A Shock Fitting Approach for Simulating the Surge Caused by Obstruction in a Storm-water Tunnel

During intense rain and flood situation, the gravity flow in sewer network transients to a partially pressurized flow, in which the air pocket is entrapped and undergone intense compression. Severe surging resulting from large compressed air pocket may lead to operational failures and structural damage. A typical approach to simulate these surge flows is interrupting the downstream end by partially or completely closing a gate valve. Besides experimental efforts, the relevant numerical simulations have attracted a lot of attention due to the cost-effectiveness. Some numerical studies simplify the governing equations by adopting a lumped inertia approach (rigid column model) to simulate the unsteady flow. Meanwhile, in other studies, the modified Saint-Venant equations were used to incorporate the pipe elasticity to take into account the effects of water compressibility. Recently, the authors performed a simulation using a shock-fitting approach, in which the free-surface Saint-Venant equations in conjunction with the rigid column model are employed to simulate the air pocket entrapment in a closed-conduit transient flow. In this paper, the performance of this approach was compared to other approaches in the previous studies. It was found that this shock-fitting approach outperforms the other methods in terms of calculating the maximum and minimum peak surge heads as well as the consecutive attenuations. This improvement was observed, particularly, for completely blocking the outflow with various ranges of the air pocket sizes and discharged flowrates.