment conditions of the middle and lower reaches of the Yangtze River,and the lower reaches of the dam have undergone a long-duration and long-distance adjustment and reshaping process.Therefore,the regulation rules of sandy channel in response to the flow-sediment variation are different and delayed,causing two-way changes of the waterway conditions,and also increasing the difficulty in predicting the development and utilization of waterway.Under the requirements of high-quality development of the Yangtze River Economic Belt,the construction of the Yangtze River Golden Waterway should not only meet the needs of regional economic and social development,but also be promoted under the premise of flood control security,water resources,river ecology,etc.Therefore,it is of great practical significance for the high-quality construction and sustainable development of the Golden Waterway in deeply understanding the changes and influencing factors of channnel stability in the middle and lower reaches of the Yangtze River after the TGP operation,clarifying the relationship between the difference adjustment of channel and the development potential of waterway,and further identifying the exploitable threshold of channel scale under the consideration of the multi-objective coordination of flood control,shipping,water supply,ecology,etc.Based on the methods of measured data analysis,theoretical elaboration and mathematical models,this paper mainly focuses on the following aspects:analyzing the impact of unbalanced water-sediment transport on the channel adjustment in the middle and lower reaches of Yangtze River,discussing the response adjustment characteristics and influencing factors of different types of sandy channel,determining and improving the evaluation method of channel stability,and analyzing the adjustment trend.On this basis,the difference impact of channel adjustment on the development potential of waterway is analyzed,and an evaluation model for the waterway carrying capacity in the middle and lower reaches of the Yangtze River is constructed,considering the cooperation of multiple constraint factors.The built model is also applied to the reach from Hukou to Nanjing in the lower reaches of the Yangtze River,and the development thresholds of waterways scale are proposed by regions.The main research contents and achievements are as follows:(1)The temporal and spatial characteristics of flow-sediment variation at the main control hydrological stations were quantitatively investigated in the middle and lower reaches of the Yangtze River,and the influence of flow and sediment non-equilibrium transport on the cumulative effect of channel adjustment was discussed and clarified.After the TGP operation,the suspended sediment load still played a dominant role in shaping the downstream channel of dam,but the proportion of bed load to suspended sediment flux in the middle and dry season was higher than that in the flood season.The midum and low diacharges(<22000m~3/s)had a larger contribution to the cumulative geomorphic adjustment in the middle reaches(Jianli hydrological station)of the Yangtze River,while the midum and high diacharges(>30000m~3/s)had a larger contribution to the cumulative geomorphic adjustment in the lower reaches(Datong hydrological station)of the Yangtze River.(2)The geomorphic adjustment mechanism of channel in response to the changes of flow-sediment fluxes was revealed in different types of downstream sandy riverbed of the dam,the evaluation method of channel stability was determined and improved,and the adjustment law of channel stability was clarified.The geomorphic dynamic process of bends in the lower Jingjiang River(LJR)was jointly determined by the incoming water and sediment conditions and the bending shape.After the TGP operation,the medium discharges(16000-18000m~3/s)had the highest contribution to shape the morphology of point bars and banks,the erosion volume of which increased with the decrease of sediment flux.Low sediment supply would lead to more upstream planform deformation,which tended to occur in the highly curved(sinuosity>2.0)or compound bends of the LJR.The morphological adjustment of point bars in these bends was more conducive to cause flow deflections,leading to the formation of concave-bank bars,which promoted the morphological transformation of bends.After the TGP operation,a power function relationship was founded between the morphological deformation of channel bars in the braided reaches downstream of dam and the flow erosion intensity,but the correlation coefficients were decreasing downstream.The changes of sandy channel stability at the downstream of the dam were affected by the combination of incoming water and sediment and channel boundary conditions,but the early incoming water and sediment regime were the key influencing factors.On this basis,the longitudinal and lateral channel stability indexes were determined.Considering the difference of flow and sediment transport capacity in different cross-section shapes,the calculation method of the comprehensive channel stability index was improved by introducing the concept of sediment-concentration-depth,and the discrimination threshold of the stability index between the curved and braided sandy channels at the downstream of the dam was determined to be 0.055,.The analysis results showed that the stability indexes of typical sandy channel at the downstream of the dam were positively correlated with the flow erosion intensity,the longitudinal and lateral stability were increasing,and the river channel was generally developing towards a stable direction,which was beneficial to waterway stability.(3)The impact of sandy channel adjustment on the development potential of waterway resources was clarified in the reaches of downstream TGP,and clarifies the background of waterway development under natural endowment conditions was determined.After the TGP operation,the adjustment trend of"convex-bank scouring and concave-bank silting"in the typical meandering bends(LJR)led to the thalweg shift and navigation channel silting in highly bends,which affected the stability of the waterway boundary and navigation safety.In addition,the channel bar of typical braided reach were scoured and shrunk in the middle reaches,causing the main flow to swing,which affected the stability of the navigation channel,while the typical braided reaches in the lower reaches were partially silted and deformed,resulting in the narrowing of the navigation channel,which was unfavorable to navigation safety.The average depths of low-flow channels were a positive correlation power function with the flow erosion intensity in the middle and lower sandy reaches of the Yangtze River,and it showed an increasing trend year by year,which was beneficial to the improvement of waterway depth resources.By comparing and analyzing the channel stability between the channel section with navigation obstruction and the excellent transition section,the fluvial facies relation of the selected excellent section and the water-depth correction coefficient were introduced,a stable navigation depth formula considering the water and sediment transport factors was established,the maximum stable navigation depth under the natural endowment conditions was estimated,and the background conditions for the development of waterway scale were clarified.(4)The evaluation model of waterway carrying capacity in the middle and lower reaches of the Yangtze River was constructed under the cooperation of multiple constraint factors.The synergetic relationship between the waterway carrying capacity and the main control factors of ecological protection,economic demand,flood control safety and water resources allocation was analyzed.A hierarchical indicators system framework of waterway carrying capacity was introduced,including four subsystems(i.e.,ecological protection,economic demand,water resources allocation and flood control safety),a total of 18 evaluation indicators.In addition,the quantitative methods of some qualitative indicators such as ecological sensitive area,channel stability,and suitability of aquatic habitats were optimized.Combined with the complementary advantages of comprehensive index evaluation and fuzzy pattern recognition methods,a fuzzy comprehensive evaluation model of waterway carrying capacity was developed,which covered a flow module,a sediment module,an ecological module and a comprehensive analysis module.(5)The temporal and spatial variation of waterway carrying capacity were revealed from Hukou to Nanjing in the lower reaches of the Yangtze River,and the exploitable threshold of waterways scale were proposed by regions.Based on the background conditions of waterway resources from Hukou to Nanjing section,it was divided into three evaluation sections,i.e.,Hukou to Anqing,Anqing to Wuhu and Wuhu to Nanjing.The evaluation models of waterway carrying capacity were established in typical representative river sections,respectively.The results showed that the carrying capacity of waterway from Hukou to Nanjing tended to decrease as the channel scale increased.In a certain future period,the exploitable thresholds of channel scales in the sections from Hukou-Anqing,Anqing-Wuhu and Wuhu-Nanjing reaches were estimated to be 8.0×200×1050m,8.0×200×1050m and 10.5×500×1050m(depth×width×radius).The evaluation results showed that the proposed evaluation model demonstrated some advantages in reflecting the trade-off mechanism between resources consumption,river-ecological pressure and socio-economic benefits.Combining the advantages of multiple evaluation methods,it could objectively clarify the assessment grade boundary and identify the carrying capacity grades of different layers.