New Metro Vancouver Outfall Tunnel to Support Expanded Treatment Capacity
Nestled at the edge of the Fraser River in British Columbia, the Annacis Island Wastewater Treatment Plant is one of the biggest in the Metro Vancouver region, discharging an estimated 480 million liters per day (125 mgd) of treated effluent into the river.
But as the population continues to increase, the plant is feeling the weight of the rising demand for sewage services. At the moment, a maximum of 14 cubic metres per second can be discharged through the plant’s aged sewage outfall pipe, built nearly 50 years ago, but this needs to be increased to 19 cubic metres per second to serve the growing need across the region.
Metro Vancouver, responsible for providing regional liquid waste services to 18 local governments, intends to boost the capacity of the wastewater facility with a major expansion that includes a new 1-km sewage outfall designed to bolster capacity, improve effluent dispersion in the river and allow for continued operations following an earthquake.
“The plant was designed to be built in stages so as the population increased, we need to expand the plant to handle those different flows and loads,” said Dave Hoffman, operations supervisor of the Annacis Island plant. “The time has also come to build another outfall to handle increased flows.”
The Annacis Island plant now serves a population of 1.2 million people from 18 local municipalities. But as Metro Vancouver prepares to welcome a million more people by 2040, there’s a need to upgrade the wastewater facility to accommodate that growth as well projected population increases in the next 100 years.
Metro Vancouver expects the outfall construction project to go to tender early 2018 and is seeking a team of contractors skilled in tunnelling and underwater construction to handle the complex project, which will face challenges related to commercial shipping activities, as it will be built near the busy Fraser River Navigation Channel, as well as migrating salmon run and other ecological concerns.
“The new outfall is going to be a lot bigger, considerably larger and it’s going to be a lot deeper,” said Ken Massé, a senior project engineer with Metro Vancouver comparing the proposed outfall to the existing one. “It is going to involve a lot of complex construction both on land and in the river.”
The new 1,200-m long outfall will see the construction of three 40-m deep shafts and a 4.2-m ID tunnel that extends to a diffuser manifold to be constructed along the river bottom, parallel to the river flow. The three on-land shafts will consist of one at 16-m ID and two at 7-m ID. The new diffuser manifold will be built west of the existing outfall and will be designed to ensure the treated effluent is dispersed over a broad area underwater to maximize dilution and lessen the adverse effects on the marine environment.
The current diffuser system, for instance, has 18 vertical risers that discharge the effluent in an upward direction toward the surface of the river, Massé said. The new system, in comparison, will feature a 240-m long pipeline buried just under the surface of the river bed and running parallel to the shore. This pipeline, with an ID of 2.5 m, will have 48 diffuser ports that discharge the treated effluent horizontally, thus allowing the river to better entrain and disperse the effluent.
Each port will be about 0.6 m in diameter and will be equipped with flexible duck-bill check valves located about 1 m above the river bed to avoid small sand waves expected to move through this area of the Fraser River.
Massé noted the new riser shaft construction and its connection to the outfall tunnel are the biggest challenges facing the project. The tunnel and shafts must be buried deep underground, where liquefaction will be limited following an earthquake, and the 3.8-m riser shaft will be built as a rigid structure keyed into deeper soils to limit deformation during a seismic event. However, the diffuser manifold will be in a shallow trench in the slope of the river bed which is susceptible to much larger liquefaction-induced deformations so it will have bendable sections to allow it to flex without breaking.
All the in-river work will be conducted in a steel-walled cofferdam to protect the interior workspace from river currents and sediment, while isolating construction activities from the river environment.
A request for qualifications is expected to be posted this summer by Metro Vancouver, followed by a request for proposals.
For further information, contact Senior Project Engineer Ken Massé at firstname.lastname@example.org. Details are also available on the website at: www.mvrd.ca.