Influence of Air Pocket Volume on Manhole Surge

Abstract

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 vertical riser 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. However, it is clear that if the air volume is sufficiently small, reduced surges must occur. Previous experiments also indicate that riser diameter is a significant parameter with smaller diameters resulting in larger surges. Experiments were conducted in which the air pocket volume and riser diameter were systematically varied. Vertical surges (when normalized by the tunnel diameter) from ~0.2 to >25 were observed, confirming that larger surges were associated with larger air pocket volumes and smaller riser diameters. Both variables have significant influence on the observed surge. The experiments suggest that a minimum air pocket volume when normalized by the cube of the tunnel diameter is necessary in order to attain the maximum surge levels. In addition, the experiments recorded transient pressures near the base of the riser. These pressure traces showed significant similarity to pressures recorded during geyser events in a large stormwater tunnel, which gives support to previous arguments that geysers observed from manholes in storm-water tunnels are associated with the release of discrete pockets of trapped air.