| There are plenty of meandering rivers in Ningxia Reach of Yellow River.The special compound and sharp bends and others have been formed gradually along with the development of meandering rivers.The existing researches currently mainly focus on the flow structure and riverbed deformation of the simple bend.Few research has been conducted on the relationship among the flow structure,geometric characteristics and bed morphology of these special bends.Therefore,the in-depth study on the special bend is of vital theoretical significance on the flood control,governance and the long-term planning of the watershed.Moreover,a potential threat exists for the safety of flood control along with the increase of the degree of curvature of the bend.In this regard,the artificial cutoff project can be used to ensure the safety of the areas on both sides.The artificial chute cutoff project implemented on the consecutive bend at Ningxia Reach of Yellow River is researched in the paper,where the threat of flooding is eliminated fundamentally while the hydrodynamics and morphological dynamics at the bend are changed.Currently,there is few research on the artificial chute cutoff.Especially,it is blank for the active chute cutoff where the original bend is intercepted and the new channel has not yet developed.The research on the flow structure and channel adjustment of the new channel after the artificial chute cutoff can make up for the deficiency of the relevant researches of the chute cutoff,and provide references for the planning,design and protection of cutoff project.With the research object of the consecutive bends of DaLiuShu-ShaPoTou and artificial chute cutoff of SiPaiKou at Ningxia Reach of Yellow River,the fluid dynamics and geometric characteristics of compound bend and consecutive sharp bend at the consecutive bend were researched by field measurement and numerical simulation.The flow structure and channel adjustment of artificial chute cutoff were also discussed,the hydrodynamic characteristics and bed morphology of the new channel(chute channel)under different hydrological conditions for chute cutoff were studies systematically.The research results can be mainly concluded as follows:(1)The geometric characteristics and flow structure of compound bend and consecutive sharp bend at the consecutive bend were analyzed systematically with the data on the consecutive bend of DaLiuShuShaPotou from the field measurement,and the influence of the bends with different geometrical characteristics on the flow structure and bed morphology were researched.The results showed that there were four single meander bends and one compound bend in the consecutive bend of ShaPoTou.There were multiple pool-riffle among the bed morphology of compound bend.The flow structure was featured with strong three-dimensional characteristics.The flow structure and bed morphology of the compound bend were more complicated than those of the single bend,but had less influence on the flow velocity.There were two consecutive sharp bends in Daliushu Section.The weak and counter-rotating secondary circulation near the outer bank was existed in the sharp bend except the primary circulation occupying most of the cross-section.A local reduction in velocity of the flow at the outer bank of sharp bends was a mechanism for decreasing bank erosion and migration of the channel.(2)Three field measurements were conducted for the artificial chute cutoff with large width-to-depth ratio at the consecutive bend via Acoustic Doppler Current Profiler and high-precision RTK and other instruments.The change rules on the flow structure,water surface shape and bed morphology and the distribution feature of the grain size of suspended sediment after the artificial chute cutoff were researched,and the interaction between flow structure and bed morphology was explored.The results indicated that the vital feature of the bed morphology with artificial chute cutoff was forming an obvious scour hole(the maximum scour depth of about 20 m)and a scour belt with the length of 2.26 km within the chute when compared with the natural chute cutoff.In addition,a reverse helical motion separated by a distinct shear layer was observed near the scour hole.In the chute channel,there were obvious recirculation zone,flow separation zone and helical motion without stagnation zone.The bed and boundary adjustment of the diversion river were very rapid,complex and changeable.The left bank was continuously eroded and retreated,and a large area of deposition was formed in the right bank.(3)Numerical simulation methods were used to build a three-dimensional hydrodynamic model suitable for SiPaiKou Section.Nine cases under different discharges were set,and the hydrodynamic numerical simulation of main channel of the chute was conducted.The effects of artificial chute cutoff on the three-dimensional flow structure,mass flux and bed shear stress of the new channel were analyzed.According to the results,it was consistent between the numerical simulation result and the measurement result of velocity at typical cross-sections.Stagnation zones were observed near the right bank during the low flow conditions,but this phenomenon was not found during larger flow conditions.Recirculation zones were found on the left and right banks of the scour hole,and a longer flux belt was formed near the scour hole and the left bank of the diversion river.The impact of the bed topography on the hydrodynamic characteristics was relatively prominent when the discharge was small,and the impact on the boundary was more noticeable at large discharge.In addition,high shear stress was observed near the left bank at the downstream of the diversion river,which indicated that the left bank at the downstream was still being scoured.In order to prevent the phenomenon of backflow,bank protection measures along the left bank were required to maintain the effectiveness of the artificial chute cutoff.(4)Based on three different topographies formed by the main channel and the beach in the chute channel,3D mathematical model of water-sediment was used to carry out numerical simulation research.The flow field characteristics,water surface morphology,longitudinal velocity distribution,three-dimensional field structure and the evolution of bed morphology in the new channel were analyzed,and the effects of the inlet discharge and bed topography on the above were studied.The results showed that there were multiple recirculation zones on the left and right banks of the chute channel,and the size of the recirculation zones varied with the change of bed topography and discharge.A larger water surface gradient was produced at the water surface morphology,and the average width of the channel and plane shape tended to readjust.The three-dimensional field structure was complex and changeable.The deposition and erosion at different degrees were displayed in the diversion river along with the increase of the discharge.In addition,the downstream deposition of the diversion river was the most obvious,causing the violent evolution of the riverbed. |
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