Effect Of In-stream Structures On Hyporheic Exchange

The interaction of surface water and groundwater in the hyporheic zone is of great significance to the hydrological cycle and biogeochemical process of river ecosystems.The in-stream structure affects the exchange process of surface water and groundwater by changing the undulations of the riverbed topography and the local spatial heterogeneity of the sediments,thereby affecting the material migration and transformation process in the hyporheic zone.Therefore,it is very important to study the influence of the in-stream structure on the hyporheic exchange characteristics.In this research,the in-stream structure across the river were taken as the research object,temperature tracing experiment was carried out in-situ river channel.Based on the Reynolds average equation,k-? turbulence model and Darcy's law,the two-dimensional groundwater model under the influence of the in-stream structure was built.And the model was verified with the measured data of the in-situ river.By changing the flow conditions of surface water and the parameters of in-stream structural,the influence law of the Darcy velocity field and the pressure,speed and hyporheic exchange flux of the sediment-water interface in the hyporheic zone were analyzed.The main conclusions are as follows:(1)The results of the in-situ river hyporheic exchange experiment showed that the hyporheic exchange flux at each point of the study reach was varied in different seasons.The hyporheic exchange in summer was mainly surface water supplied groundwater,and the hyporheic exchange flux of each point varied obviously over time.The hyporheic exchange in winter was mainly groundwater supplied surface water,and the change of the vertical exchange flux was relatively stable.The temperature results of the in-situ test were used to verify the model.The results showed that the simulated value and the measured value fitted well,and the surface water-groundwater coupling model was reliable.(2)Numerical simulation results showed that when the in-stream structure was applied,the pressure distribution on the sediment-water interface and inside the hyporheic zone would be affected by the changes of surface water velocity and water level,which in turn affected the hyporheic exchange characteristics.Within a certain range,with the increase of surface water velocity,the pressure value,Darcy velocity and the exchange flux of submerged flow on the sediment-water interface were gradually increased.And with the increase of the ground water level,The pressure value,Darcy velocity and exchange flux of the sediment-water interface were gradually reduced.(3)The local pressure in the hyporheic zone would be increased by setting up in-stream structures and induced up-welling and down-welling in the hyporheic zone.At the same time,the local Darcy flow velocity and the flux on the sediment-water interface would be also accelerated.With the increase of the structure height,the pressure value of the sediment-water interface,the Darcy velocity,and the flux on the sediment-water interface were gradually increased.As the structure width increased,the Darcy velocity field range gradually increased,but the extreme value of the pressure on sediment-water interface and Darcy's flow velocity,the water exchange flux would decreased slightly.Therefore,if the local hyporheic exchange flux of the river bed was intended to increase in engineering application,the in-stream structure could be applied and the structures height enhanced appropriately.(4)The deeper the structure was buried in the sediment,the pressure value,Darcy velocity and the exchange flux of the hyporheic at each point on the sediment-water interface became smaller and smaller.The flow path and the hyporheic exchange flux would be affected by structure permeability coefficient.When the flow by seepage through a structure with high permeability coefficient,the water gathered at the lower end of the structure,wich had a higher flux in this area.With the increase of structure permeability coefficient,the total flux of sediment-water interface and x=2?5m research area were also increased.When the water flowed through a structure with a low permeability coefficient,it tended to bypass the structure.As the permeability coefficient of the structure gradually decreased,the water flow through the structure gradually decreased.But at this time,it had little effect on the total flux on the sediment-water interface and x=2?5m research area.Therefore,appropriate lower buried depth of the in-stream structure or the high-permeability structure should be used to increase the local hyporheic exchange flux.(5)Orthogonal test analysis results showed that the main influencing factor on the hyporheic exchange was in-stream structure height and buried depth,followed by permeability coefficient.The structure width had the least influence on the hyporheic exchange.