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Editorial contribution to Special Issue Air-Water flows in urban drainage systems

Air-water flows in urban drainage systems are among the most interesting and complex research topics. The phenomena are present everywhere in roof drainage systems, gully pots, sewers, pressurised wastewater mains, at treatment plants and stormwater outfalls. Air-water flows are hardly visible to the general public, until an incident occurs, like a pipe burst or geysering event. I find the topic very interesting, because the research on airwater flows can be very fundamental with very practical consequences. Nevertheless the research community on this topic is still small and spread out over a wide range of research fields. These were the main reasons for me to ask the Editorial Board of the Urban Water Journal to support the publication of a special issue on air-water flows in urban drainage systems. The objective of this special issue is to provide a stateof-the-art overview and current research activities on the topic. The first paper by Pozos et al., describing a failure analysis of a severe incident during rapid filling of a gravity stormwater tunnel, emphasises the relevance of the topic. Beecham et al. provide an overview of air-water flows in building drainage systems. Two other papers by Lopes et al. and Granata et al. present recent advances to improve our understanding of flow phenomena in gully pots and drop manholes. Vasconcelos et al. describe the current state-of-the-art on flow regime transition modelling with entrapped air pockets and Chanson et al. focus on air entrainment in hydraulic jumps. Hereby, this Special Issue covers the air-water flow phenomena in key components of an urban drainage system. The Special Issue also shows that a lot of work still needs to be done, before accurate predictions can be made with computer models. One of the generic issues is the air entrainment, mixing and coalescence of bubbles, which is driven by turbulence in the decelerating airwater flow in a hydraulic jump or plunging jet. This fundamental knowledge contributes to better understanding of air transport through gully pots, air accumulation in pressurised sewers and water quality of receiving water bodies (downstream of outfalls). Another generic issue is associated with the wide range of scales in urban drainage systems. Pipe diameters in building drainage systems hardly exceed 0.2m, whereas stormwater tunnels, subject to similar air-water flow phenomena, may have diameters up to 7m. Hence combinations of experiments at different scales and numerical models will be required to further improve our understanding of these flow phenomena and make more reliable, safer and more cost-efficient designs. Another topic, related to air-water flows, is the ventilation of sewers and associated gas transfer between the liquid and gas phase with important consequences for sulphurrelated corrosion processes. I would like to thank the editors of Urban Water Journal for the opportunity to publish this special issue. I hope that this special issue will serve as a useful starting point for future research on the intriguing challenges of air-water flows in urban drainage systems.