Investigation of Air Pockets Compression and Motion in Stormwater Storage Tunnels

During the rapid filling of stormwater storage tunnels, an event anticipated to occur during intense rain events, a number of different mechanisms may lead to the entrapment of air pockets. Entrapped pockets may cause operational issues such as damaging surges, loss of storage and severe geysering upon their ventilation through water-filled vertical shafts. Most previous studies focused either on determining surges associated to air pocket compression or in obtaining minimum flow velocities required to expel entrapped pockets in water mains. However, mechanisms behind the motion of air pockets, controlled by a balance between drag and buoyancy forces, still require further investigation in conditions where ambient flow exists. Unlike water mains, inflows in stormwater tunnels cannot be fully pre-specified, and the tracking of pockets is important in the context of numerical simulation to assess the risk of operational issues such as geysering. This work presents preliminary results of an experimental investigation performed at Auburn University in which different air pocket volumes are released in pressurized closed-pipe flow at selected slopes and flow rates. Kinematics of entrapped air pockets is studied for the different flow conditions, with the goal of gaining insights on the relationship between the pocket celerity and the relationship with its volume as well to flow velocity and pipeline slope.