Influence of Air Pocket Volume on Manhole Surge
Large discrete air pockets trapped along the crown of a nearly horizontal stormwater tunnel have been shown in laboratory experiments to produce large vertical surges of water as the air is expelled through a manhole connected to the tunnel crown. The phenomenon has been hypothesized as the source of observed geysers from manholes in such systems. A previous theoretical framework that assumes an unlimited supply of air has been shown to reproduce the essential details of laboratory experiments although deviations were observed in tests with smaller air volumes. It is clear that if the air volume is sufficiently small, it must result in reduced surges. The experiments also indicate that manhole diameter is a significant parameter with smaller diameters resulting in larger surges. A set of experiments in which the air pocket volume and manhole diameter were systematically varied were conducted. Vertical surges (when normalized by the tunnel diameter, from about one to over 25 were observed, confirming that larger surges were associated with larger air pocket volumes and smaller manhole diameters; both variables have significant influence on the observed surge. In addition, the experiments recorded transient pressures near the base of the manhole. These pressure traces showed significant similarity to pressures recorded during geyser events in a large stormwater tunnel, lending support to previous arguments that geysers observed from manholes in stormwater tunnels are associated with the release of discrete pockets of trapped air.
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