| The fluvial process downstream a reservoir is one key issue in river dynamics and river engineering,and it has always caused considerable attention.The construction of large dams could essentially change the flow and sediment conditions,which could cause series of fluvial process downstream,and even lead to the transforming of river pattern.The evolution of riverbed is not only affected by the utilization of the reservoir,including its regulation capacity of flow and sediment and its siltation stage,but also depends on the boundary conditions and channel characteristics of the original river channel.In generally,the impacts of reservoir on the downstream fluvial process can be ignored when the riverbed consists of bedrock or pebbles,however,the effect is always significant when the riverbed consists of sand.So far around the world,in the studies on the fluvial process downstream a reservoir,more attention has been paid to the suspended load transportation in alluvial rivers.For mountainous rivers which mainly affected by bedload movement,the research on fluvial process downstream a reservoir is still naive,because the movement of bed load is more complex than suspended load.Jinsha River is the upper reaches of the Yangtze River,which is a typical mountain river in southwest China.It is characterized by steep gradients,large flow velocity,and adequate gravel bedload.The fluvial process in this river was mainly determined by bedload movement,and suspended load was mainly transported as wash load,which rarely involves with bed-building process.Downstream of Jinsha River is a typical mountain river,after the construction of large reservoirs,the flow and sediment conditions in there are dramatically changed.The downstream scouring of the dam,and especially the local scouring of important wading buildings may affect the operation and dispatching of the reservoirs,wading buildings and shipping safety.Therefore,it is of great practical value to carry out the research on the riverbed evolution in the lower reaches of the Jinsha River.For mountain rivers have a wide grading of bed material,which could vary over 7 orders of magnitude,and the motion of different size of sands differ greatly in mechanical properties,the sediment transportation mechanism are very complicated,which incurred that studies on the fluvial process downstream the reservoir in mountain rivers are relatively rare.In this paper,the downstream reach of Xiangjiaba dam which is located in the lower part of the Jinsha River is selected as a typical case,whose fluvial process is systematically studied through prototype observation data analysis,numerical computation,and hydraulic model test.And advanced experimental techniques and monitoring system are developed,through which the flow-sediment characteristics are analyzed.The long term scouring process of reach downstream the reservoir in mountain river are simulated,whose mechanism and the ultimate scouring depth are explored,and the evolution rule of the reach near the dam downstream the reservoir has been revealed.The impacts on the reaches downstream a reservoir that aroused by gravel bed-load are determined.The local scouring of important wading buildings are identified.It is of great theoretical significance for enriching the theory of fluvial processes study,and could contribute to the understanding of the mechanism of the river reformation downstream a reservoir in mountain river,and it is of great practical value to ensure the safe operation during reservoir regulation,which has filled the gap of the fluvial process study in the downstream reach of the large reservoir in the lower reaches of Jinsha River.The main results are as follows:(1)After the impoundment of mountain river reservoir,the bed of downstream reach are likely to be disturbed,which is characterized by scouring quickly,decaying rapidly along the reach,and has big local scouring intensity.Based on measured hydrological and terrain data,the runoff,sediment and response characteristic of the channel in different period are compared and analyzed,the results showed that:Before year 2008,the downstream reach of Xiangjiaba Dam is in its natural state,there is no tendency variation of channel erosion and deposition in it.After the closure in year 2008,to year 2012,within the scope about 14 kilometer downstream the dam site,the sediment has been scoured up to 940,000 m3,the riverbed averagely declined about 0.56m,the maximum scouring depth is about 5m,and the scouring is mainly concentrated around the highway bridge and the Shuifu wharf.From October 2012,the Xiangjiaba reservoir has been put into operation,to year 2013 the maximum scouring magnitude is relative smaller than that of year 2008 to year 2012,but more than 90%sediment is deposited in the reservoir,and the scoured range,and the scope of erosion is increased along the river and the total amount of erosion is increased.