Kinematics of Entrapped Air Pockets in Stormwater Storage Tunnels

When stormwater storage tunnels undergo rapid filling during intense rain events, many different mechanisms can lead to the entrapment of air pockets within these tunnels (Vasconcelos and Wright, 2006). Entrapped air pockets have been linked to operational issues like damaging surges, loss of storage capacity, and severe geysering upon release through water-filled ventilation shafts. Tracking of entrapped air pockets and their celerity is thus important in the context of numerical simulation to assess the risk of these operational issues. Previous studies focused on quantifying the magnitude of surges associated with air pocket compression or in obtaining the minimum flow velocities required to expel these entrapped pockets in water mains. However, the mechanisms behind the motion of these finite volume pockets following entrapment require further investigation. While a balance between drag and buoyancy is expected to control the motion of discrete air pockets in closed conduits, there have been limited studies in terms of how factors such as varying slopes, background flows, and air pocket volumes affect the air pocket motion. Past research by the authors presented at the 2012 ASCE-EWRI Congress indicated that for adverse conduit slopes, entrapped air pocket celerities can be approximated as the summation of the celerity measured in quiescent conditions and the ambient flow velocity. The present work aims to advance this research by including a wider range of experimental conditions, particularly horizontal and favourable slope conditions. Results of pocket celerities and trajectories are systematically compared for various tested slopes, flow rates, and pocket volumes. These experimental results will be useful to the future development of numerical models that can include the motion of entrapped air pockets in tunnels.