Research On The Interaction Between The Lower Reaches Evolution And Estuary Evolution Of Yellow River

At present, many researchers disagree with each other on the problem of feedback influence in the lower Yellow River due to the estuary extension and changes of the course. After analyzing the exist results, the author thinks that there are many factors that influence the riverway in the lower Yellow River and estuary evolution, and their interaction is extremely complicated. But by the limit of methods, previous research failed to recognize the interaction between different factors and to analyze the relation scientifically, and as the researches are based on different aspects of the problem, the results are diverse from each other. The estuary of Yellow River has weak tide like a inland estuary. For a long period, because the amount of incoming sediment is very big, estuary evolution mainly displays as the river course extension and changes of the course, and the alternate action of these two factors comes into being present delta. One obvious characteristic of riverway and estuary evolution in Yellow River is that the incoming flow and sediment change greatly and the erosion and deposition of riverbed in the lower reach including estuary, adjust strongly. This is the most obvious characteristic that any other river can't compare with. For example, in initial stage of river changing its course, as the sediment inflow is very large, the effect of sediment deposition conceals the development of scouring which traces to the source. So a false appearance is that river changing its course takes little effect on the lower river scouring. Therefore, the results based on simple and the one-sided field data analysis are not correct. In this paper, by using hydrodynamic sediment mathematic model that can reflect relation among each factor, the relation between estuary and riverway scouring and deposition is studied, and influence of incoming flow and sediment also is considered in quantities.Aiming at the characteristics of the lower river evolution and estuary evolution, based on field data analysis and academic discussion, the formula of roughness, the distribution equations of sediment concentration and suspended across the river width, exchanging equations about suspended load and bed load are set up in this paper. And the relevant parameters in exchanging equations, the formula of sediment transport capacity, simulation method of river width changes in the process of erosion and deposition, and bed load diameter adjustment, estuary deposition and extension are studied. Finally, a flow and sediment mathematic model is proposed, which is used to simulate the interaction between the lower river and estuary in the process of erosion and deposition, and it is systematically verified by the data from July 1976 to June 1996.Several research methods are used in this paper, including field data analysis and mathematical model calculation of multiple combination schemes. The research takes the incoming flow and sediment, erosion and deposition, estuary extension and changes of the river course as a whole. And the interaction between the lower river evolution and estuary evolution is also discussed. The results and the innovation viewpoints are listed as followed:(1) The annual incoming flow and sediment of the lower river change greatly so that the riverbed evolution appears accumulative deposition, but in a certain period it also experiences a process of erosion and deposition alternatively. The riverbed evolution from the downstream of the LiJin station is both influenced by the incoming flow and sediment, river changing its course and estuary extension. There is a direct proportion between the incoming sediment and the distance of estuary extension. And the proportionality coefficients are different between courses and evolution stages of the same course.(2) The incoming flow and sediment, as the main reason, influences evolution of the lower river and estuary. The annual mean deposition amount between HuaYuanKou and LiJin is inversely proportional to square of the incoming flow amount from HuaYuanKou, and it is direct proportional to the incoming sediment amount. And with the increase of sediment grain size, the amount and rate of sediment deposition increases too. According to regression analysis of field data, an empirical formula is established, which is used to calculate the amount of erosion and deposition of this reach and this reflects the average amount for a long time. When the incoming sediment increases or decreases at HuaYuanKou, the corresponding sedimentation amount of each reach will increase or decrease. The degree of erosion or deposition reduces gradually from upstream to downstream, and the longitudinal gradient becomes steeper or slower. By the self-adjustment of riverbed about several hundreds kilometer, the amount of sediment into the estuary still can't achieve to the level in equilibrium state. With the increasing or decreasing of incoming sediment at HuaYuanKou, the speed of estuary extension changes. But the rate of change of estuary extension is less than that of incoming sediment at HuaYuanKou, and they have the exponential relation with the coefficient of 0.58.(3) At present, compared with the historical situation, the boundary condition in the lower river has changed obviously. Under the present boundary condition, if there recurs a same flow and sediment process with an old one, the erosion amount of the lower river will reduce, the deposition amount of will increase, and the distribution of erosion and deposition along the river will adjust too. The influence of water and sediment diversion on lower river erosion and deposition is dual. On the hand, when the proportion of water diversion to total incoming flow is very high, as result of losing sediment with big proportion simultaneously, the river deposition will reduce. On the other hand, when the proportion is not very high, the river deposition will increase. For a long time, under the influence of water and sediment diversion along the river, the sediment deposition will increase in the river, and the distance of estuary extension will shorten. The water division along the river is the main reason that leads to the fact that the bankfull discharge is relatively higher in upstream of GaoCun station and downstream of AiShan station, and it is relatively lower between these two stations. This phenomenon is the concurrent result of water division, which led to the increase of river deposition, and river diversion to QingShuiGou course in 1976, which led to decrease of river deposition. For the reach between GaoCun and AiShan, the influenceof water division along the river, which leads to river deposition and the water level increase, is relatively great, and the influence of river diversion to QingShuiGou, which leads to decrease of deposition, is small.(4) As numerical simulating results show that, during the period of 20 years after Yellow Rvier changing its course to QingShuiGou in 1976, compared with running the course of DiaoKou, the deposition amount of its upstream reduced. From 1976 to 1995, the reduced deposition amount from HuanYuanKou station to YuWa station is about 15.3% of the total in the reach. And the reduced deposition amount is smaller in the upstream and larger in the downstream. Before the flood season in 1996, the reduced-deposition-reach has extended to the reach of HuangZhuang Station, and its distance is about 370km far from the diversion point. Compared with non-extension of estuary, its extension in the QingShuiGou's course leads to increase of the deposition amount. And the increased deposition amount between HuanYuanKou and QingThree is about 16.2% of total amount from 1976 to 1995. The increase amplitude of deposition amount decreases gradually from downstream to upstream. Before the flood season in 1996, the increase-deposition-reach has extended to Zhangcun cross section upstream of LuoKou Station, and its distance is about 275km far from the diversion point.(5) From the point view of practicability for engineering application, the range of feedback influence due to the estuary extension is confirmed, and two dominate indexes, which are obvious influence and relatively serious influence, are proposed. And a new concept of increment rate of the feedback influence due to the estuary extension is brought forward. The course of influence distance increase can be partitioned into two periods. In the first period, the extension distance is about 28-30 km from division location, and the feedback influence distance is about 160~170km. In this period, dominant factor that influences the increment rate is the estuary landform at the beginning of division, and subordinate factor is the landform of influenced-reach. In the second period, the estuary extends farther, and dominant factor is the landform of influenced-reach. Base on this, a formula that is used to calculate the influence distance due to the estuary extension is established. When the extension distance from the XiHeKou is about 98km, the feedback influence will extend to GaoCun reach, and when the extension distance from the XiHeKou is about 125km, it will extend to HuaYuanKou reach. With the increase of longitudinal gradient, the increment rate of the feedback influence due to the estuary extension decreases from downstream to upstream. If the incoming flow and sediment change, the longitudinal gradient will adjust, and the relation between the estuary extension distance and feedback influence distance will change too, but the amplitude of change is small. And according to the comparison of river course evolution between ShenXianGou, DiaoKou and QingShuiGou, it is concluded that feedback influence due to estuary extension will not exceed to AiShan, but downstream of LuoKou will been influenced greatly.