Response Mechanism And Simulation Of The Bifurcated Channel Adjustments Under New Flow-sediment Regimes

The construction of Three Gorges Reservoir（TGR）has significantly changed the downstream flow-sediment regimes,resulting in responsive channel adjustments in both long-distance scale and reach scale.Under the new flow-sediment regimes,diversified changes characterized by“primary branching channel prevails”and“secondary branching channel prevails”have occurred in bifurcated reaches of the Middle Yangtze River.These adjustments have significantly affected flood control and waterway navigation.Furthermore,there are large uncertainties in determining river resistance and its distribution in bifurcated reaches due to the complicity of various controlling,which affects the accuracy of two-dimension numerical simulation of bifurcated channel processes.Therefore,a systematic analysis of the response characteristics of channel adjustments to new flow-sediment regimes and a reasonable method applied to determine the river resistance distribution within bifurcated reaches are urgently needed.The results have important theoretical value for improving the accuracy of two-dimensional morphodynamic model and practical value for flood control and waterway navigation.In this thesis,comprehensive methods combining field data analysis,theoretical analysis and numerical simulation are employed to systematically investigate the responsive characteristics and driving forces of channel adjustments in bifurcated reaches of the Middle Yangtze River under the new flow-sediment regimes.Additionally,new methods of calculating movable bed resistance and flow division ratio in bifurcated channels and resistance distribution for numerical simulation of bifurcated channel evolution are developed.Moreover,the effect of flow-sediment regime variations on the channel adjustments in bifurcated reaches is quantitatively discussed.The principal conclusions of this thesis are as follows:（1）After the impoundment of TGR,erosion and deposition adjustments between bankfull channel and medium flow channel in the bifurcated reaches of Middle Yangtze River have decreased,and the erosion are mainly concentrated in medium flow channel.The medium and low bars within bifurcated reaches suffer significant erosion,while minor changes occur in high bars.The increases in cross section area（A）and average water depth（H）,the decrease in river facies relation index（W/H）,the increase in elevation difference between bar and deep channel and the overall trend towards a V-shaped narrow and deep cross section are the main characteristics of the post-dam channel geometry adjustments in bifurcated reaches.The duration of medium and low flows has increased,and higher proportions of water-sediment transport as well as channel-forming processes are completed mainly by medium and low flow discharges smaller than the channel-forming discharge.These are the primary reasons why post-dam channel adjustments and morphological changes mainly occur within medium flow channel.（2）Bifurcated reaches with different channel planforms have no consistent adjustment trend of"primary branching channel prevails"or"secondary branching channel prevails",but share the generic trend of"short branching channel prevails"after the dam construction.The greater the channel curvature is,the more likely"short branching channel prevails"occurs.After the impoundment of TGR,channel adjustments of bifurcated reaches in the Middle Yangtze River behave a spatial variation in reaches below and above Chenglingji.Specifically,more secondary branching channels prevail over primary branching channels from Yichang to Chenglingji,while more primary branching channels prevail over secondary branching channels from Chenglingji to Jiujiang.The relationship between sediment saturation degree and discharge shows different trends upstream and downstream of flow discharge variation between the reaches upstream of Chenglingji;this is one cause of the spatial difference in bifurcated channel adjustments downstream of the TGR.（3）The magnitude and duration of flow discharge,changing processes of sediment saturation degree and branching pattern with flow discharge variations as well as riverbed anti-scouring ability are the main factors that trigger the post-dam“anabranch wax and wane”in bifurcated reaches.During this process,different driving factors play different roles and the final result of channel adjustments are collaboratively affected by all the driving factors.（4）Applicability of the existing movable bed resistance formulas and flow division ratio formulas are compared with field observed data and physical model experiment data.The comparison results of movable bed resistance formulas show that existing formulas can to some extent describe the change laws of resistance coefficient,but will generate relatively large errors when applied to bifurcated reaches in the Middle Yangtze River due to the lack of parameters reflecting the characteristics of bifurcated reaches.The comparison results of the flow division ratio formulas show that the existing formulas will cause relatively large errors when calculating division ratio under low flow conditions.The accurate reflection of the difference in roughness and water surface gradient between primary and secondary branching channels is the key to ensure calculation accuracy.（5）Using field data of bifurcated reaches in the Middle Yangtze River,a formula to calculate comprehensive resistance in bifurcated reaches is established after considering the influence of the Froude number（Fr）,relative water depth(H/D₅₀),river regime coefficient（ε）,area ratio of the branching section to upstream single section（A/A₀）,and then calibrated and verified using field data and physical model experiment data.The test result shows that this formula can calculate comprehensive resistance more accurately.The average water depth ratio between primary and secondary branching channel can comprehensively reflect the difference in roughness and water slope,and thus is highly related to their roughness-gradient comprehensive coefficient ratio.By introducing average water depth ratio,a flow division ratio calculation formula with a significantly improved accuracy of less than5%maximum absolute difference is established.（6）The comprehensive resistance calculation formula can be applied in numerical models to accurately calculate comprehensive roughness under different discharges and channel boundary conditions through a procedure of trail calculation.By further calibrating the roughness ratio of two anabranching channels using the correlation relationship between comprehensive coefficient ratio and water depth ratio,transverse distribution of river resistance can be determined automatically under any channel boundary and flow condition,which can improve the accuracy of simulating bifurcated reach evolution,thus improving the accuracy of the existing two-dimensional hydro-and morpho-dynamic numerical model from the aspect of river resistance calculation.（7）Different aspects of flow-sediment regime variations can lead to different impacts on channel adjustments of the downstream bifurcated reaches.For instance,flow hydrograph variations can directly alter the adjustment area of channel erosion and deposition,and reduction of flood peak caused by reservoir operations can reduce net channel erosion amount and is the main external cause leading to the more intensive erosion within medium flow channel.The decrease in saturation degree can intensify the channel scouring and reduction of saturation degree after TGR construction is the principal cause of the intensified channel scouring.Both the increase in flood discharges and decrease in saturation degree can make the channel cross section narrower and deeper.（8）Different driving factors triggering“anabranch wax and wane”play different roles in channel adjustments of bifurcated reaches of different types.For example,in straight branching reaches,branching pattern is sensitive to flow discharge variations and thus flow discharge and its duration exert major influence.By contrast,in slightly curved or curved branching reaches where difference of inflow condition between primary and secondary branching channel is significant,branching pattern under the initial topography determines the overall trend of“anabranch wax and wane”adjustments and variations in flow hydrograph can strengthen or weaken parts of the adjustments.Under a fixed flow hydrograph,the increase in sediment saturation degree is more favorable to the development of secondary branching channel but the impacts are relatively minor.