| In the report of the 17th National Congress of the Communist Party of China in 2007,the goal of building ecological civilization was clearly put forward for the first time.It emphasizes the importance of ecological civilization construction,which is related to the overall strategic layout of the country.With the increasing awareness of ecological protection of rivers and lakes,people realize that rivers are closely related to urban development and human settlement environment,and that urban ecological environment,natural ecology and urban landscape are inseparable from the role of river system.In order to solve the problem that most rivers in northern China are seasonal rivers,and the flow is less or even cut off in dry season,even in some rivers in flood season,a series of river regulation actions have been carried out in various places,and many cascade dams have been built to solve the problems of small flow and strong seasonality in rivers,so as to achieve the purpose of creating ecological landscape of rivers.Under normal circumstances,cascade dams can retain river water to form water surface.However,when floods occur,they will increase the water surface level during flood discharge,lower the flood control standard of dikes,and have a certain impact on river flood discharge,and may even cause river dykes to break and endanger people’s lives and property on both sides of the river.Therefore,it is very important to study the impact mechanism of cascade dams on river flood discharge and reduce its impact.The influence of cascade weirs on river channel flow is complex and multifaceted.However,in the current practical engineering design,the calculation of water level and energy dissipation is generally based on simple hydraulic formula,and other factors such as scouring and silting and the optimization of weir layout are seldom considered.In the simulation calculation of scientific research,two-dimensional simulation is usually used to calculate the influence of flood discharge,and three-dimensional numerical simulation is rarely used to simulate the calculation.Studying the influence mechanism of cascade weirs on river flood discharge and exploring the layout scheme that can coordinate the adverse effects caused by cascade weirs with the landscape effect has an important guiding role in regulating the design of weirs,and is very important for implementing the concept of harmonious coexistence between man and nature.Therefore,this paper has carried out the following experimental studies in a specific river with different weirs layout schemes:(1)Through indoor hydraulic model test,this paper sets a number of schemes combining different weir heights and different weir distances for a river project with a longitudinal slope of 1%,and measures the water level at key positions in the cascade weir channel to find out the local highest water level;(2)Observe the flow pattern in the river channel and its changes under different flow rates;Establish the mathematical model of cascade weir and set the key parameters of simulation;(3)Carry out mathematical simulation,compare the measured data of hydraulic model test with the simulated data,verify the accuracy of simulation calculation,and study and analyze the influence laws of hydraulic model test and three-dimensional numerical simulation calculation methods on the along-way water surface line,local maximum water depth,surface velocity and river bottom velocity of each scheme under different flood discharge.The results of comparative analysis are as follows:(1)Under four different arrangement schemes of weir height and weir spacing,when encountering 41.7m3/s(once every 10 years)and 116m3/s(once every 20 years)floods,the relative improvement degree of the maximum water depth is 6.89%,7.53%,10.02%,11.22%and 6.87%,7.09,respectively.Compared with other schemes,scheme 1 has a lower backwater degree on water level,and the improvement on the overall roughness of the river channel is smaller than other schemes,which is conducive to the rapid passage of floods.(2)Under four different arrangement schemes of weir height and weir spacing,the higher the weir is,the larger the weir spacing is,and the velocity in the rapid velocity zone behind the weir is larger and more concentrated,which is easy to cause scouring and silting in the area behind it.On the whole,the distribution of flow velocity zone produced by scheme 1 is less prone to siltation and local erosion than other schemes.(3)In the river course with 1%slope,the layout of Scheme 1 can achieve the purpose of retaining water and creating landscape at low flow rate,and is more conducive to flood discharge than other layout schemes of water retaining weir.In the actual engineering design,the layout scheme of the weir should be determined by simulation calculation or experimental optimization according to the river flow where the actual project is located,the landscape effect to be created,the bottom slope of the river,etc.,so as to create the landscape and minimize the negative impact on flood discharge. |
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