Why Flood Damages Are Increasing In Canada - Rain or Runoff?

Runoff is the key factor, not rain. Drainage system design comes into play as well.

Intensification of subdivisions has more than doubled since the 1950's increasing from less than one third impervious area coverage to more than two thirds in the last 15 years.

Extreme rainfall is decreasing as shown in Environment and Climate Change Canada's Engineering Climate Datasets. Are scientists being muzzled? In Southern Ontario for example, more statistically significant decreasing trends are observed than increasing trends:

Most changes in rainfall intensity are mild trends, not statistically significant. Version 2.3 Engineering Climate Dataset.

Note that some increased intensity observations are a function of the intrinsic sample bias of the underlying skewed rainfall probability density function. This is important for drainage design and should be considered, but does not indicate a change in the underlying 'population' being observed/sampled (i.e., increases not necessarily a change in fundamental rainfall characteristics due to climate change).

Because flood damages are increasing, could it be the increased runoff due to intensified development contributes to flooding? Yes. The GIS analysis of a city in York Region up top shows how the percentage of impervious area has increased from the 1950's when less than one third of land was hardened with road, rooftops and driveways, to the early 2000's when over two thirds of land was hardened.

Ironically though, the pre 1980's subdivisions may have the greater basement back-up risk because of the partially separated sanitary sewer systems and high wet weather inflows during extreme storm events. These unwanted inflows are sometimes 10 times the inflow allowance in new subdivisions where systems are fully separated (i.e., foundation drains connected to the storm sewer instead of sanitary). Also the overland drainage in the pre 1980's subdivision may not safely follow the roadway but instead travel across private lots, into window wells and basement walkouts aggravating impacts to the sanitary sewer, despite the lower overall runoff rates.

The lesson? Changing rainfall patterns do not explain increased flooding in urban areas in Canada. Increased runoff is dramatic but may be managed in new subdivisions with more resilient design standards (fully separated sanitary sewers, and controlled overland grading patterns). Look for a future post on intensification in existing serviced areas and impacts on the original sewer and drainage systems.

It is time to move past infographics (below) and consider real data when assessing flood risks and developing evidence-based flood mitigation policies in Canada:

Sorry. We don't know what this means or what it is based on ... just that it is from the Ontario government and is a distraction to for any meaningful analysis or discussion on flood management.