Since about the 1970’s, Canadian cities have a adopted a two-tiered approach to handling stormwater. An underground system that conveys the precipitation from relatively frequent, low-intensity events and an above-ground conveyance system that handles water from less frequent but more extreme precipitation events. In most communities, this system is designed to be able to effectively convey water from a 100-year storm event. That is, the storm with the most rainfall that occurs in a 100-year span (statistically speaking). Even relatively common storm events, the 20-year or 50-year storm for example, can still cause modest disruption to daily life by inhibiting traffic and public transit due to ponding on city streets, among other minor complications. However, events that result in widespread damage and basement flooding should be relatively rare. However, the last few decades have shown that these events are more common than previously anticipated, and only promise to get worse as a result of global climate change. Some projections indicate that the previously 100-year storm event may occur five to ten times more frequently than in the past.
In response to a major flooding event, and predicting the increasing occurrence of extreme-precipitation events due to climate change, in 2004 the City of Stratford, Ontario, created a City Wide Storm System Master Plan that prepared it for a 250-year storm event. Most other plans of this type only take into account the 1-in-100 year storm event, so Stratford’s plan is exceptionally ambitious. However, it may also be prescient. At the time it was written, climate projections were much less precise than they are now. Current projections indicate that once-in-a-century storms may soon occur five or even ten times in a century by the year 2100. Accordingly, the heightened protection offered by this plan will likely prove extremely beneficial in the decades to come.