Digging in against sewer overflows

Ottawa Combined Sewage Storage Tunnel

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.