Study On The Hydro Thermal Coupling Model Of Saturated-Unsaturated In Riparian Zone Downstream Of Dam And Its Application

Reservoirs are considered to result in significant changes to river water temperatures.Particularly,the discharge temperature of large reservoirs is quite different from the natural water temperature.The discharge of deep water from reservoirs has an extremely important impact on the aquatic organisms and even the aquatic ecosystems in the lower reaches of the dam and along the banks of the river.Based on the analysis and summarization of relevant research results at home and abroad,this paper adopts theoretical analysis,existing generalized sand tank model test,field test and numerical simulation to carry out research on saturated-unsaturated hydrothermal coupling model in the downstream riparian zone of the dam.The main research results are as follows:(1)From the laboratory test to verify the three analytical indicators(RMSE,PCC and NSE)of a single measurement point,the results showed that the use of the Lu(2007)model based on COMSOL Multiphysics and the Chung&Horton model based on HYDRUS-2D have their own advantages.From the global analysis,the Lu(2007)model based on COMSOL Multiphysics can better simulate the riparian zone temperature field under our experimental conditions.(2)Laboratory tests and numerical simulations shows that the soil near the infiltration boundary and bottom boundary will be rapidly descends and reaches steady state in a relatively short period of time,and its temperature is close to the infiltration water temperature.On the contrary,it takes more time for the temperature near the upper and right boundaries to reach steady state.The more close to the infiltration boundary,the more obvious the influence of infiltration water temperature.With the increase of infiltration water head,the average temperature in the soil decreases,and the low temperature region of the soil expands gradually,and the temperature gradient increases.(3)The engineering example verification results show that the root mean square error(RMSE)obtained through COMSOL Multiphysics simulation ranges within 1.14 to 2.78.The Pearson correlation coefficient(PCC)ranges from 0.84 to 0.99.The Nash-Sutcliffe model efficiency coefficient(NSE)ranges within 0.61 to 0.96.The deviations between temperatures simulated through COMSOL Multiphysics and measured ones at different measuring points are small.The simulated results agree well with the measured ones,indicating an accurate prediction.The hydrothermal coupling model based on COMSOL Multiphysics has a good performance in simulation and is able to show the dynamic change of temperature in riparian zone accurately.(4)The diurnal variations of temperature and water level have a slight influence on the temperature field distribution in riparian hyporheic layers.Seasonal climate change has a big influence on the temperature field distribution in riparian hyporheic layers.In summer and winter,the temperature field of the hyporheic layers in the riparian zone presents the phenomenon of"warm up and cold down" and "warm up and cold down" respectively.On the vertical section,it can be divided into low-temperature region,intermediate-temperature region,and high-temperature region as well.(5)In the calculation of the vertical seepage velocity of the riparian zone using four analytical models in the VFLUX 2 program,the velocities calculated by Hatch phase method and Keery phase method approximately accord,while those calculated by Hatch amplitude method and Keery amplitude method are different.It indicates that thermal dispersion ? greatly influences the groundwater velocity calculated by the amplitude method when the velocity is low.For the amplitude-phase combination method,the velocity calculated by McCallum solution and Luce solution coincides basically,and the value is between the amplitude method and the phase method.After comparing the results of amplitude method,phase method and amplitude-phase combination method with those of COMSOL simulation,it is found that Hatch amplitude method has a higher accuracy in calculating the groundwater velocity in the study area.The calculated vertical seepage velocity of the riparian zone is about 2.47×10-6 to 3.15×10-6m/s.(6)Based on the Morris sensitivity analysis method,the effects of various parameters in the hydrothermal coupling model on the model are analyzed.The results show that the hydraulic conductivity KS is a parameter that has a great influence on the output of model temperature value,while the,? value of solid specific heat capacity cs,porosity n,residual water content ?r,saturated water content ?s,van Genuchten model parameters ? and ? are relatively smaller,it means that these parameters have little influence on the output of model;the larger ?value of hydraulic conductivity Ks indicates that when the temperature value of model is output,it will have a relatively larger influence on the output of model temperature value.In addition,the influence of each parameter on the temperature value of the model has both positive and negative effects,which can provide a reference for model calibration.