Study On The Effect Of Yangtze River Jacking On Hydrodynamic Weakening To Lower Han River And Its Effect To The Water Bloom

As the largest tributary of the Yangtze River,the Han River is the main source of water for the South-to-North Water Diversion project.In recent decades,large-scale bloom outbreaks occurred frequently in the middle and lower reaches of the Han River,which affected the production and life of residents along the river.In this paper,mathematical statistics methods,HEC-RAS model and MIKE21 model were used to simulate the hydrological situation of Han River during bloom outbreaks,focusing on the study of the law of water jacking in the middle and lower reaches of Han River,and the relationship between the water jacking in the middle and lower reaches of Han River and the Yangtze River.Through hydrodynamic model simulation and other methods,the aim was to discover the response law of hydrodynamic weakening effect of Han River water bloom outbreaks under the influence of Yangtze River water level top support,and to study the law of water blooms outbreaks in our country,and to provide some scientific reference for middle and lower Han River joint scheduling and Chinese river type water blooms prevention and control.(1)Analysis of hydrological regime change and its driving factors in the lower Han River.Using MK test method and Morlet wavelet periodic analysis,the hydrological evolution law was analyzed.The IHA-RVA method was used to evaluate the hydrological regime change quantitatively.Budyko theory and GRNN model were used to analyze the hydrological regime change quantitatively.The results show that the annual runoff of the Han River has a significant decreasing trend,while the annual runoff of the Luoshan-Hankou section of the Yangtze River presents an increasing trend,but not significant.The first main cycle of Han River annual runoff is 10 years.The periodicity of Luoshan and Hankou stations in the main stream of the Yangtze River is the same,and the first main periodicity is 22 years.The overall change degree of Luoshan,Hankou and Xiantao all reached moderate change,and human activities were the dominant factor of the change of discharge in the Han River Basin.(2)Analysis of water level topping law between Yangtze River and Lower Han River.Based on the hydrological data analysis of the Yangtze River to the lower Han River water jacking law;The HEC-RAS model was used to simulate and invert the water level of the lower reaches of the Yangtze River and Han River.The results showed that the water jacking of the upper reaches of the Han River was the strongest in June,July and August,and gradually decreased from the end of the year to March of the next year.When the jacking effect is the strongest,the farthest distance of the water level influence is about 192 km away from the mouth of the Han River.When the jacking effect is weak,the process of the water surface line is similar to a natural river with no water flowing into it.(3)Analysis of hydrodynamic weakening effect of the lower Han River under the influence of the Yangtze River on the water jacking effect of the Han River.Based on the HEC-RAS one-dimensional hydrodynamic model,through the simulation of different discharge and water level combination conditions,the changes of the flow velocity along the lower Han River under the water jacking of the Yangtze River water level,and the relationship between the upstream discharge and the downstream water level and the hydrodynamic weakening effect were studied.The results show that the higher the upstream discharge is,the faster the flow velocity is.The lower the water level in the downstream,the higher the velocity along the river.The supporting effect from January to March in water bloom year was significantly greater than that in non-bloom year.(4)Analysis of hydrological and hydrodynamic conditions of water depletion in typical reaches of the lower Han River.The Mike21 model was used to simulate and invert the hydrodynamic conditions during previous blooms,and the critical flow velocity to inhibit the bloom outbreak was determined to be 0.74m/s and the critical flow velocity to subside the bloom was 0.87m/s.The combination scenario of the inflow rate and the Xiantao-Hankou water level difference required for the typical section to reach the critical velocity was put forward,and the minimum Xiantao-Hankou water level difference required for the Xiantao section to reach the critical velocity condition to inhibit the bloom outbreak and promote the critical velocity condition to subside the bloom was determined under the conditions of different levels of inflow rate.