Sensitivity analysis on transient pressure of entrapped air pocket in deep stormwater tunnel system

In deep stormwater tunnel systems (DSTSs), entrapped air pockets are prone to pressurization and deformation during rapid filling, resulting in pressure surges that threaten system security. This study investigates the effects of structural and inflow parameters on peak pressures. To simulate the pressure surges of entrapped air pockets, a rigid-column model was developed for a simplified DSTS configuration comprising two shafts and one tunnel. The global sensitivities of the air pocket, system structure, and inflow parameters were calculated using the Extended Fourier Amplitude Sensitivity Test (EFAST), which employed indicators of the maximum pressure and relative increment. The results indicate that parameters related to the shape and initial state of the air pocket exert a substantial and direct effect on pressure surges in rapid filling, whereas tunnel system parameters tend to exhibit minimal influence. Notably, compared to the total inflow discharge, the flow difference between two shafts imposes a more significant and direct impact. For the maximum pressure of air pocket, the initial pressure and the maximum water level height of shafts demonstrate more pronounced effects. The proposed sensitivity analysis could be integrated into methodologies for system safety assessment, while the rigid-column model may be extended to accommodate multiple shafts and air pockets.