| Ningxia plain section of the Yellow River is wide and shallow,and the main stream swings frequently.In order to stabilize the river and smooth the mainstream,the flood control system based on dikes and river regulation works has been established.Because of the low anti-scouring ability of the riverbank,the nearshore of the vulnerable spot may be eroded during the flood,thus threatening the stability of the bank slope.In order to study the impact of sediment laden flow on the safety of vulnerable spot,numerical model of water and sediment in typical river sections was established,riverbed erosion and siltation under different conditions of incoming water and sediment were simulated,the erosion law on the toe of slope was analyzed,and further the stability of vulnerable spot slopes and its influencing factors were studied.The specific research contents and results are as follows:(1)The research area was selected from Qingtongxia to Mianjiawan.The one and two dimensional water & sediment mathematical models were combined for modeling.Firstly,the one-dimensional model of the Qingtongxia to Shizuishan section was established,the measured data during typical floods in 2012 was used to fix the model and ensure the accuracy of the model.Then the two-dimensional model of Qingtongxia to the Mianjiawan section was established.The one-dimensional model calculation results were used to provide the downstream boundary conditions for the twodimensional model and verify the model.The foundation of the study on the law of riverbed erosion and sedimentation has been laid.(2)In order to study the scour and erosion of the vulnerable spot under different conditions,six kinds of calculation conditions were proposed: low sediment,normal sediment and high sediment based on design flood and medium flood conditions.According to the numerical simulation results,the variation law and influencing factors of the riverbed scouring depth were discussed,and the lateral erosion at toe of slope was calculated and analyzed.The results showed that the scouring depth of the riverbed will reach equilibrium basically at the end of flood.The maximum scouring depth of the riverbed is related to the average sediment concentration and the average flow velocity during the flood period.When the average flow velocity stays constant,the scouring depth increases with the decrease of average sediment concentration;when the average sediment concentration stays constant,the scouring depth increases with the increase of the average flow velocity.In addition,the average shear stress of the nearshore flow and the lateral erosion length of the slope during the flood also increase by the increase of the average flow velocity.(3)Based on the saturated-unsaturated seepage theory and the Morgenstern-Price limit equilibrium method,the finite element model of the seepage-stability analysis of the vulnerable spot slope was established.Firstly,the seepage field inside the bank slope was simulated,and then the safety factor of the slope stability was calculated.Taking the Caijiahekou 15# spur dike with the largest scouring depth and lateral erosion length at toe of slope as typical object,the stability of bank slope during the water-rising stage,flood peak period and water-falling stage under the design flood and medium flood conditions were analyzed,The safety factors of stability were compared by considering and not considering the action of erosion.(4)Based on the seepage-stability analysis finite element model,the effects of different rising and falling rates of water-level,different scouring depth and lateral erosion length at toe of slope on stability of the slope were discussed.Based on this,the bed depth of the slope is studied quantitatively.The relationship between the scouring depth,lateral erosion distance,relative water level,the changing rate of water-level and the safety factor of bank slope stability,then the safety factor prediction formula was proposed. |
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