Modeling mixed flow in storm sewers

The blowing off df storm manhole covers may be attributed to the severe pressure transients that can occur during and after the transition from free-surface to pressurized flow in a sewer system. Observations from a physical sewer model with a submerged outlet indicate that the pressure transients were composed of an initial flow frequency water hammer type pressure surge and a subsequent high frequency pressure fluctuation due to the release of trapped air at the upstream manhole. A mathematical model, which was based an the assumption of rigid water columns and a compressible air bubble, was derived to simulate the pressure transients. The frequency of pressure transients predicted by the calibrated mathematical model is in close agreement with that recorded during the laboratory experiments. The attenuation of the pressure fluctuations, however, is underestimated. This may be attributed to the superposition of various air release processes observed during the experiments and the assumption of a steady-state friction factor used in the mathematical model.