Case study: Numerical evaluation of hydraulic transients in a combined sewer overflow tunnel system

The potential for hydraulic transients in the West Area Combined Sewer Overflow Tunnel System of the City of Atlanta was investigated by numerical modeling. Different design alternatives were evaluated to mitigate pressure oscillations, backflows, and flooding during storm events. A dynamic transient model was developed to simulate the transition from gravity flow to pressurized flow that would occur during a storm event. The model is able to simulate pressurization with pipe-filling bore or gradual-flow regime transition. Numerical results compare well with experimental data on single pipe flows. Following a brief description of the model, the paper discusses the pressure transients simulated during the filling of the tunnels for both the original and improved designs.