Study On The Subglacial Water Flow Structure And The Variation Law Of Riverbed Flushing And Siltation Of Shisifenzi Bend In The Yellow River

Ice flood is the flood season when the Yellow River is prone to danger,and the sharp bend of the river is the main area where danger occurs frequently.Therefore,it is of great theoretical significance and practical application value to clarify the influence of the structure and flow state of the subglacial water flow and the evolution of the riverbed in the ice-covered bend to ensure the safety of ice prevention and reduce the risk of ice flood.Based on the field prototype observation tests combined with theoretical analysis,this paper investigates the structural characteristics,flow patterns and sediment movement distribution of the water under the ice in the Shisifenzi curve.It further reveals the evolution mechanism of the bend river bed during the ice period under the combined effect of ice,water,sand and bed.The main research results are as follows:（1）During the stable ice period,the maximum flow velocity point（mainstream area）of the subglacial water flow is located on the side of the convex bank of the bend from the entrance to the top of the bend,and deflects to the central axis after exiting the bend.The maximum flow velocity point is at the position of relative water depth y/h=0.4.In addition,the water flow in the subglacial bend does not completely present a double helix structure,and it is mainly on the concave bank side of the top of the bend,which is basically the same position as the bend circulation in the free flow period,but the scale is smaller in the glacial period.The swirl distribution of different flow layers at the same position of the curve is basically the same,the second half of the curve is larger than the first half,and the concave bank before the top of the curve is larger than the convex bank.In the water depth range of the bend,the circulation curl in the circulation area increases linearly along the water depth,and decreases linearly in the non-circulation area;the circulation curl is smaller near the deep groove and the main flow area.（2）Except that the intensity of lateral turbulence in the circulation area is slightly greater than that of longitudinal turbulence,the intensity of longitudinal turbulence is the largest in the others.The vertical distribution of turbulence intensity and turbulence energy is approximately"S"shape,and the two inflection points are located near y/h=0.2 and y/h=0.8 respectively.In three directions,the Reynolds shear stress of the convex bank is greater than that of the concave bank.Two inflection points are used to divide the subglacial water flow divisions.The water flow in the ice sheet area is dominated by outward interaction and inward interaction;the water flow in the riverbed area is dominated by jetting and sweeping.The distributions of the four flow regimes in the whole water depth are not much different.In the process of turbulence burst,the self-transformation probability of the quadrant is the highest,the river bed area is dominated by jetting P_2（?）2 and sweeping P_4（?）4,and the ice sheet area is dominated by outward interaction P_1（?）1 and inward interaction P_3（?）3.（3）The distributions of sediment concentration,median diameter of suspended sand and median diameter of bed sand in different periods are basically the same along the course,and reach the maximum value near the top of the bend.The vertical distribution of sediment concentration shows the distribution law of"thin at the top and thick at the bottom",but the linear distribution along the water depth is more obvious during the stable seal period.The sediment concentration in the stable sealing period is about 1/10～1/15 of that in the free flow period,but the suspended sand particle size is about 10μm larger than that in the free flow period.After the water flow enters the bend,the shear force on the bed surface begins to increase,the convex bank side is greater than the concave bank side,and the maximum shear force appears at the narrow section of the S4 bundle.The shore side is large,and the convex shore side is small.The river bed of the bend in the three stable seal periods was in a state of silting every year,and the river bed of the S2 section changed most drastically,and the river bed change process was greatly affected by ice plugging.During the ice-covered period,the curved riverbed is affected by the ice cover（ice plug）,which increases the resistance of the river flow,reduces the flow area,significantly reduces the flow capacity under the ice,and weakens the ability of water to transport ice and sand.,the riverbed is silted up;however,for the ice-slugged section,due to the smaller starting flow velocity of the sediment under the ice,the starting particle size is larger,and the riverbed of the ice-slugged section is easier to scour.