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Ottawa Combined Sewage Storage Tunnel

Digging in against sewer overflows

  • 43K

    M3 of Storage

  • 6.2KM

    Of Tunnels

  • 3M

    Pipe Diameter

  • Ottawa, Ontario

    Ottawa, Ontario

Virtually eliminating Ottawa sewage overflows and keeping the river clean

A combined sewer system uses the same infrastructure to transport sewage and stormwater drainage in wet weather. So, when a big storm fills up that system, the water comes back up and discharges—relieving the system and reducing the risk of flooding. That’s called a combined sewer overflow (CSO), and systems are designed so that CSOs happen infrequently to protect the environment while still protecting the community against flooding.

Ottawa’s goal was ambitious. They wanted to drastically reduce CSOs into the Ottawa River. We started with an environmental assessment, and we found (through a triple bottom line analysis) that the preferred solution was deep tunnel storage. Basically, there would be tunnels acting as storage reservoirs inside the combined sewer system, which would capture overflow and hold it until the system could handle the excess.

The tunnels are 4.2 kilometres (2.6 miles) and 2 kilometres (1.2 miles) long respectively and 3 metres (9.8 feet) in diameter each, adding approximately 43000 cubic metres (1.5 million cubic feet) of storage. The tunnels range from 20 metres (65.6 feet) to 35 metres (114.8 feet) deep with six major drop shafts transferring the flow into the tunnel. This project was constructed from 2016 to 2020 and commissioned for operation in 2020.

A previous project of ours upgraded the city’s flow-regulating structures and implemented an automated real-time control system. That project reduced annual CSO volume by 60-70%, and with the addition of the combined sewage storage tunnels, Ottawa will virtually eliminate combined sewage overflow during a typical year.

At a Glance

ACEC Canada Canadian Consulting Engineering Awards Award of Excellence, Water Resources, 2022
OPWA Project of the Year Awards Project of the Year, Structures, 2022

Ottawa CSST: A life on the water and a personal project

Ottawa’s Combined Sewage Storage Tunnel is now in operation, protecting the community’s beloved Ottawa River. Colin Goodwin explains the benefits of the CSST project and what it means to him.

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