Experimental Analysis Of Different Hydraulic Gradients On Flow Structures Near A Spur Dike

The natural river is always in the process of continuous erosion and deposition.On the one hand,water scours the river bed and makes the river bed deform.On the other hand,the deformation of the river bed causes the change of the flow structure.After constructing hydraulic structures in a river,it will have a great influence on the evolution of the river bed.As a common hydraulic structure,spur dikes are frequently used in river regulation projects,coastal protection projects and reclamation projects.It changes the original section configuration of river bed,thus causes the change of flow structure and sediment scouring and eventually leads to the adjustment of riverbed slope.Studies on the principle of flow moving around a spur dike and influence of river regime with spur-dike project are helpful to optimize the structural design of spur dikes,improve the stability of hydraulic structures,and strengthen the protection measures of river shoreline.In this paper,after reviewing and summarizing the research status about flow around spur dikes from both domestic and foreign scholars in recent years,pointing out the inadequacies that the current experimental study of spur dike is mostly carried out in single bed slope and studies on groin project of the Yangtze Estuary lack of analysis about river regime of Hengsha Passage.Therefore,this paper combined with the experimental analysis and practical engineering.At the micro level,the flow structures in the near-field of submerged and non-submerged spur dike under different hydraulic gradient were studied by flume experiment.At the macro level,the hydrodynamic influence of Nanhui East Tidal Flat Reclamation Project to the main river of the Yangtze River Estuary was calculated and analyzed through the numerical simulation.In the physical experiment part,according to different hydraulic gradient,we studied the flow structure near the spur dike under submerged and non-submerged conditions by using PIV velocity measurement system and automatic water level measurement system.Experimental results show that,(1)Flow velocity distribution near the spur dike is fan-shaped.From the upstream of the spur dike to the downstream,the flow velocity increases gradually along the counter clockwise direction,and then decreases gradually.The velocity reaches the maximum in the turbulent mixing region in front of the groin head and the minimum velocity is at the root of the spur dike.(2)The change amplitude of Reynolds stress near the submerged spur dike is greater than that of the non-submerged spur dike with different flume slope.In the descending slope,the strong Reynolds stress areas of the submerged spur dike and the non-submerged spur dike are concentrated in the upper part of the turbulent mixing zone behind the spur dike.(3)After water flows through the submerged spur dike,it extends a relatively wide and "kelp-shaped" area of strong turbulence intensity to the downstream.When the flume slope changes from-2‰ to 0,the strong turbulent area behind the submerged spur dike decreases with the increase of flume slope,and this strong turbulent area increases with the increase of the flume slope when the flume slope changes from 1‰ to 2‰.However,a relatively slender and ?strip-shaped? area of strong turbulence intensity occurs when water flows through the non-submerged spur dike to the downstream,and this region gradually spreads to the mainstream and upstream with the increase of the flume slope.(4)There is no obvious relationship between the change of the recirculation zone in the downstream of the dike and the flume slope.However,after analyzing the relationship between the flow velocity near wall area of dike head and the recirculation length behind the dike,a mathematical formula of them has been deduced and it has the high calculation precision.Therefore,it is demonstrated that the flow velocity in the boundary layer of the dike head determines the recirculation length behind the dike within the relatively small velocity range.In practical engineering,the roughness of the dike head and the disturbance degree of water flow will directly affect the size of the recirculation zone behind the dike when the velocity of river water flow is relatively small.(5)There is a good corresponding relation between the increase extent of the water level of observation points near the dike and the change of the flume slope.In the descending slope,the spur dike has a greater impact on the flow of water.With the increase of hydraulic gradients,the position with the highest water level in the upstream of the submerged dike moves downward,while the water level recovery point in the downstream moves upward;And the lowest point of water level behind non-submerged spur dike gradually moves down after hydraulic drop occurring.In numerical simulation part,according to the implementation of the Nanhui East Tidal Flat Reclamation Project,a two-dimensional tidal flow numerical model of the Yangtze estuary was established based on the MIKE21 model.The model is calibrated with field data and is used to simulate the hydrodynamic changes of the branching channels in the Yangtze River Estuary before and after the project.Simulation results show that,(1)After the reclamation project,the tidal discharge greatly decreases and the velocity significantly increases in the South Passage due to a reduction in river width.This results in river bed scour and the river regime developing downstream from the middle of the South Passage.(2)After the reclamation project,although the Jiuduansha and Jiangya South Shoal will be eroded,the Jiangya North Passage will be developed.A relatively strong river system is composed of the North passage,Jiangya South Shoal,upper middle of the South passage and the Hengsha Passage.Thus,the sedimentation in the middle of the North Passage will be intensified and it will have a negative effect on the middle and lower of the North Passage.(3)After the reclamation project,the tidal discharge in the Hengsha Passage shows a high correlation with that in the South Passage and dramatically decreases.It directly weakened sediment transporting capacity of the Hengsha passage and this may have a negative effect on the development and utilization of the Hengsha Passage as a navigation channel.This paper reveals the mechanism of the regulative action of the dike to the flow structure under different hydraulic gradient.At the same time,we give a clear answer to the difference of the length of recirculation zone obtained by previous researchers' measurements.The research achievements have a certain innovation.In the analysis of the numerical simulation results,?A relatively strong river system is composed of the North passage,Jiangya South Shoal,upper and middle of the South passage and the Hengsha passage? is first put forward.The research method and achievements have some innovation.