Research On The Response Mechanism And Waterway Regulation Countermeasures After Water Diversion Of Han River

Han River is the largest tributary in the middle Yangtze River, so it is the main trunk line connecting the Yangtze River waterway. However, there will be some significant changes in the flow and its process after the construction of middle route of South-North Water Diversion Project, which leads a new round of channel restructuring downstream the Danjiangkou reservoir. Therefore, there are also many problems and challenges in the navigation development of Han River under the new situation. Based on the systematic analysis of the flow and sediment characteristics before and after water diversion in the middle and lower Han River, the complex response mechanism of channel evolution is studied in the downstream reaches of the Danjiangkou dam. This paper also put forward the concept of water demand for channel recovery after flood season and analyzed the changes in water quantity according to the statistical data. In addition, waterway regulation parameter calculation model was established on account of the complicated water and sediment conditions after water diversion. The main research results and conclusions can be summarized as follows:(1) The main characteristics of runoff change in the middle and lower Han River are as follows:Stream discharge reduces seriously in non-flood season, and the duration of average discharge is shortened obviously in the water recession process; There is little difference between the discharge in the dry seasons and the current situation, so a long-time lower flow duration will be appeared after the water diversion; Small and medium-sized flood peak were cut sharply or even disappeared, runoff changes during the whole year shows serious imbalance.(2) The water diversion influence on channel evolution will develop from the upstream to downstream reaches both in space and time. The magnitude of bed scouring and sedimentation in the upstream reaches was significantly greater than the downstream ones. As time goes on, the changes of scouring and silting in riverbed develop downstream gradually. The fluvial process not only shows the repetitive phenomenon with space and time, but also reflects the cumulative effect during the riverbed deformation adjustment. Furthermore, it also presents the typical characteristics of space differentiation between the deep channel and shoals during the channel evolution. The changes of flow process not only have an important influence on erosion and deposition of the typical shoal reaches but also have a profound impact on the water level, water depth, etc. With the reduction of upstream flow dynamic conditions, the horizontal adjustment phenomena of channel such as the bank collapse along the river will be eased in the middle and lower Han River.(3) Taking the reaches from Xinglong to Xiantao in the lower Han River as an example, water demand for channel recovery after flood season was calculated by the methods of erosion-deposition analysis and artificial neural network. The calculated results were also made a comparison with the water volume of non-flood seasons before and after water diversion. The research indicated that the channel evolution under natural conditions is hard to meet the designed channel scale requirements. Taking some waterway regulation measures is quite necessary, which could change the section shape of flow and sediment transport so as to achieve the aim of channel recovery after flood season. (4) Based on the analysis of backwater influence on the lower Han River, the reaches in the lower Han River are divided into three parts: no influencing reaches, changed influencing reaches and perennial influencing reaches. According to the different influencing time and range, the methods of sediment transporting capacity during effectual time and the guaranteed rate of water level were proposed to compute the regulation stage. For the changed influencing reaches whose action time in receding period, the regulation stage could be calculated by gradient method, the method of correlation between stages of upstreadm and downstream and so on.(5) For the reaches that bed-forming due to suspended load transport, a new computing method of regulation width was put forward based on the energy dissipation law of sediment-laden flow and equilibrium of sediment transport. Moreover, the maximum transport rate was presented to discriminate the combination of regulation parameters.