(2)The scouring mechanism and the ultimate scouring depth in the reach downstream a reservoir in mountainous river are explored.By using the numerical sedimentation model that based on the non-uniform no-equilibrium sediment transportation theory,the fluvial process from the dam site to Lizhuang in the next 50 years was simulated,and the sediment erosion-accumulation,the amount of scoured suspended load,the amount of scoured bedload,the process of scouring and silting,the depositions distribution,and the variation of d50 in that reach were obtained.The mechanism of the scouring intensity decay rapidly over time,the scouring magnitude decreased rapidly along the reach,the variation in size distribution of bed material,and the coarsening of the river bed are revealed.In addition,according to the theory of extreme erosion and coarsening of the riverbed,the ultimate scouring depth and variation of grain size are predicted.(3)Advanced experimental techniques and monitoring system are developed.In consideration of the gravel bedload play the leading role in the riverbed deformation in mountain rivers,and the river channels are narrow and deep there,a normal movable-bed model is adopted to simulate the fluvial process in the reach downstream the reservoir(Xiangjiaba hydropower station),and the non-uniform natural sand are adopted as model sand.In the experiments,dynamic similarity,kinematic similarity and geometric similarity are ensured at the maximum degree.Meanwhile,by using advanced equipment,a monitoring system that realizes the automated measurement in the experimental parameter,data measurement and data acquisition are developed,which improves the stability,repeatability,and reliability of the data and the measurement efficiency.(4)The rule of the fluvial process of the reach near the dam downstream the reservoir is revealed.By using a normal movable-bed model which adopted natural model sand,the flow and sediment motion kinematic similarity are ensured at the maximum degree.And series of experiments were carried out.The rule of the fluvial process of the reach downstream a reservoir in mountainous river are revealed.?after the impoundment of Xiangjiaba reservoir,the downstream flow pattern and velocity filed are not only involves with the reservoir discharge,but also involves with the locations of release structures and their enablement conditions,which lead to that the velocity decreased with the increasing of the discharge or the backflow appears in some places.This state of flow field may affect navigation on some level.?for the typical flow rate of 15000 m3/s and general series of flow and sediment series,there is no scouring in the river channel downstream.For the discharges of 15000 m3/s,20000 m3/s and 28200 m3/s,the riverbed averagely declined by 0.1m,0.6m and 1.3m.Under wet series,the riverbed averagely declined by 0.6m.? when the discharge is smaller than 15000 m3/s,there is no obvious scouring downstream the stilling pool apron,when the discharge is reaching 20000 m3/s,there are two separated scouring pits in the riverbed downstream of the stilling pool apron,and the left one has a length of 128m,the right one has a length of 184m;when the discharge is reaching 28200 m3/s,the left and right scouring pits will connect together,and its maximum length can reach 280m,whose area is about 3000m2,and the maximum scouring depth is about 10m.? For the discharges of 15000 m3/s,20000 m3/s and 28200 m3/s,the depth of the scouring pits outside the approach channel is about 2m,5m and 8m,in addition,the silting will occur near the entrance of the approach channel.? The scour always occurs near the Shuifu wharf under different flow conditions,which leads to the formation of scouring pits in the upper reaches of the pier and the upper part of the pier in the mainstream side along the river.When the discharge is 28200 m3/s,the width of the scouring pit is more than 10m,and its maximum depth is about 3.5m.The presence of Shuifu wharf will exacerbate the local erosion or reduce silting of the riverbed,which contributed about 50%to the scouring and silting of the riverbed from the wharf to the railway bridge.? around the left bank piers of the railway bridge,the maximum scouring depth varies over 5-8m,and the radius of the scouring pits varies over 8?10m.Around the right bank piers,the maximum scouring depth is less than 4m,and the radius is less than 8m.(5)The suggestions and strategies to regulate the critical location are proposed.The research shows that,under 5%frequency flood condition(28200 m3/s),the scouring for the riverbed downstream was only about 3m,which means that the riverbed is generally stable.Considering this fact,we just need field monitoring rather than massive river controlling projects.Specifically,under general flood condition where the discharge is smaller than 15000 m3/s,the number of the riverbed scouring pits is limited in the lower reaches of the stilling pool apron,the entrance of the approach channel,the Shuifix wharf and the railway bridge of the Neikun line,which means that the field monitoring is enough;under the large flood condition(about 28200 m3/s),large scouring pits will occur near the above mentioned wading structures,which needs to strengthen the field monitoring,and should take riprap measures to ensure the operation safety if it's necessary.In addition,the silting will occur in the area near the entrance of the approach channel,therefore,it is advisable to strengthen the siltation monitoring,and,if necessary,to carry out dredging. |
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