| With the increasing development and utilization of rivers,the natural hydrological situations of rivers have been significantly changed by the construction of large-scale water conservancy works.The long-established self-adaptation relationships between flow-sediment transport and river morphology have been broken while these water conservancy works bring a series of comprehensive benefits,including flood control,electricity generation,navigation and ecological environment.The Yangtze River where has abundant fresh water resources and huge amount of shipping capacity is the longest river in China and the third longest worldwide with around 6300 km.With the intensifying human activities in the Yangtze River including water-soil conservation projects,reservoir construction,waterway regulation works and sand extraction,the variation trends in streamflow and sediment have been obviously altered.Particular in the construction of large-scale water conservancy works,the water discharges and sediment transport in the mainstream and tributaries both varied markedly.The operation of the Three Gorges Dam altered the hydrological processes of the inner-annual flow-sediment transport and thus the adjustment characteristics downstream the dam also changed.Riverbed rebuilding is the result of the mutual adjustment and adaptation between riverbed and the varied flow-sediment regimes.Different operation rules of dams change the processes in streamflow and sediment in varying degrees,and river morphology determines the flow-sediment transport in turn.Revealing the interaction mechanisms between the altered flow-sediment regimes and the riverbed adjustment under the impacts of large-scale water projects has important theoretical and practical significance in deeply understanding the river evolution,improving the principle of river self-regulation,assessing the impacts of reservoir construction on riverbed evolution reasonably,as well as optimizing the operation rules of dams.The river self-regulation caused the riverbed evolution to adapt to the varying flow discharges and sediment transport,developing toward a certain state of equilibrium and hydraulic geometry until reaching a relative equilibrium state between water and sediment processes and sediment-carrying capacity.Riverbed evolution includes one-dimensional,two-dimensional and three-dimensional variation characteristics of vertical scouring and silting,changes in the geometric elements of cross sections,vetical adjustment area and micro-deformations of bars.Based on a large number of prototype observations,taking the Jingjiang reaches for instance,the variation characteristics of flow-sediment transport and the changes in vertical scouring and silting,micro-deformations of bars downstream the Three Gorges Dam are studied by analyzing the measured data and application of mathematical models from multi-angles,including from the whole to the part,from long-term to short-term and from phenomenon to mechanism.The interaction mechanisms and quantitative relationships between the altered flow-sediment regimes and the riverbed evolution,and the prediction of the variation trends in scouring and silting in the typical river patterns under with,without and different regulation rules of the Three Gorges Dam were all studied.The research findings improved the research content of the flow-sediment transport and riverbed adjustment in those large alluvial rivers,which can provide the basis for prediction of riverbed evolution and estimation of the Three Gorges Dam's benefits.The main findings are as follow:(1)The characteristics of flow-sediment transport under the effects of the Three Gorges Dam.Taking the periods of the TGD operation in 2003 and the 175 m impoundment in 2008as boundaries,the variation characteristics of streamflow and sediment before and after the TGD operation were studied.In the aspect of the changes in runoff,the effects of the TGD operation on discharge processes mainly focused on the storage period at the end of the flood season.Especially after the 175 m impoundment in 2008,the reduction of the monthly discharges during the water-falling stage at the end of the flood season increased significantly and the hydrological processes of the inner-annual runoff have been further flatted.Based on the results of the storage routing,the percentage of the water discharges between 5000m3/s and 10000m3/s significantly increased due to the TGD operation,with an increase from 29.3%to 54.6%.The effects of the different discharges at Yichang station on the processes of the inner-annual runoff mainly reflect on the adjustment in the flood peak reduction and flooding processes.In the aspect of the changes in sediment transport,the reduction of the monthly sediment load mainly concentrates on the flood season(from May to October),with the decreased percentage up to 90%.After the 175 m impoundment in 2008,the sediment load downstream the TGD further decreased,which primarily the downward trend of the outflow sediment load during the flood season and the water withdrawal period,and the reduction on September was the largest.The results of the changes in the effects of water discharges and sediment load on riverbed evolution show that the impacts that corresponding to the middle water discharges obviously enhanced after the TGD operation,especially after the 175 m impoundment.This variation is closely related to the changed the processes of the inner-annual runoff caused by the TGD operation that is the flood peak reduction and the increased duration of the middle water discharges.The temporal and spatial variations of