Author(s): J. G. Vasconcelos; S. J. Wright
Linked Author(s):
Keywords: Storage tunnels; Peak surges; Air dynamics
Abstract: Storage tunnels are one solution to mitigation of stormwater sewer overflows. Even these systems may fill under major rainfalls and experience a transition between free surface and pressurized flow with a subsequent surge depending on the system characteristics. Previous research has focussed on numerical model development with laboratory studies directed primarily for model calibration and verification. This study was conducted to investigate the role of principal variables in determining the magnitude of surges generated during the transition from free surface to pressurized flow. The laboratory model consisted of a nearly horizontal conduit filled to an overflow level at one end and a surge riser at the opposite end. The effect of an inflow hydrograph was simulated by initiating flow into a partially full system. Principal variables investigated were the inflow rate into the system, the initial water level in the system, and the conduit slope. Over 150 different experiments with different combinations of these three variables were investigated. For a given slope and inflow rate, the maximum observed surges occurred at intermediate initial water levels such that the hydraulic bore formed by the inflow just filled the pipe cross section. For the case of horizontal slope and when the fill end of the pipe was lower than the surge riser, the surges behaved in a similar fashion. When the inflow end of the pipe was higher than the surge riser, a sudden drop in the surge magnitude was observed for initial water levels just slightly above those that resulted in the maximum surge. Subsequent investigation indicated that this behavior was caused by pressurization in the air as it was expelled by the moving bore. A simplified numerical model was constructed to predict the peak surges in the experiment, and it is fairly accurate as long as the air pressurization is negligible. Since the role of the air phase was determined to be relevant under certain conditions, new experimental investigations are being conducted in order to understand the aspects of the air dynamics in rapid filling tunnel applications, so that its effects can be incorporated in future numerical modeling.
Year: 2003