A Numerical Model for Hydraulic Transients in Drainage Systems

Most Cities count on networks of underground tunnels for the conveyance of storm water and wastewater. When a storm exceeds the design event, the flow in the tunnels may transition through free-surface flow to pressurized flow with the potential of generating unacceptable hydraulic transients. A numerical model of the flow during the filling of the tunnels, including transition from free-surface flow to pressurized flow, was developed at the IIHR. The code was used to predict the hydraulic transients in the West Area CSO tunnels system located in the city of Atlanta. Different design alternatives were evaluated to mitigate the pressure oscillations and overflows during a heavy storm. In this paper, a brief description of the numerical model and some results of the simulations are presented.