Limits of Aggregate Discharge for Inferring Local Water Pressure Behind Tunnel Linings: A Reduced Hydraulic Framework Accounting for Capacity and Topology

Tunnel drainage deterioration reduces groundwater release capacity and may increase local water pressure behind linings, whereas aggregate outlet discharge can conceal different local pressure states after capacity or connectivity changes. This study determines when aggregate discharge remains an informative proxy for local pressure and when that inference fails. A reduced framework coupling groundwater supply, finite component capacity, and network topology was evaluated using cross-head blockage-discharge data, a 6 × 6 blockage length–area dataset, and multi-outlet blockage cases. Increasing blockage from 60% to 80% reduced normalized discharge from 0.73–0.84 to 0.26–0.29. The excluded-head normalized-discharge root-mean-square error (RMSE) was 0.041, whereas the row-/column-level pressure RMSE was 0.242 kPa. The fitted area exponent (0.748) exceeded the length exponent (0.153). Two outlet-blockage layouts differed by only 0.019 in discharge ratio but by 30.6 kPa at the same invert location. Capacity loss governs overall drainage, whereas blockage position and active paths control local pressure allocation. The framework supports quasi-steady screening under measured or stable source and tailwater boundaries; when topology or boundaries may have changed, aggregate discharge should be supplemented by local pressure monitoring or geometry-specific analysis.