Design standard adaptation trumps climate change mitigation !

The grade 10 kid who first said "“Our underground storm
sewers can’t handle the rainfall we get these days.”
also gave us this very lovely art project.
It looks like the guys at ICLR is getting on track with some much awaited myth-busting ..err ... "chestnut" cracking as they call it :

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

Nutty Myth: climate change is causing flooding in our cities, 100 year storms are getting worse

Nut-Free Truth: many of our drainage systems were only designed for 2 year storms

Of course the engineering community has know this for years. But it is a revelation to the insurance industry that the chestnut " “Our underground storm sewers can’t handle the rainfall we get these days.” ... was actually written by a grade 10 geography student who read it first on Wikipedia. The insurance industry has claimed that we are getting 20 times more storms than we used to - 20 time more storms chestnut cracked - thankfully the CBC Ombudsman checked with Environment Canada after our complaint and fixed that nutty story.

Here is my response to the ICLR inblog post (since usually doesn't publish my comments that push the envelope in any way):


Thank you Glenn for promoting this ("Our storm sewers can't handle today's extremes, they never really could") – we all need to emphasize design standard adaption to prevent flooding, not climate change mitigation.

As an example, storm sewer capacities have been increased up to 400% to adapt to a higher level of service (up to 100 year) in the GTA municipality where I have developed the long term flood control program. Local rainfall intensities are decreasing for most durations at our local airport, just like many southern Ontario rain gauges. So rain is not the issue.

1960's development with 5 year storm capacity.
This is how it was designed to work with a very
low level of service compared to new standards.
One factor not noted in the post is the interaction of the major overland system with the sanitary sewer system in older, pre 1970’s areas, particularly because of the risk of extraneous inflows that overwhelm the sanitary system during extreme events. Our experience is that late-1970’s areas with somewhat limited major overland drainage networks have relatively low flood risks if the sanitary sewer is fully separated (that is foundations drains go to the storm and not the sanitary).

In the municipality where I work we have mapped the overland flow paths, and determined estimated ponding depths using an elevation model to identify areas to seal sanitary manholes from overland inflows and have disconnected downspouts in the high inflow areas to manage that risk factor. This has proven to be an effective approach in an area that had extensive flooding in August 2005 but very little in July 2014 – this was achieve with relatively low cost measures (i.e., downspout disconnection and pick hole plugs). And this risk reduction was achieved prior to storm sewer upgrades.

I’m guest lecturing in the Civil Engineering class at Ryerson this Monday and I will highlight the nod to their site!

Rob M


The real irony here is that the chestnut that ICLR is busting is actually from their own 'research' into rainfall trends that stated that a "rising frequency and severity of extreme weather events" is responsible for floods. That nutty myth has been thoroughly cracked:


Cracking nuts since 2013
The other truth is that the previous major flow paths we used to have have been compromised (filled in, enclosed, etc.) due to urbanization and the residual lost rivers have been correlated with basement flooding risks. Here are some examples of that:

Yesterday's Lost Rivers are tomorrow's flooded basements

More Toronto Lost Rivers

If you are interested in more data, tables and mapping on extreme rainfall trends in Canada then take a gander at the following links:

Static Maps:
Interactive Map:
Table Summaries:
Chart and Table:
Long-term Station Table:
Environment Canada Denies Changes:
Contradicting Insurance Industry Claims:


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