Experimental Study on Stormwater Geyser in Vertical Shaft above Junction Chamber

Stormwater geysers are commonly observed as explosive releases of air-water mixtures from municipal systems, which can cause property damages and human safety concerns. In Edmonton, Alberta, Canada, a number of geyser events were reported in recent years from a 27-m-deep vertical shaft above a junction chamber connecting a 3.5-m-diameter incoming pipe to an outgoing pipe with a 4-m drop. To understand the conditions for the geyser formation and select suitable mitigation methods, an experimental study was conducted using a simplified conceptual $1\text{:}20$ scaled model. Three series of experiments were conducted with a sudden increase of the inflow rate and different initial flow conditions: Series A with an initial downstream open-channel flow, and Series B and C, both with an initial downstream full-pipe flow, but an air pocket was deliberately entrapped in the upstream pressurized pipe in Series C. A geyser was not observed in Series A mainly due to the available large outflow capacity in the downstream pipe. With the downstream pipe initially full, a geyser event was observed in Series B as pressure surged in the chamber caused by the increased flow rate. Severe geyser events were observed in Series C, with the first phase triggered by the transient pressure wave and the second phase triggered by air released from the air pocket. The relationship between the measured maximum pressure and the geyser height was established. An analytical model is proposed for predicting the magnitude and period of the pressure oscillation induced by the sudden inflow rate increase. The amount of water splashed out of the riser was also measured.