Groundwater Recharge Modeling In The Alpine Area Of Southeastern Tibet And Apply To Prediction Inflow In Duomuge Tunnel On The Sichuan-Tibet Railway

In the simulation of groundwater seepage fields,the model rainfall recharge conditions are generally approximated as the average annual precipitation,and snow accumulation is not considered separately.In the Sichuan-Tibet Railway Duomuge tunnel area located in the Duomuge mountain,the temperature difference between winter and summer is about 16.7°C,and snowfall in winter accumulates in mountainous areas with higher altitudes and melts in summer.Snow cover has a regulation effect on groundwater recharge in the Duomuge mountain range.The snow accumulation-thaw process changed the groundwater recharge method and recharge time,and formed a unique periodic significant change in groundwater flow field in the study area.Seasonally fluctuating seepage fields have a direct impact on the tunnel water inflow,and the commonly used calculation methods for tunnel water inrush do not take into account the rich seasonal snow conditions in the mountainous area where the Duomuge tunnel is located.There is a large error in the supply near the peak of the dry season.To more accurately evaluate the characteristics of groundwater recharge and the water cycle in the study area,and to more accurately predict the centralized drainage problems that may occur during the construction of the newly constructed multi-mug tunnel and the water environmental problems that may occur in the tunnel drainage impact zone,this article is based on the unique water cycle pattern of groundwater under snow replenishment conditions.Using the Landsat 8 high-resolution remote sensing image,the differential snow index?NDSI?index was used to calculate the monthly snow distribution range in the study area to establish rainfall and snowmelt The relational expression of groundwater recharge under dual-source recharge is used as a numerical model recharge parameter.The finite element groundwater runoff simulation software?FEFLOW?was used to establish a three-dimensional groundwater seepage model in the study area.The field survey of borehole water levels and water balance verification verified the model's reliability in fitting the groundwater seepage field in the study area.Based on this,we set up heavy and dry seasons and long-term drainage conditions,and compare and analyze the dual-source rainfall and snow-melt recharge conditions and single-source rainfall replenishment conditions.The newly constructed Duomuge tunnels have different gushing water flows at different periods and the possible centralized drainage sections.Causes of drainage belts to provide suggestions for tunnel construction.The main research results obtained in this paper are as follows:?1?Landsat8 remote sensing images in the study area were interpreted using the NDSI index to obtain high-resolution monthly snow distribution.Concerning relevant literature and on-site survey results,calculate the monthly distribution of groundwater rainfall and snowmelt recharge in the study area monthly,and summarize the groundwater recharge rule under the snow storage in the study area:When the ambient temperature is lower than the critical snow melting temperature₍?8?,The surface conditions are snow and frozen soil,which isolates the groundwater and accepts vertical replenishment conditions.Within the snow cover,there is almost no vertical recharge of the groundwater.When the ambient temperature is higher than the critical snow melting temperature₍?8?,the underlying surface melts and forms an open surface.The channel allows groundwater to accept vertical infiltration recharge.Within the snow-covered area,the groundwater receives the dual sources of atmospheric precipitation and snow melting vertically.In non-snow-covered areas,the groundwater mainly receives atmospheric precipitation recharge.?2?Summarizing the characteristics of the water cycle caused by seasonal fluctuations in groundwater under the snow accumulation in the study area:The water cycle in the study area is generally based on hydrological years.Due to snow accumulation during the dry season,the groundwater is mainly stored in abundant water.Season to receive the supply of melting runoff.?3?Based on the borehole test and hydrogeological survey data,through analysis,induction and calculation,the hydrogeological structure of the study area where the Duomuge tunnel is located is described in detail.According to the annual average rainfall method adopted by traditional numerical model recharge and the characteristics of dual sources of rainfall and snowmelt recharge in the study area,the conditions for single-source recharge and dual sources of snowmelt recharge were set to simulate the natural seepage field distribution under the two recharge conditions.The borehole water level and water balance verification were used to compare the borehole water level of the simulated natural seepage field with the actual borehole water level under the two recharge conditions.The results show that the dual-source recharge conditions of snowmelt and rainfall are suitable for the groundwater seepage field simulation better.?4?Two types of recharge conditions for groundwater natural seepage field models were used to set the excavation conditions of the drainage tunnel in the high and low seasons and perennial year,respectively,and the distribution characteristics of water influx and drainage influence range of the Duomuge tunnel under the two recharge conditions were compared and analyzed.The study area is affected by snow accumulation and the groundwater level fluctuates significantly during the high and low seasons,and the water inflow from the tunnel during the high and low seasons is significantly greater than during the dry season.Under the conditions of dual sources of rainfall and snowmelt recharge,the difference between the initial water flow in the tunnel during the high and low seasons is greater than that of the single source of rainfall,which reflects the role of snow accumulation.The tunnel drainage is concentrated in the C₂P₁l stratum at the entrance end and the high buried depth section of the F3-5 fault crushing zone.Among them,the F3-5 fault crushing zone has a high water pressure and a relatively large permeability coefficient,and a large amount of water per unit length is required in tunnel construction.Pay attention to drainage and drainage.It is recommended that a tunnel construction design should avoid exposing the F3-5 fault fracture zone during the flood season.