RC121 NovDec 2025 - Magazine - Page 26
FLOOD MITIGATION
Ottawa’s $232-million Combined Sewage Storage Tunnel (CSST) was
designed specifically to prevent the oldest parts of the city’s sewer
system from overflowing into the Ottawa River during heavy downpours.
Tunnel vision that’s paying off
To its credit, the city has been prioritizing separate sewer
and stormwater systems dating back to 1961 when it
was mandated that all new developments have separate
sewers. And since then, there have been ongoing e昀昀orts to
progressively separate sewers from older neighbourhoods.
Most noteworthy of those e昀昀orts has been the $232-million Combined Sewage Storage Tunnel (CSST), designed
speci昀椀cally to prevent the oldest parts of the city’s sewer
system from over昀氀owing into the Ottawa River during
heavy downpours.
Capable of handling up to 43,000 m3 of water per event
(approximately 18 Olympic-sized swimming pools),
planning for the CSST’s 4.2-kilometre network of tunnels
commenced back in 2009.
26—RENEW CANADA –NOVEMBER/DECEMBER 2025
That was followed by an environmental assessment by
Stantec, commencement of construction in 2016 a year before that 昀椀rst “century storm” hit the city, with the project
昀椀nally opening in 2020. Stantec Consulting Ltd. and Jacobs
Engineering Group (formerly CH2M Hill) completed the
detailed design, contract administration and construction
support services for the project, with geotechnical support from Golder Associates Ltd. Construction was led by
Dragados Tomlinson Joint Venture.
Since that time, Sandanayake observes that “our performance data clearly shows that the combined sewerage
storage tunnel is working. Events that would have caused
over昀氀ows no longer cause over昀氀ows.”
In addition to reducing the likelihood of over昀氀ows, the
tunnel temporarily stores the water so it can be sent back
and treated at the wastewater treatment plant. And after
the wastewater treatment plant it’s cleanly discharged
back into the river. “So the high level story is pollution
prevention,” says Sandanayake.
He adds that consistent with the city’s goal of maximizing the impact of such a major investment, the CSST can
be used to essentially backstop future maintenance e昀昀orts.
“If the large collector sewer that crosses downtown
were ever needed to be maintained in a very disruptive
RENEWCANADA.NET
CITY OF OTTAWA (INSET), PUREPAVE TECHNOLOGIES INC.
PurePave’s surface for a new condo project—the site of former flour
plant—in Carleton Place eliminates the need for a stormwater pond.
