Evolution Of Water Runoff In A Typical Railroad Mountain Tunnel And Its Water Environment Risk Assessment

In recent years,the country has been deeply implementing the "Belt and Road" strategic policy and focusing on the development of mountain railroad construction,which is affected by the endless mountain ranges and therefore requires the construction of a large number of tunnels to ensure the smooth passage of the railroad line.Mountain railroad tunnels in the construction process usually need to cross the deep and large active fracture zone,and the tunnel site area of the underground water system is crisscrossed,rich in water,which brings a great risk of sudden surge of water to the construction of mountain railroad tunnels,more importantly,the surface runoff formed by the surge of water will have a negative impact on the water environment of the tunnel site area,so the risk assessment and prevention of water environmental risks caused by sudden water surges is a key issue to ensure the smooth implementation of the tunnel project.Therefore,this study takes the typical mountain railroad tunnel in the southwest of China as the research object,based on the production and convergence theory,combined with the natural environmental characteristics of the tunnel site area,the water environment risk of tunnel surge runoff is evaluated and the differentiated surge prevention and control measures are proposed.The results of the study provide a scientific and systematic technical means for the simulation of railroad tunnel gushing water runoff discharge from the theoretical level,and innovatively use the hierarchical analysis-simulation results superposition analysis function to evaluate the water environment risk of tunnel site area caused by tunnel gushing water runoff,which provides a new research idea for water environment risk assessment.In this study,surface runoff formed by gushing water from a typical mountain railroad tunnel in the southwest mountainous region is taken as the research object,and the water environment risk assessment algorithm of gushing water runoff is constructed by using hydrological model and hierarchical analysis method comprehensively to classify water environment risk for each section of runoff,and based on the vegetation cover before（2006）,in（2012）and later（2016）tunnel construction,the center of gravity migration model,the Standard deviation ellipse and interannual difference study analyzed the change pattern and trend of vegetation cover in the tunnel site area,so as to verify the reasonableness of the water environmental risk level division of the tunnel surge runoff;finally proposed differentiated surge prevention and control measures.The main research results achieved are as follows.（1）The normal situation of the basin area and water surface width of the tunnel surge runoff trends show consistency,while the flow velocity and its opposite trend.At the beginning of the runoff（near the tunnel mouth）and the end（into the downstream river）flow rate is faster,the central flow rate is slower,but the watershed area and water surface width in the middle of the runoff appears maximum.（2）Runoff in the middle section（construction site area）produces the greatest water environment risk.（3）There is a strong correlation between land use category and vegetation cover.The degraded vegetation cover of the tunnel site area is mainly concentrated in the construction and residential land,which is also a high risk area for the water environment of the tunnel.The change of land use type on the construction,residential land above ground and groundwater environment have caused a certain degree of impact,and thus affect the regional vegetation cover value,the same superposition analysis function calculation results also indicate that the high risk of water runoff water environment is concentrated in the low vegetation cover area.This paper’s hierarchical structure-simulation results superposition analysis function calculation results and vegetation cover spatial and temporal change analysis results are basically consistent.This study advocates the adoption of a multi-faceted and comprehensive strategy to improve the accuracy of runoff discharge calculation by compiling "one tunnel,one file" electronic basic information with the help of tunnel management platform,and by using environmental models and high-resolution spatial data.From the tunnel construction point of view,the tunnel gushing water-prone section should strengthen the initial concrete support,set up over-the-top slurry blocking measures.In the tunnel gushing water control,combined with the Internet and big data and other technical means to form a scientific and reasonable new tunnel gushing water control mode.