Characteristics Of Ice-Flow-Sediment Transport And Its Coupling Mechanism Of Bed Evolution At Sharp Bend In Inner Mongolia Section Of The Yellow River

Ice flood disaster in the Yellow River(Inner Mongolia section)is the most prominent and significant natural disaster in China in winter and spring,the prevention and mitigation of ice flood has become the urgent need to promote the high-quality development of the Yellow River.The sharp bend is the main area of ice flood,due to the comprehensive influence of ice cover and ice jam,the characteristics of flow and sediment transport are greatly changed,which leads to bed evolution.However,the influence of river ice movement on sediment transport and channel evolution in winter is often ignored.River bend involve many processes in freezing period,such as ice generation and extinction,water level rise and fall,bed erosion and deposition,and bank erosion,et al.,there are complex ice-flow-sediment interaction,which is the frontier and difficulty of river ice research.Focusing frontier issues,based on field data,river ice dynamics,hydraulics,sediment motion mechanics,river dynamics and other disciplines,integrated static ice jam theory,fractal theory,quadrant analysis,dimensional analysis,minimum energy consumption rate principle and other theoretical methods,the movement and transport characteristics of ice,flow and sediment in the Shisifenzi bend of the Yellow River are studied,and the evolution mechanism of the river bed under ice are further revealed.This study can provide a scientific basis for channel improvement and ice flood disaster prevention in cold regions.The results are as follows:(1)The law of ice transport and the formation mechanism of ice jam is clarified.The river channel with a wide width at the upstream and a narrow width at the downstream is easy to form ice jam.The smooth bank ice on both sides of the river gradually develops into an ice bridge to accelerate the freezing of the river channel.The frozen of bend apex and the larger flow velocity in the downstream,which promotes the formation of open water leads.Under the effect of the centrifugal force,the ice is concentrated to the concave bank side,and is compressed and thickened to form tight ice,and there is obvious ice accumulation phenomenon.In the stable freezing period,the ice thickness on the concave bank side of the bend is about 50?90cm,which is about 10?30cm thicker than that on the convex bank side.The ice thickness on both sides of the open water leads was uniformly distributed,with an average of about 50cm.Due to the influence of the backflow and river regime in the upper reaches of the bend apex,the flow is not enough to carry away the frazil ice,the ice jam toe formed in the bend,the ice transport capacity reduced,and the ice jam accumulated in the channel of the concave bank.(2)An simple method for deriving the velocity formula under ice is presented,the distribution and transport characteristics of flow in bend are determined.The flow velocity under ice has fractal characteristics,and the formula of the flow velocity under ice can be derived based on the fractal theory.The vertical distribution of the flow velocity under the ice follows the double logarithm distribution or two power law distribution,while flow velocity in the core region does not follow logarithmic distribution.Under the influence of ice jam and backflow,the main stream line of the bend swings.Longitudinally,the flow velocity decreases and begins to recover near the bend apex.Radially,the main stream is compressed by ice jam and tends to convex bank gradually,resulting in the translocation of the main stream,the open water leads then shifts to the convex bank.Affected by the ice cover,the flow turbulence intensity and turbulent kinetic energy along the depth is"S"shape distribution,Reynolds stress distribution has no obvious regularity,but all of them near the bottom have a large amplitude,which indicates that flow near the bottom is active.In addition,due to the countercurrent gradient of the flow velocity under ice,the coherent structure of the flow changes:in the upper layer are mainly outward interaction and inward interaction,while ejection and sweep are mainly in the lower layer.(3)The characteristics of sediment transport in bend with or without ice conditions are distinguished,and a more applicable and accurate formula for threshold velocity of sediment under ice is derived.The suspended sediment concentration(SSC)in the free-flowing period is significantly higher than that in the stable freezing period,but the median suspended sediment particle size(d₅₀)is smaller than the latter,the bed sediment particle size is similar.The SSC and d₅₀is proportional to the flow velocity,and reaches the maximum near the apex of the bend during free flow period,but near the ice jam reach during stable freezing period.In the vertical direction,the SSC/d₅₀ in the two periods showed the distribution pattern of thin on the top and thick on the bottom/fine on the top and coarse on the bottom,but the vertical distribution of SSC/d₅₀ in the stable freezing period was more uniform,the sediment carrying capacity as well.The change of sediment physical properties affects the dynamic process.The sediment threshold velocity formula under ice derived from Einstein hypothesis shows that,the threshold velocity of sediment under ice is affected by the roughness ratio of ice sheet to bed.The higher the roughness of ice sheet,the lower the threshold velocity,which can be summarized as:open flow is the largest,ice sheet flow is the second,ice jam is the smallest,which indicate that the ice jam section is the easiest to scour.(4)The basic characteristics and influencing factors of riverbed evolution during freezing period are clarified,and the riverbed evolution mechanism of bend is revealed.The existence of ice sheet changes the distribution of flow,which leads to the imbalance of sediment transport and further causes the riverbed evolution.The riverbed evolution in the freezing period is the result of the coupling effect of ice,flow and sediment,among which the ice jam has a more obvious effect on the riverbed change.Under the influence of ice jam,the main stream of the river was forced to the non-ice jam area,and scour appeared in the non-ice jam area.The response of the riverbed to the ice jam showed that the increase of ice jam caused the riverbed scour,while riverbed deposition with ice jam decrease.The adjustment process of bend riverbed follows the principle of minimum energy consumption rate.Compared with the free flow period,only about 50%of the channel energy is used for sediment transport and bed forming in the freezing period,the total available energy decreases.Ice jam causes a higher upstream water level and a larger difference between upstream and downstream water levels,resulting in an increase in longitudinal energy consumption and a decrease in transverse energy consumption,the bend circulation effect decreases,the transverse slope decreases as well,and the transverse sediment transport weakens.Finally,the bend shows the law of"the convex bank was scoured and the concave bank deposited".The variation of the riverbed under ice is obvious,which cannot be ignored in the riverbed evolution of seasonal rivers.It is of great significance to grasp the riverbed evolution process in ice period for the prevention of ice flood in the Yellow River.