Flow in stormwater conduits may undergo transition from free surface into pressurized flows regimes during intense rain events and in this transition process air pockets may be entrapped. While some different mechanisms leading to air pocket entrapment have been identified, pressure surges following sudden air pocket entrapment have been less studied and there are some outstanding questions as to these surge strengths following realistic entrapment scenarios. This work represents an ongoing experimental and numerical investigation on the effects of sudden air pocket entrapment to an initially steady flow with variable volumes of air. A reservoir supply steady flows to a 101.6-mm diameter, 12-m long clear PVC pipeline placed in horizontal slope. At the downstream end there’s free discharge conditions through a knife gate valve. An initial atmospheric air cavity exists at discharge, with a volume that decreases with larger discharging flow rates. Sudden closure (partial or total) of the knife gate valve creates near-instantaneous air pocket entrapment, ensuing pressure surges. Pressure and flow velocity measurements were performed at selected locations along the pipeline. Such measurements were compared with predictions from a lumped inertia model that accounts for air pressurization. The general behavior of the total and partial valve closures was fairly well predicted by the proposed model. Moreover, predicted pressure peaks matched experimental measurements after appropriate calibration of energy and discharge coefficients. The model, however, underestimated the energy damping, and it is speculated whether the additional losses comes from the flow constriction represented by the entrapped pockets. Future work aims to understand in separate the role of flow rates and air pockets in surge strengths, as well to gain further insight in the interactions between air and water phases.