Experimental Study on the Compression of Entrapped Air Pockets in Stormwater Tunnel Systems

Abstract

Stormwater storage tunnels may undergo rapid filling pipe conditions during extreme rain events. Such conditions are relevant as adverse conditions may develop, such as surging due to entrapment and compression of entrapped air pockets. Operational issues such as structural damage, geysering and return of conveyed water to grade, among others, have been linked to air pocket entrapment. This work presents results from experimental investigations on pressure surges caused by sudden air pocket entrapment. Steady flow was supplied in a pipeline in such a way that pressurized flows existed at the upstream end while the downstream end experienced free surface flow due to free discharge conditions at the downstream end. A combination of flow rates and slopes resulted in several gradually varied flow profiles at the discharge, with various volumes of atmospheric air at the discharge end. By sudden closing the downstream discharge valve, an air pocket was entrapped and surges were recorded. To emulate conditions in which surge relief is provided during air pocket compression, valve maneuvering also included cases with partial valve obstruction. Among the obtained results, one can notice significantly different surges for cases with and without pressure relief (e.g. with total or partial obstruction), and that the larger is the obstruction degree the larger are the surges. Such findings are useful in the development of numerical models to simulate rapid filling of stormwater tunnels incorporating these findings.