Fremont Siphon replacement project

What is a siphon and how does it work?

The Fremont Siphon is an inverted siphon. An inverted siphon is used to pass flow through a valley or under a waterway. King County’s Fremont and Ballard siphons run under the Lake Washington Ship Canal.

Inverted siphons take advantage of the basic rule that water flows downhill. Wastewater is pushed through the siphon by the upstream flow that is at a higher water level than the downstream level.

In a wastewater system, inverted siphon pipes are smaller diameter than the connecting pipes upstream and downstream. This “pinch point” increases the speed of flow to help keep solids moving through and avoid plugging the siphon pipe.