| The Yellow River is a well-known silt-laden river in the world.In recent decades,under the combined influence of human activities and climate change,the water and sediment in the Ning-Meng reach have decreased sharply,and the river channel has been seriously silted up,gradually forming a’new suspended river’.In this thesis,the Ning-Meng reach of the upper reaches of the Yellow River is taken as the research object.Based on the observation data of water and sediment and large cross-section in the Ning-Meng reach from 1960 to 2016,the interannual and intra-annual variation characteristics of water and sediment are analyzed by mathematical statistics method,hydrological method and graphic method.Incorporating’bed-forming flow’and’flat bank flow’to discuss the response relationship between riverbed evolution and water and sediment conditions,and the effects of human activities and climate change on water and sediment conditions,revealing the Ning-Meng reach siltation attribution.This study is of great significance to maintain the health of the Yellow River and the sustainable development of the basin.The main research contents are as follows:(1)In this thesis,taking Xiaheyan station,Shizuishan station,Sanhuhekou station and Toudaoguai station in the Ning-Meng reach as representatives,the Mann-Kendall trend test method was used to analyze the trend of runoff and sediment transport at each station.It was found that the annual runoff and sediment transport at each station were significantly reduced((6=0.01).Through the cumulative anomaly method and the Mann-Kendall mutation test method,it was found that the mutation time of annual runoff was1986,and the mutation time of sediment transport at Xiaheyan and Shizuishan stations was 2000,while the Sanhuhekou and Toudaoguai stations were 1984.Through the variation of characteristic parameters such as the dispersion coefficient C_v of runoff and sediment distribution during the year,the proportion of runoff and sediment in flood season,and the graphic method,it is found that after the mutation of runoff and sediment transport,the annual distribution is flat.(2)In order to explore the response relationship between riverbed evolution and water and sediment conditions,the bed-forming flow of each station was calculated by the Makawiev method.The results show that the bed-forming flow decreased significantly after 1986,and the multi-year average value from 1960 to 1986 was 2200-2900 m~3/s.There was no obvious siltation in each section,but the shape of Sanhuhekou and Toudaoguai section was adjusted violently,and the bankfull flow of each station was maintained at a high value.From 1987 to 2016,the multi-year average bed-building flow decreased to 700-1250 m~3/s.The evolution of the riverbed was dominated by siltation and atrophy,and it tended to be dynamically balanced after 2000,resulting in a decrease in bankfull flow.(3)In this thesis,the main influencing factors of runoff and sediment transport in different stages were discussed.It is found that the primary and secondary relationships between the influencing factors in different stages were different,and the possible causes of runoff and sediment transport process flattening were preliminarily analyzed.The contribution rates of precipitation and human activities to runoff and sediment variation were analyzed by double cumulative curve method.The results showed that the contribution rates of human activities to runoff and sediment reduction were about87%-98%and 83%-98%,respectively,which were the main factors for the sharp decrease of water and sediment in Ning-Meng reach. |
