Brightwater Outfall

As consultants for design-builder Triton Marine Construction, Vancouver engineers Dayton & Knight designed the Brightwater Outfall for a large sewage treatment plant under construction in Washington state. The plant is designed to handle 643 Megalitres of wastewater per day and serves north Seattle and King County, discharging the effluent into Puget Sound, south of B. C.

In order to ensure that the effluent meets the minimum dilution criteria before it reaches shore under the most adverse winds and ocean currents, the outfall pipe stretches 1.6 kilometres long and is set at a depth of 200 metres in the water.

The project is unique for being among the largest size of HDPE pressure pipe in North America and one of the deepest municipal outfalls. It also required extreme loading during its deployment, and had to be designed for a 1:2500 earthquake event.

Laid out in a dogleg alignment, the outfall consists of both a steel and a twinned HDPE pipe. The 120 metres of 2,100-mm diameter steel pipe is buried inshore and designed to minimize the impact on the ecology, (such as eel grass beds that support many types of marine life). The remaining 1,530 metres of twin HDPE pipe is 1,600 mm diameter.

Steep offshore slopes of 25-35% and a tendency to liquefaction and sliding soils complicated the design and required careful modeling. To keep the HDPE pipe on the bottom and control its buoyancy (about 32 kg/m when full), the pipe was weighted using four different weight sizes and spacing: type A for minimum area, type B for minimum lateral resistance, type C for lighter weight and ring stiffeners, and type D for stable orientation of the diffusers.

Normally inshore pipe and connections are installed “in the dry” by dewatering a sheet pile excavation. However, dewatering was not permitted on this project because there was a concern that organic contaminants could gravitate to the site. Consequently, all the pipes and connections were installed underwater with divers or remote operating vehicles. The HDPE pipes with 80 mm thick walls were thermally welded together in 18 m lengths, and the pipe had to be pressurized (tested to 80 psi) and kept under tension to control the bending radius during its deployment in deep water.

The US $27.5 million project was completed in December 2008, a year ahead of schedule and at a third less than its original budget. It won a Consulting Engineers of B. C. award of excellence this year.

Principal design: Dayton & Knight, Vancouver (John Boyle P. Eng., Dennis Harrington, P. Eng., Jonathan Knudsen P. Eng.) Client: Triton Marine Construction. Owner: King County, Washington.