Conference Agenda

Session 14A: Collection, Pump Stations, and Conveyance: Hydraulics
Tuesday, 10/Sep/2019:
8:00am - 9:30am

Session Chair: Adam Crafts, Murraysmith;
Location: E145

8:00am - 8:45am

Bringing Sewers To Un-Sewered Urban Areas: The Tukwila Loop Project

John Hendron, Zach Schrempp

RH2 Engineering; ,

In 2015, the approximately 70-acre Tukwila Loop neighborhood in Tukwila, Washington was one of the last large, un-sewered urban areas within Valley View Sewer District’s service area. Although located near a new light rail mass transit station, it was difficult for the existing residential neighborhood to redevelop due to lack of sewers. Additionally, many septic systems in the neighborhood had failed, and high coliform counts were detected in downstream surface water. Rather than gradually connecting this neighborhood to the collection system in a piecemeal fashion as redevelopment demands increased, the opportunity to construct gravity sewers throughout the Tukwila Loop as a single large project presented itself as the more viable option.

The project was designed to be constructed in two phases. The first phase involved constructing the sewer mains, primarily using traditional open-cut methods. Directional drilling was used for certain challenging alignments. The second phase consisted of side sewer construction on private property. Acquiring the necessary utility easements from property owners and permits to build sewer mains in areas of WSDOT right-of-way with historically unstable slopes required extensive coordination.

A Washington State Department of Ecology grant/loan application for this project was submitted in 2017. The project was ranked third out of 160 applications and ultimately received $4.5 million in grant funding, significantly defraying the $6.9 million total project cost. Connection charges for property owners were set to increase over time, incentivizing property owners to connect at time of construction. Community Block Development Grant funding was also available for low-income households. As of construction completion in early 2019, approximately 84 percent of property owners in the Loop area had connected to the system.

This project serves as a model of the steps necessary to successfully conduct public outreach, design, fund, and construct a collection system in a large un-sewered urban area.

8:45am - 9:30am

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

Ben Nelson1, James Chae1, Sibel Yildiz2

1Jacobs; 2King County Wastewater Treatment Division; ,

King County’s Mercer and Enatai Interceptors were built in the 1960’s and extend over 14,000 feet from Mercer Island into the Enatai neighborhood of Bellevue, Washington. Large portions of the existing system are designed to operate under a surcharged condition, and the system includes an inverted siphon crossing of the East Channel of Lake 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 was completed and an alignment and profile were chosen that addressed the combination of technical, environmental, permitting, and construction challenges presented by the project. This new system will include three inverted siphon segments. The first siphon will be the 96th Ave Siphon which will cross under Interstate 90 and will convey local sewerage to the North Mercer Interceptor via two 450 LF long, 8-inch and 14-inch barrels. The second siphon will be a 2,460 LF crossing of Lake Washington’s East Channel which will use three barrels (12”, 18”, 18”) to convey flows ranging from the 2.5 MGD to 13.9 MGD. The third siphon segment will operate in conjunction with a rehabilitated existing line to convey flows from the East Channel Siphon and additional local flows from the City of Bellevue to the Sweyolocken pump station. Low to moderate flows will continue to go to the rehabilitated existing line and peak flows of up to 11.95 MGD will be diverted to the a 3,000 LF 30-inch diameter Enatai Siphon.

This presentation will review the unique set of constraints and hydraulic design challenges presented by each of the siphons and discuss the different strategies and tools used during design. Key topics will include: system design approach, sedimentation strategy, control structure design, steady state hydraulic modeling, air management review, and computational fluid dynamic modeling.