Our storm sewers can’t handle today’s extremes, they never really could

3 April 1, 2016 at 2:43 pm by

sewer geyserYou’ve probably heard this little chestnut on at least a few occasions: “Our underground storm sewers can’t handle the rainfall we get these days.” The implication is that under climate change and with increased runoff in urban centres from the proliferation of impermeable payment, our storm sewers can no longer handle the load from extreme rainfall events.

This is a misnomer.

While, true, underground storm sewer systems weren’t designed to handle some of the extremes we experience today, the truth is, they really never where designed to handle large extremes. It is not as though the systems were able to handle really severe rainfall events when they were first designed and built in the 1940s, 1950s or 1960s, but can’t now due to climate change and increased urbanization. This is a wild mischaracterization of how storm sewers are designed and built, what they are intended to do, and perhaps even the current impacts of climate change on precipitation patterns in Canada.

Canada’s storm water infrastructure can essentially be divided into two major categories, the minor system and the major system (with both essentially representing different storm water management eras in Canada).

For many decades, the minor system – i.e. the system of eavestroughs, downspouts, service connections, catchbasins and underground pipes – was the only formal infrastructure in existence that could be relied upon to convey storm water away from homes and other buildings, and prevent ponding and street flooding during rainfall events. Most areas built before the early 1970s could only really rely on this system to manage storm water in a systemic way.

Then, from about the early 1970s onward, the major system came into play. This system can broadly be characterized as those storm water management assets that use the surface of the earth to convey, collect and store storm water. These can include both natural waterways (such as streams and rivers) and man-made assets, including roadways, drainage ditches, swales, channels and wet/dry ponds.

The underground portion of the minor system is largely designed to handle more frequent (less extreme) 2 year, 5 year and, sometimes, 10 year events (i.e. events whose probability of occurring are 1 in 2, 1 in 5 and 1 in 10 in any given year, respectively). In some cases, a few municipalities are replacing old underground systems with new infrastructure that can handle 1 in 100 year events (Toronto is a prime example), but such efforts are relatively few and far between. (It is  important to note that even these more robust systems can – and have been – overwhelmed by extreme rainfall events.)

The major system, conversely, is largely designed to handle runoff from less frequent (i.e. more extreme) rainfall events, often in the range of 1 in 100 year.

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Examples of recurrence intervals based on probability of occurrence (Source: climatica.org.uk)

Hence, and as noted by the faculty of engineering at Ryerson University, “By providing the major and minor system for urban drainage, a higher level of flood protection can be provided and the chance of basement flooding can be reduced.”

So, the underground portion of storm water management systems in Canada were most commonly designed and intended to handle more frequent, less severe, rainfall events. In the event of severe rainfalls in older parts of cities (where a large amount of rain falls in a relatively short time and there is no engineered major system in place), flooding often would occur.

So, while it is accurate and truthful to say that our underground storm water networks can’t handle the kind of extreme rainfall events we have been experiencing the last few years; it is equally truthful to say that they never really could handle extreme rainfall events.

This is an important distinction.

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