Monday, June 24, 2013

Canada's "1 in 100 year" flood planning information

Most people would have seen the images coming out of Calgary right now, where flooding has caused some of the worst problems in living memory.  Despite the disturbing pictures, I find some of the information around this to be a little troubling as well.

Take Calgary's flood planning information.  It points out on the map where the river is.  Then it points out where the flood plains are.  After you reconcile in your head the fact that so much of the city has been built on known flood plains, you then spot the "flood plain based on 1 in 100 year plain" note...

Given we know that the whether is getting worse, rainfall is getting higher and extremes are getting more extreme, why did Calgary base it's current information on past numbers and data, instead of the actually saying what's possible?  To me, this is a very thought jarring oversight.

It's not just Calgary though.  I've been in Toronto for 14 years and experienced numerous "once in a lifetime" events.
  • the 2003 blackout, 
  • two storms with “100 yr” rainfalls, 
  • snowfall big enough to call in the army, 
  • and the SARS breakout that shut down many essential services.  
As I joked before, Toronto's "lifetime" definition must be close to that of a hamster lifespan (5 events in 14 yrs = one "once in a lifetime" event every 2.8 years.)

Looking at the Toronto Flood Plain map, we see the same thinking as in Calgary...  

Here, they used the wording "the Regulatory Flood Plain is based on the regional storm, Hurricane Hazel, or the 100 year flood; whichever is greater."  

This "retroactive-looking" mentality flies in the face of the currently known facts where we are seeing greater rainfall in summer storms due to higher temperatures creating more warm air (which in turn holds more moisture and leads to more rainfall).  This increase is supposed to continue for many decades (see the City debate for the 2040 to 2049 decade as an example).

Conclusion
It appears that Canada's cities only tell you a part of the story.  This is likely because nobody would want to live anywhere that people knew would have issues, so the cities would rather keep the areas populated and then deal with the flood claims and issues afterwards.

My advice therefore is to take what the cities do tell you, then add a multiplier of 3 or 4 to the data.  So, if they say they base things on a 100yr event, you'll probably be closer if you multiply that by 4 so that event is likely every 25 years.  It'll likely be more informative for planning purposes than what we're being told "officially" right now.