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Posts Tagged ‘flooding; storm drains; Montréal 1987 flood

The Montréal flood of 1987

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Thirty years ago, there was a huge cloudburst that paralyzed the city.  Ici Radio-Canada has put some video footage on-line, and it’s quite interesting.  It’s in French, of course, but the images are quite eloquent.  Here is a link to the article that features the video; the translation is from me.

The Décarie Expressway, thirty years ago.

The city had been under a heat wave, and when a cold front came a huge storm was created: over 100 millimetres of rain fell in an hour. A storekeeper witnessed the storm:

The wind started blowing, and all of a sudden you couldn’t see anything, it was like white smoke.  But it was rain! Trees were all bent.  Water started coming here through the roof, through the windows, even through the toilet.  We have three feet of water in the basement!

Weatherman Pascal Yiakouvakis explained that there was wind, rain and hail.  For hail to form, the storm cloud has to be very tall, at least ten kilometers – one can imagine how much water that represents!  More water fell in one hour (100 mm) than normally falls in an average July in Montréal (90 mm)  That is also much more than the stormdrains can handle: their design capacity is for 40 mm/hr.

Traffic was paralyzed, even emergency vehicles could not move.  Drivers had to be rescued, using ladders, from the below-ground level highways like the Décarie Expressway.  Hydro Québec reported a loss of power to about 350,000 households, without knowing exactly where: the storm had damaged the main computerized data control system.  The Montréal Métro had to shut down, the tunnels were completely flooded.

In the wake of the flooding of Gatineau, Rigaud, and areas around Montréal this past spring, one wonders what, if anything, has been learned.  The two instances of flooding cannot be compared, of course; the 1987 flood was a local cloudburst (Longueuil, on the south shore, got barely 10 mm during the storm) of short duration, while this year’s flooding was due to relentless rain over a large area for over a month.

But still, would it not be a good idea to berm the subway entrances so that flooding is minimized? The same scenario happened in New York City after the storm Sandy, and this is something considered. Does it make sense to put a highly in a large ditch?  Not really – actually, any highway is a long scar through a neighbourhood, and it traps users whenever something goes wrong.  Surface streets with regular intersections are far more resilient.

Does the engineering approach of stormdrains work?  Yes, it does, but it has its limitations.  Whether a system can drain floods according to its design specs depends on maintenance, among other things; today (July 15th) CBC reported about a flood in a Montréal underpass because sand and gravel had accumulated in the combined sewer pipe.

But even should the system work as designed, the design criteria are no match for a storm from hell.  Green roofs can shave off a few millimeters from a storm, preventing the worst; cities as diverse as Chicago and Paris have now mandated them on new construction. Ditto for infiltration basins and rain gardens; we are starting to see these here and there (for instance, nearby Township of Langley has some interesting plans).  And our combined sewers could do with expanded storage: much larger pipes can be used in strategic locations, creating underground reservoirs that let sewage flow freely but can store storm waters when necessary (and also prevent release of combined sewage and strom waters, or CSOs; that`s their key design function).

But to find really aggressive approaches, one has to look overseas – and nowhere better than the Netherlands.  Rotterdam, for instance, has created a good number of, well, holes under the ground surface, where cloudburst water can accumulate before flooding streets and houses.  For instance, the underground parking lot of the biggest art museum in town (Boijmans van Beuningen) is designed to flood automatically under cloudburst conditions.  In another instance, a local school has a sunken outdoor basketball court; as much as two meters of rain can gather there.

Vancouver has at least one large tunnel, abandonned and condemned, that could serve as a temporary receptacle for deluge-like storm waters.  I expect this is also true in Montréal as in many Canadian cities that have been flooded in the last decade.  But I rarely hear anything like that mentioned.  How about you, Calgary?  Toronto?  …anyone?


Written by enviropaul

July 15, 2017 at 5:23 pm