Experimental analysis of geyser formation in urban drainage systems

Geyser formation in the storm drainage system can occur during extreme precipitation events when there are rapid filling conditions. In these conditions, entrapped air pockets can move within the systems displacing the water in front of the air. Upon reaching a water-filled shaft or manhole, the uncontrolled release of entrapped air pockets can trigger geysering, with negative impacts on the environment, public health, and costs associated with structural damage. This is worsened by inadequate geometries in stormwater systems and the lack of a planned urban drainage that are typical of Latin American urban areas. Despite research in the last years, there is yet no complete understanding of mechanisms leading to air pocket entrapment, and consequently, stormwater systems designers do not have enough guidelines to deal with phenomena such as geysering. In this context, this work presents results from experimental modeling of a scaled stormwater tunnel. Two pressurization conditions (fast and slow) and two air volumes (25 and 45L) were evaluated in the system with a single diameter of 100 mm, totaling 4 experimental tests. Slow pressurization of the drainage system was observed to cause a higher elevation of water in the ventilation tower; however, the fast pressurization causes higher pressures in the system. In both cases, geysers' piezometric pressure heads did not exceed grade elevation. This laboratory observations are in agreement with observations of Wright et al. 2011 performed in an actual stormwater system. This is an important and troublesome characteristic of geysering phenomena because it indicates that even if surge levels do not exceed grade elevation, geysers may occur. The results indicate the need to extend the investigation of such phenomena, to provide better guidance to designers relating to the excess and oscillation amplitude of the pressure in the drainage network.