Simulation Study Of Water And Heat Transfer On Lateral Hyporheic Zone Between The Rriver And Groundwater In Xi'an Section Of Weihe River

Based on the National Natural Science Foundation of China and Key R&D Projects in Shaanxi Province,this article selects The lateral hyporheic zone between river and groundwater in Xi'an section of the Weihe River as the research object,carried out in-situ monitoring exper-iments,comprehensively used the hydraulic gradient method,temperature tracing method and hydrothermal coupling simulation method,calculated the vertical water exchange volume in the lateral hyporheic zone between river and groundwater in the study area,clarified the cou-pling effect of the hydrodynamic field and the temperature field in the lateral hyporheic zone and revealed the mechanism of the influence of environmental changes on the water and heat transport in the lateral exchange zone.The main results are as follows:(1)The hydraulic gradient method was used to determine the vertical water exchange vol-ume in the lateral hyporheic zone between river and groundwater.The calculation results of the hydraulic gradient method show that the vertical exchange between river and groundwater is between 0.003 and 0.102 m~3/m~2/d.In the vertical direction,the average value of water exchange in the shallow layer(0-50cm under the riverbed)of the hyporheic zone is higher than that in the deep layer(75-150cm under the riverbed),which are 0.047m3/m2/d and 0.038m3/m2/d,re-spectively.In the horizontal direction,the average water exchange capacity of the interzone in the river channel is twice that of the average water exchange capacity of the interzone below the river bank.The results of water level dynamics indicate that the study area is groundwater replenishing river water laterally,and rainfall changes the distribution of water head in the lat-eral hyporheic zone without changing the recharge direction.(2)The temperature tracing method was used to determine the vertical water exchange volume in the lateral hyporheic zone between river and groundwater.The vertical water ex-change of the lateral hyporheic zone is between 0.01?0.18m3/m2/d.In the vertical direction,the average value of the shallow water exchange in the hyporheic zone is higher than the deep water exchange,0.087m3/m2/d and 0.079m3/m2/d,respectively;In the horizontal direction,the amount of water exchange in the hyporheic zone at the river channel is higher than the amount of water exchange in the hyporheic zone below the river bank.The temperature moni-toring results show that the vertical influence range of environmental temperature changes at the river channel is between 30 and 50 cm below the surface of the river bed,and the vertical influence range at the river bank is between 50 and 75 cm below the surface.The temperature in the lateral hyporheic zone during the rainfall period is more stable than that in the non-rainfall period,showing a trend of"cold up and warmer down".(3)Using COMSOL Multiphysics,a variable saturation two-dimensional hydro-thermal coupling model of the lateral hyporheic zone in the study area was established.The hydro-thermal coupling simulation results clarified the influence of groundwater flow on the transfer of ambient temperature in the lateral hyporheic zone.The simulation result of groundwater velocity in the lateral hyporheic zone is between 0?0.21m/d,the simulation result of vertical Groundwater velocity is between 0?0.17m/d,and the horizontal Groundwater velocity is be-tween 0?0.12m/d.The groundwater flow in the river channel is mainly vertical flow,and the groundwater flow at the river bank is mainly horizontal flow.Due to the replenishment of groundwater to river,the vertical transmission depth of ambient temperature at the river channel is less than the vertical transmission depth at the river bank.(4)A two-dimensional hydrothermal coupling model was used to simulate the coupling effect of the seepage field and temperature field in the lateral hyporheic zone,revealed the mechanism of the influence of environmental changes on the water and heat transport in the lateral hyporheic zone.The results show that when the head difference between the inflow boundary and the outflow boundary is 0.5m,1m,and 1.5m,the horizontal influence range of the infiltration flow on the temperature field of the lateral hyporheic zone is 18m,48m,and81m,respectively.The change of the inflow boundary temperature only has a significant effect on the temperature in the lateral hyporheic zone.The seepage field in the lateral hyporheic zone has a significant effect on the temperature field,while the temperature field has no obvious effect on the seepage field.