the non-uniform sediment reflected the adjustment in the riverbed downstream the TGD in response to the varied flow-sediment regimes.The transport of size-specific sediment shifts from upstream to downstream in space.There existed the interactive relationships among the size-specific sediment transport,riverbed morphology and the processes of the flow discharges.The fine sand mainly originated from the upper reaches and the tributaries along the mainstream,and the coarse sand primarily originated from the replenishment of the riverbed.The river bed armoring has been basically completed in the reaches between Zhicheng and Shashi reaches and main scouring zone has been moved to the reaches between Shashi and Jianli.During the different periods of the TGD operation,the replenishment of the coarse sand in the reaches between Shashi and Jianli did not alleviated over time and with the river bed armoring.The intensity of hydrodynamic force is the main driving factor in affecting the replenishment of coarse sand.Based on the condition of the driving force for scouring at each discharge interval and the changes in the frequency of flow before and after the TGD operation,the flow range that has primary effects on the erosion of coarse sand is10000~25000m~3/s.This new finding revealed the interactive relationships among the dynamic conditions of sediment transport,river bed morphology and the duration of flow discharges.(2)The interaction mechanisms between the alterd flow-sediment regimes and the adjustment characteristics of the Jingjiang reach.The Jingjiang reach of the middle Yangtze River has undergone significant adjustment mode alterations,with intensive channel erosion and major adjustment area changing from the bankfull channel to the medium flow channel at the time boundary of the Three Gorges Dam(TGD)impoundment.The volumes of medium flow to bankfull channels adjustment exhibited rapid changes after dam completion and then stabilized at a relatively high percentage after the 175 m impoundment in 2008.The frequency of each flow range and the variation characteristics of flow-sediment regimes have been changed due to the TGD operation.Based on the correlation relationships between the medium flow channel dimensions and the cumulative sediment transport capacity of each discharge interval,the discharge of 16500m~3/s produces relatively high correlation coefficients,and when the flow discharge exceeds this critical value,the correlation decreases.From the geomorphic impacts of the altered flow-sediment regimes,the critical discharge(16500m~3/s)possesses high undersaturation degree.At the same time,small and medium discharges under 16500m~3/s possess increasing geomorphic impacts,resulting in fluvial processes occurring mainly in the medium flow channel of the Jingjiang reach after damming.From the aspect of discharge shaping medium flow channel morphology,the average reach-scale bar top is 35.28 m and the correspondent discharge at Shashi station is16500m~3/s based on the water level and discharge relationships.The discharge 16500m~3/s corresponds to the top of medium bar restricting the flow in the channel,which is of great significance for the shoal management and channel regulation of the Jingjiang reach.In general,changes in flow and sediment transport resulted in river erosion and deposition as well as variations of cross-sections configurations,while the river morphology adjustment also affected the flow-sediment regimes in the Jingjiang reaches.(3)The micro-geomorphologic deformation characteristics of bars under the impacts of the Three Gorges Dam.Due to the flood peak reduction,the increased duration of the medium flow and the sharply decreased sediment,the sandy bars showed the head scoured and the shrink of area.After the 175 m impoundment in 2008,the erosion of the sandy bars further enhanced.After the TGD operation,the erosion of the bars in the Jingjiang reach ranks the strongest when the discharges fall in 15000~25000 m3/s.Based on the reconstructed water discharges,the duration of this flow range further increased after the 175 m impoundment in2008,and the consequence is that the bars still presented an erosion state.Due to the impacts of river bed armoring and the significantly reduced sediment,there existed certain interactive relationships between the adjustment in the erosion and deposition of bars and the changes in the percentage of the grain size belonging to 0.125<d<0.25mm.The reduction of the fine sand had a negative impact on the sedimentation of bars after erosion.Based on the application of the two-dimensional mathematical model of flow and suspended sediment transport in the reaches between Tashiyi and Chenglingji,the variation characteristics of the riverbed adjustment and the bar evolution have been given under with and without the TGD operation.Under the condition of non-TGD operation,the focused reaches presented a minor siltation state,while that under the TGD operation scoured.The distributions of erosion and deposition of river channels and bars were consistent no matter with or without the TGD operation.The main reason is that the interactive mechanisms between flow-sediment regimes and riverbed evolution did no change before and after the TGD operation.However,the reduction of sediment had effects on the amplitude of erosion and deposition.With the increase of controlled peak flow,the erosions of the point bar of the straight channels,the convex bank of the curved reaches and the middle bar of the braided channels present slight reductions in bar areas and tops. |
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