It’s All True—Solid, Liquid, and Gas Don’t Mix in Inverted Siphons!

This paper explores the performance parameters of inverted siphons (also known as depressed sewers) in sewerage and raw water conveyance systems. It focuses on the management of water, air, and sediment transport in two design case studies in the Pacific Northwest. The paper discusses the mechanisms of air/gas and sediment development in conveyance systems, literature review findings, lessons learned from historical installations, the design steps adopted in the case studies, and what can be done to minimize the risks of air and sediment capacity restrictions. A focused understanding of sediment characteristics, mechanisms for development, and how air and sediment behave at critical flow rates will assist in maintaining conveyance capacity through profile development and infrastructure placement and geometry. The design solution for air and sediment transport need to consider the transport impact on both. It is economically impractical to eliminate particle and air retention in inverted siphons, but through appropriate design processes, their behaviors can be better understood and impacts reduced.