Prediction water inflow in a karst tunnel based on MODFLOW-CFP model

ObjectiveTunnel construction in karst areas often faces the high-risk sudden water inflow, posing serious threats to construction and operational safety. This risk is particularly critical when tunnels intersect underground river systems, where accurately prediction of water inflow is not only a key concern in engineering design but also a major challenge for existing prediction methods. MethodsThis study develops a tunnel water inflow prediction approach based on MODFLOW-CFP, using the Houwan underground river system in western Hubei as a case study. Particular emphasis is placed on several foundational tasks that are often overlooked during model construction and calibration, including detailed karst hydrogeological surveys, monitoring of rainfall-underground river discharge responses, and groundwater tracer tests. On this basis, a physically reasonable MODFLOW-CFP model for predicting tunnel water inflow in karst areas is constructed, and simulations under different tunnel design schemes are carried out to compare inflow processes and provide a basis for optimizing tunnel design. ResultsThe study delineates the spatial extent and boundary conditions of the Houwan karst water system, clarifies its groundwater circulation characteristics, and rainfall-runoff response patterns, and accurately identifies the key parameters of the underground conduit network, thereby providing a foundation for the accurate application of the MODFLOW-CFP numerical model. The calibrated model yields a linear correlation coefficient (R2) of 0.831 and a Nash-Sutcliffe Efficiency (NSE) of 0.71, indicating that it can satisfactorily reproduce the hydrological behavior of the system. Under an extreme heavy rainfall scenario (P=325 mm), the predicted peak tunnel inflows differ markedly for design elevations of 1350, 1330, 1318 and 1270 m, with peak values of 0, 0.52, 8.17 and 14.88 m3/s, respectively. ConclusionThe results demonstrate that raising the tunnel design elevation can effectively reduce the magnitude of sudden water inflow and thus lower the risk of water inrush during tunnel construction and operation. The integrated methodology of karst groundwater investigation, monitoring, testing and numerical simulation presented in this paper can provide a useful reference for similar projects in other karst regions.