Study On The Dynamic Response Mechanism Of Hyporheic Exchange Characteristics To Seasonal Variation In Island Riparian Zone

Dam operation is an important cause of changing the water level in downstream rivers.The fluctuation of water level promotes the exchange between the downstream river and the groundwater in the island riparian zone,and redistributes the temperature and water fields in the island riparian zone.Temperature and water are two important indexes of soil environment,which directly affect the survival of animals and plants inhabiting or parasitizing them and the activities of microorganisms.Taking a island riparian zone of Yueyang in the Dongting Lake downstream of the Three Gorges Dam as the research object.Through laboratory experiment,field experiment,analytic model and numerical simulation,Carry out research of the hyporheic exchange changes during seasonal and water level fluctuations in island riparian.The main research results are as follows:(1)Selecting four analytical models:amplitude method(Hatch model and Keery model)and amplitude-phase combination method(McCallum model and Luce model)to calculate the vertical seepage velocity of a one-dimensional homogeneous sand column,and verified by laboratory experiment,the results show that:The magnitude of the vertical velocity and the overall change rule obtained by the four analytical models are relatively consistent with the results of the laboratory experiment.It further illustrates that the four analytical models are reliable for the calculation and analysis of vertical seepage velocity.The calculation of the vertical seepage velocity of the four analytical models under the influence of the heterogeneous sand column is further verified by the heterogeneous sand column experiment,the results show that:using the monitoring data of t₂ and t₃ to calculate that the flow velocity at 0.2 m is smaller than the measured value,which is consistent with the rule that the flow velocity at 0.2 m in the vertical direction of the homogeneous sand column is smaller than the measured value,indicating that four analytical models can be used to calculate the vertical seepage velocity which affect the heterogeneity.(2)Under seasonal conditions,the temperature field of the beaches has obvious stratification in the vertical direction.The difference is that the spring is "warm up and cool down",The opposite of autumn and winter.The temperature gradient below the ground level of 0.3 m is higher than that below 0.3 m.External factors can cause uneven temperature distribution in the shallow layer,the temperature field of the island riparian near the river is greatly affected by the temperature of the river water and the difference in water content of each layer of the island riparian shows a certain degree of vertical difference,The surface water of the island riparian is affected by external factors.As the depth increases,the impact weakens.(3)In winter,the change of groundwater level will delay the time for the internal heat flux of the island riparian to change from a negative value to a positive value.In winter,no matter the water level rises or falls,0.5-1.0m below the ground is the heat loss during the complementary process of river and the groundwater in the island riparian zone.The heat loss in the low water level period is greater,mainly concentrated at 0.5-0.7m below the ground.As the vertical depth increases,the heat loss decreases gradually,and tends to be stable below 0.8m.(4)The experiment area is greatly affected by human activities,and its geological distribution is relatively complex,which leads to deviations in the simulation results of some monitoring points.Generally speaking,The saturated-unsaturated flow-thermal coupling numerical model of the island has a good simulation effect.and can more accurately describe the dynamic change process of the island riparian temperature.The temperature field distribution of the island riparian is affected by various factors such as water level and temperature changes,air temperature changes and so on.With the change of water level,the 17? temperature contour of the island riparian temperature field has the most obvious change trend.(5)The four analytical models of amplitude method(Hatch model and Keery model)and amplitude-phase combination method(McCallum model and Luce model)and the numerical model simulated the vertical seepage velocity of the island show the same magnitude and the same change law,it shows that the saturated-unsaturated flow-thermal coupling numerical model of island riparian is reliable.During the monitoring period,the period of time for river water to recharge the groundwater in the island riparian is much longer than the period of river water replenishment by the island groundwater,and the amount of groundwater in the river replenishment in the island riparian is greater than that of the river.The exchange volume varies greatly in different months.The exchange volume of surface water and groundwater is the largest in December,and the exchange volume of surface water and groundwater is smaller in January and April.This paper discusses the temporal and spatial characteristics of the hyporheic exchange and clarifies the heat and water migration of the hyporheic zone of the island riparian under the operation of Dam.Which is expected to provide a theoretical basis for the study of the water and heat exchange process in the riparian zone of the river downstream of the dam.