Citation: Allasia, D.G.; Böck, L.É.; Vasconcelos, J.G.; Pinto, L.C.; Tassi, R.; Minetto, B.; Persch, C.G.; Pachaly, R.L. Experimental Study of Geysering in an Upstream Vertical Shaft. Water 2023, 15, 1740. https://doi.org/10.3390/w15091740 Academic Editors: Marco Franchini and Chin H. Wu Received: 21 March 2023 Revised: 14 April 2023 Accepted: 26 April 2023 Published: 30 April 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). water Article Experimental Study of Geysering in an Upstream Vertical Shaft Daniel G. Allasia 1 , Liriane Élen Böck 1 , Jose G. Vasconcelos 2, * , Leandro C. Pinto 1 , Rutineia Tassi 1 , Bruna Minetto 1 , Cristiano G. Persch 1 and Robson L. Pachaly 2 1 Department of Sanitary and Environmental Engineering, Universidade Federal de Santa Maria, Santa Maria 97105-900, RS, Brazil; dallasia@gmail.com (D.G.A.); leandro.pinto@ufsm.br (L.C.P.); ruti@ufsm.br (R.T.); bruna.minetto@gmail.com (B.M.) 2 Department of Civil and Environmental Engineering, Auburn University, Auburn, AL 36849-5337, USA; rlp0046@auburn.edu * Correspondence: jgv@auburn.edu; Tel.: +1-334-844-6280 Abstract: Transient flows in stormwater systems can lead to damaging and dangerous operational conditions, as exemplified by geysering events created by the uncontrolled release of entrapped air pockets. Extreme rain and associated rapid inflows may result in air pocket entrapment, which causes significant changes in flow conditions and potentially geysering. Stormwater geysers have been studied in different experimental and numerical modeling studies, as well as from limited data gathered within real systems. However, there is still no complete understanding of geysering events, as stormwater system geometries vary considerably. Most past studies involved releasing air from an intermediate shaft, in which a significant fraction of the entrapped air may bypass the release. This work advances the understanding of geysering by considering uncontrolled air release through an upstream shaft or manhole. In such cases, the entire air pocket is released upon reaching the shaft, worsening the occurrence of geysering. Pressure and water level measurements were performed for various combinations of initial water pressure, trapped air pocket volume, and vertical shaft geometries, indicating the higher severity of these geysering events. The results obtained also corroborate previous studies in that the measured pressure heads were lower than the grade elevation. Future studies should include larger-scale testing and the representation of this geometry using CFD. Keywords: stormwater systems; air pockets; sewer pressurization; two-phase flows 1. Introduction and Objectives Intense rain events leading to rapid filling conditions of stormwater collection systems may create operational problems such as pressurization of conduits, entrapment of air pockets within the pipes, pressure surges, and even water hammer [1–4]. Large entrapped air pockets spread and move within stormwater conduits as gravity currents [5–7], and as they reach the bottom of water-filled ventilation, uncontrolled air release ensues. Entrapped air will move upwards due to buoyancy, and as it moves, it displaces the water initially present in the structure. When the displaced water reaches grade elevation, an explosive release of an air–water mix is observed, referred to as stormwater geysers. Stormwater geysering events have been investigated as a single-phase flow phenomenon since the 1990s [8]. Still, two-phase flow geysers have been investigated more recently [4,9] and, since then, have been studied by many authors. In some episodes, stormwater geysers were reported to reach more than 30 m [10]. These events can negatively impact the environment and public health and create additional costs related to the damage of the drainage infrastructure [1,4,8,11,12]. The severity of the observed geysering events is more consistent with two-phase flow interactions than with single-phase mass oscillation phenomena [13]. As such, research continues to be devoted to air entrapment and release processes in stormwater systems, as exemplified in [10,14,15]. Past contributions included experimental investigations [9,16–19], numerical investi- gations [14,15,19–22], and, more rarely, field observations [3]. The severity of geysering is Water 2023, 15, 1740. https://doi.org/10.3390/w15091740 https://www.mdpi.com/journal/water