Numerical Simulation Analysis Of The Influence Of Partial Pressure Of CO₂ And Rainfall On Karst Evolution

China has a large area of karst,and the karst area in southwest China,such as Yunnan and Guizhou,accounts for about one-third of the country.As an important tourism resource,the geomorphology formed by karst development plays a guiding role in the study of karst development.During the simulation of karst evolution,the dissolution evolution under different CO₂partial pressures was carried out,and the saturated calcium ion concentration corresponding to different CO₂partial pressures was analyzed.By changing the value of saturated calcium ion concentration in the software,the influence of pressure of CO₂on karst evolution and watershed disappearance was analyzed indirectly.Therefore,the analysis of CO₂partial pressure further improves the exploration on the factors of karst evolution.In this paper,we will discuss respectively the influences of rainfall and CO₂partial pressure in the model area on karst development and evolution and watershed disappearance.During the operation,the parameters of the fracture should be set,connected and treated.all nodes should be discretized,and the dissolution dynamics and seepage model should be coupled.According to the water equalization equation,using the formula of underground water laminar flow-cubic law,the water head equations of each node are established and carried out simultaneously.According to the formula of carbonate karst erosion rate,the dissolution rate and flow rate of each fracture section are calculated.According to the principle of mass conservation,the width of each crack in the first time step is obtained,and the new width is finally obtained.After the new gap width is calculated,the above steps are repeated,and the water head distribution and gap width are calculated again,so as to obtain the simulation process of karst fracture evolution.Taking carbonate rock mass as the model area,its length and width are 1000meters and 200 meters respectively.The morphology,movement characteristics,disappearance factors of watershed and karst evolution are simulated.Numerical simulation shows that karst development is the strongest and fissure opening is the greatest near the diving surface.The disappearance time of watershed is related to the range of simulated area,partial pressure of CO₂and rainfall.When the range of the simulation area increases,the watershed is less likely to disappear.As the partial pressure of CO₂increases,the content of CO₂dissolved in water increases,and the dissolution ability strengthens.the speed increases,and the disappearance time of the watershed decreases.The influence of rainfall on the disappearance time of watershed is complex and mainly divided into three stages.When the rainfall is less than 200millimeter,there is no watershed.The disappearance time of the watershed is longer with the increase of rainfall between 200 and 600 millimeter.When the rainfall is 600millimeter,the disappearance time of the watershed reaches its maximum.When the rainfall is between 600 millimeter and 1600 millimeter,the disappearance time of watershed decreases with the increase of rainfall.By using the simple analytic solution of deterministic fracture network,the correctness of the software simulation of plane flow and profile flow model is verified.