Siphons Galore: Crossing under a Lake, a Hill, and a Highway by Designing Inverted Siphons for King County’s Mercer Enatai Project

King County’s Mercer and Enatai Interceptors were built in the 1960s and extend over 14,000 feet from Mercer Island into the Enatai neighborhood of Bellevue, Washington. The Interceptors primarily receive flows North Mercer Pump Station but also collect flows from the local city of Mercer Island and city of Bellevue sewer systems. Some parts of the system are reaching the end of their useful lives, and future peak flows are projected to exceed the system’s capacity shortly. After the county chartered the need to upgrade the existing interceptors, an in-depth alternative analysis and preliminary design process resulted in a new system design including three inverted siphon segments. This paper reviews the unique set of constraints and hydraulic design challenges presented by each of the siphons and discuss the different strategies and tools that were used to develop and evaluate the proposed solutions. Some of the key items that will be covered include: concept development, system design approach, sedimentation strategy, control structure design, steady state hydraulic modeling, air management review, and computational fluid dynamic modeling.