City of Ottawa celebrates completion of Stantec-designed Combined Sewage Storage Tunnel

The City of Ottawa recently celebrated the completion of its Combined Sewage Storage Tunnel (CSST), a ten-year, CAD$232 million investment. The CSST, the marquee project of the Ottawa River Action Plan (ORAP), will greatly reduce the frequency of combined sewage overflows, reduce the risk of flooding in the downtown core, and add resiliency to the operation of the system. The CSST can hold up to 43,000 cubic meters of sewer overflow during major rainfalls, the equivalent capacity of approximately 18 Olympic-sized pools. Stantec was the City’s lead design consultant from the initial environmental assessment through design, implementation, and construction support.

The Ottawa River has always played a vital role in the history and economy of Canada’s national capital, whose one million residents depend on the river for tourism and recreation. Like many historic North American cities, Ottawa’s downtown has a combined sewer system which uses the same pipes to convey both sanitary and surface water to its treatment facilities. During heavy rains or snowmelts, these systems can become overwhelmed, resulting in untreated water flowing into the City’s namesake river. The CSST – an engineering feat, built with precision and beneath Ottawa’s downtown core – will significantly decrease the amount of untreated water that spills into the river, protecting the health of people and wildlife. This project places the City amongst the world’s leaders in wet weather flow management, targeting a performance level that well exceeds current industry regulations and guidelines.

“The CSST’s commissioning is an important milestone that will result in significant benefits for the City and the people who live, work, visit, and play in our nation’s capital,” says Adrien Comeau, Stantec’s CSST project manager. “We’re extremely proud of this project. Working in tandem with the City and project partners, this creative tunnel design maximizes the functionality and resiliency of the system, protects public health and the environment, and as people continue to fall in love with all that Ottawa has to offer, our City can continue to accommodate a growing population.”

CSST by the Numbers
The newly completed project has several important features, including two inter-connected storage tunnels – one running east-west, and the other running north-south. The tunnels are a total of 6.2 kilometers (3.9 miles) long, with an interior diameter of 3 meters (10 feet), and range from 10 to 31 meters (32 to 101 feet) below ground. Additionally, there are 15 major access shafts – including five drop structures and six flow diversion chambers – and four odour control facilities. All elements are now operating, but they do their most important work during and after moderate to heavy rains. Once rainfall has subsided, water captured in the tunnel will be treated and returned safely to the river.

The system will also lessen the risk of basement flooding for several low-lying lands in Ottawa’s Centretown/Glebe/O’Connor area, and increase operational flexibility and redundancy to major collector sewers. Additionally, the east-west tunnel was designed to “twin” a 60-year-old sanitary sewer so the City can divert its flow into the tunnel and more easily conduct inspections and maintenance on the aging pipe.

Integrated Holistic Green/Grey Optimization
As part of its project role, Stantec conducted a triple bottom line analysis – looking at environmental, social, and economic factors – and concluded deep tunnel storage through the core combined sewer area was the best solution. Multiple location and storage alternatives were developed and assessed to comply with municipal, provincial, and federal requirements. Environmental studies, hydraulic analyses, as well as laboratory and geotechnical testing programs supplied data for the design of multiple project components. Stantec developed the conceptual design and then completed the preliminary design, detailed design, and tender documents, as well as contract administration. A comprehensive risk management strategy and an innovative procurement process provided added value to the project.

With major weather events (i.e. “100-year storm events”) happening more frequently, communities are redesigning the urban environment to better manage stormwater. A previous Stantec project upgraded the City of Ottawa’s flow-regulating structures and implemented an automated real-time control system. Since being put into operation, the system led to a 60 to 70 percent reduction in the volume of overflows into the river. Stantec is also serving as the lead designer and program manager for Phase III of the Narragansett Bay Commission’s CSO Program – where upon completion, Rhode Islanders will benefit from cleaner water and a reduction in annual beach and shellfish bed closures, better supporting the local economy for generations to come. Stantec is also working with the City of Toronto on their Basement Flooding Protection Program to reduce the risk of flooding by making improvements to the City’s sewer system and overland drainage routes to protect businesses and residential communities.

About Stantec
Communities are fundamental. Whether around the corner or across the globe, they provide a foundation, a sense of place and of belonging. That's why at Stantec, we always design with community in mind.

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Media Contact                                               
Toni Fink
Stantec Media Relations
Ph: 216-298-0708
toni.fink@stantec.com