One Story...

November 30 to December 1, 2006.  Hamilton (Canada) received more rain on those two days (57.0 mm / 2.3 in) than the average for the whole month of December (43.7 mm / 1.7 in).  These were also the greatest rainfalls for each of those two days since at least 1959 (22.4 & 34.6).  Can you see what's coming?

There are a few factors to consider that make this torrential downpour a potential disaster.  The water treatment plant destined to have problems on the 1st services approximately 380,000 people.  Over 600 km of the city's pipelines (370+ miles) are combined sewers, both sewage and runoff into the same pipes.  With an average temperature hovering just above freezing, there is little evaporation.  Torrential downpours often result in immediate runoff (into sewers) because the ground does not have the time to absorb the rainfall.

To make a long story shorter, the treatment plant had an increasing wall of dirty water bearing down on it, filling it's holding tanks and maxing out their outfall and diversion structures and still the wet water well was increasing in height, threatening to breach the upper limit.  As the water level approached the limit, the height differential between the well and several dozen residential basements starts to become equal. 

If this could be worse, it was.  If the water was to breach the top of the well, it would flow into the dry well area of the plant.  This is the location of the pumps and (of course) all of the electrical equipment servicing the pumps.  If the electrical shorts, the equipment fails, the waters rise uncontrollably and a significant portion of the lower city becomes flooded with dirty water.  A public works, public health, civic disaster.

If it had happened, you would likely have known as it would have made BIG news in North America.  The treatment plant had 5 wet well pumps and 3 dry well sump pumps fully operating before a fix was found.  There was one sump pump left.

So... an isolated incident?  An unfortunate synergy of circumstances?  A product of aging infrastructure?  Poor planning from decades past?  Does the cause matter?  The fact remains that many cities worldwide still have a significant percentage of combined sewerage.  That funding for municipal infrastructure is not keeping up with maintenance in many cases.  That science is telling us that global warming will give many of us an increase in torrential rainfalls.

While the incident may be isolated from a historical perspective, it is quite likely a harbinger of things to come.

PS:  it is heartening to know that the 'fix' found was reportedly offered up by a couple of old timers in public works who pointed out a little known, unused gate in the system.  "Institutional memory" saved the city.  A different lesson for another day.