Manhole lid displacements created by the release of entrapped air pockets

Stormwater systems undergoing rapid filling may present a variety of operational problems, among which it is the displacement of manhole lids, posing a threat to traffic and pedestrians. While there scarce documentation and measurement of flow conditions in rapid-filling stormwater systems, it is speculated that the air pressurization within manholes may lead to such occurrences. There are different causes for air pressurization in manholes, among which is the release of entrapped air pockets through water-filled shafts. Research has indicated that such releases result in water displacement within shafts and manholes, which in turn create conditions for air pressurization to develop. Air pressures in excess of few kilopascals are enough to overcome the weight of manhole lids and create displacement. This work presents results from an ongoing CFD simulation of air pocket release in shafts with limited ventilation at the lid. Results indicate that releases of large air pockets can create pressure surges that far exceed the threshold for manhole lid displacement, and such pressure rises can occur in few seconds following the air admission in the shaft. The ultimate goal of this investigation is to provide better guidance on the management of air in junctions within stormwater systems, and prevent operational issues that are linked to air-water flows.