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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. [ABSTRACT FROM AUTHOR]

A response to a comment on the article on numerical evaluation of hydraulic transients in a combined sewer overflow tunnel system published in the previous issue is presented. The authors' agree that the model described in the paper could not predict that geysering events can be affected by air pockets entrapped in the tunnel system. They even agree on the fact that pressurization takes place when the energy contained in the inflow is insufficient to create a hydraulic jump.