Influence of Plunging Shaft Inflow on the Formation of Geyser in Underground Drainage Systems

Vertical drop shafts are used for transferring storm water to underground drainage tunnels. Significant air entrainment is usually observed for plunging-type drop shafts, which could lead to entrapment of air pockets in the surcharged underground drainage tunnel. The entrapped air pockets reduce flood conveying capacity and may be released explosively in the form of geysers. This paper studied the effect of plunging dropshaft inflow and air entrainment on the formation of geysers by physical modeling. Experiments have been performed on a simplified drainage system, which consists of a vertical plunging dropshaft connected to a horizontal pipe with a constant head tank. A steady inflow is established in the dropshaft. An air pocket is released into the horizontal pipe and then release out of the system through the dropshaft. The dynamics of the air pocket in the dropshaft is measured with high speed imaging. When the inflow from plunging dropshaft is small, air entrainment is confined to the top part of the water column. The net rising velocity of air pocket is reduced, due to a net downward velocity. As the inflow increases, the influence of air entrainment extends towards the entire length of dropshaft. The air pocket is much more rapidly accelerated and released, causing strong spewing and expulsion of air and water. A simple theory is postulated to account for the impact of dropshaft inflow on the occurrence of geyser. Observations and theory suggest that the dropshaft inflow could lead to the occurrence of geyser even when the dropshaft to tunnel diameter ratio is above the critical value of 0.6.