| Scouring and siltation changes caused by water and sand changes is the main reason leading to the evolution of river channel morphology,and it is also one of the important reasons affecting the stability of dikes in the basin.Therefore,the dike safety in the Dongting Lake Basin under the new water and sand situation will be an important part of the security of Dongting Lake area.Using the Dahukou River,the eastern branch of Songzi,as a typical river section in Dongting Lake Basin,the characteristics of water and sand changes in recent years in the typical river section and the risk and causes of the embankment were analyzed,the scouring and siltation characteristics of riverbed and bank slope in the typical river section under different water and sand conditions were studied,and the bank stability under different degrees of the riverbed scouring and bank erosion was explored,the influence of water and sand conditions on the scouring and silting evolution of the riverbed and bank slope and bank stability in the typical river section is revealed,providing a theoretical basis for the study of scouring and siltation characteristics and bank safety in the Dongting Lake basin under changing water and sand conditions.The main research contents and conclusions are as follows:(1)Based on the measured water and sand data of the Dahukou River from 1991 to 2007,we analyzed the trends and abrupt changes of the recent runoff and sediment changes during the period,and applied the Mann-Kendall trend test and abrupt change analysis method to verify the trends and abrupt changes of the long-term runoff and sediment transport series of the Dahukou River.The results show that the overall trend of runoff and sediment volume change in Dahukou River in recent decades is decreasing,and the decreasing trend of sediment volume is more significant than the runoff volume,and there are obvious abrupt change points of runoff and sediment volume change in the river section;the significant difference in the variation of runoff and sediment volume will increase the sand-carrying capacity of the water flow,which intensified its scouring of the riverbed and banks of the Dahukou River,by changing the hydrodynamic conditions of the river channel,the water and sand changes have an impact on the evolution of the river channel morphology and the bank stability of the river section to some extent.(2)A dike-bed local three-dimensional generalized model of a study section of the Dahukou River was established,and the scouring and silting evolution law of the riverbed and bank slope of the Dahukou River under different water and sand conditions was studied.The results show that the riverbed is undercuted vertically under the effect of water flow,at the same time the foot of the slope moved back laterally in the direction of the embankment,resulting in a steepening of the bank slope and an increase in bank height,which is detrimental to the overall stability of the embankment;the impact of water scour on the bank slope is more remarkable than river bed,and as the river level rises,the depth of the scour pit at the foot of the slope increases,and the lateral backward displacement of the foot of the slope in the direction of the embankment is greater,it is necessary to consider both the scouring of riverbed and bank when performing bank stability analysis.(3)Based on the simulation and analysis of local three-dimensional scouring and siltation in Dahukou River,two-dimensional generalized models of the scoured bank section was established,the effect of different scour degree on the overall stability of the embankment section in the typical river section was analyzed using the strength-discounting finite element method.The results show that the maximum stress of the embankment section in the ultimate damage state increases with the increase of the riverbed scouring depth and the erosion distance of the embankment,and the change of the embankment stability in the typical river section is negatively correlated with the degree of the riverbed scouring and the bank erosion,the coefficient of safety of the embankment after the scouring of water flow in the dry and flat water periods are reduced by 10%and 21%respectively compared with the initial form,the coefficient of safety of the embankment after the scouring of water flow in the high water period is reduced by up to 42%compared with the initial form,which is close to the critical value of the coefficient of safety for slope instability,the embankment has a great safety risk,should take the corresponding de-risking and strengthening measures. |
