Understanding and Mitigating Overland Flooding Risks

Overland flooding, also known as pluvial flooding, is a significant natural hazard that occurs when intense rainfall overwhelms drainage systems, causing water to flow over land surfaces. This phenomenon can lead to substantial damage to property, infrastructure, and pose risks to human safety. 

Types of Overland Flooding 

1. Flash Floods: These occur rapidly due to intense rainfall or dam failures. They offer little warning time and can carry debris and contaminants 

2. Areal Floods: These develop more slowly as water bodies gradually overflow their banks due to prolonged rainfall or snowmelt 

Causes of Overland Flooding 

Overland flooding primarily results mainly from two big factors: 

Extreme Weather: This includes 

• Intense rainfall, especially during spring when the ground is frozen or saturated 

• Storm surges from hurricanes 

• Rapid melting of snow and ice 

• Ice jams in rivers 

Urban Development: Changes in land use associated with urbanization can exacerbate flooding by: 

• Removing vegetation and soil 

• Grading land surfaces 

• Increasing impervious surfaces 

• Poor lot grading, directing water flow toward buildings 

• Lack of proper overland flow routes in older neighbourhoods 

High-Risk Areas 

Certain areas are more susceptible to overland flooding: 

• Floodplains near rivers 

• Coastal regions 

• Areas with heavy seasonal rains 

• Regions prone to freeze-thaw cycles 

• Low-lying areas below sea level 

Over 1.5 million homes across Canada are located in areas of high flood risk.*  With 80% of Canadian cities built wholly or partially on floodplains, the potential for damage from overland flooding continues to grow. 

Overland Floodings are On the Rise 

The alarming increase in overland flooding poses a significant threat to communities worldwide. Especially driven by the recent climate change, extreme weather events are causing widespread flooding, resulting in substantial damage and disruption. 

Canada is no exception. Overland flooding events are becoming more frequent and severe across Canada. 

Increasing Frequency and Intensity 

• Climate models project that by the end of the century, extreme rainfall events that currently occur once every 20 years in Canada could happen as often as every 5 years.* 

• The amount of 24-hour extreme precipitation occurring once in 20 years is projected to increase by 12% on average.* 

• Parts of southern British Columbia, Ontario, Quebec, and the Atlantic provinces have already seen an increase of two to three heavy rainfall days per year on average. 

  
  

Rising Economic Costs 

The financial toll of overland flooding has been steadily climbing: 

• In the past decade, floods have averaged nearly $800 million in insured losses annually in Canada.* 

• Experts estimate that for every dollar of insured flood losses, there are two additional dollars in uninsured damage borne by households and taxpayers. 

• By mid-century, the annual costs of flood damage to homes and buildings in Canada could grow to over $5.5 billion, potentially reaching as high as $13.6 billion by the end of the century.* 

Preventing Overland Flooding 

Preventive measures focus on directing water away from properties and ensuring proper drainage: 

• Lot Grading: Ensure the ground slopes away from building foundations to prevent water accumulation near walls 

• Swales: Use grassy depressions to direct stormwater away from your property 

• Downspouts and Gutters: Disconnect downspouts from weeping tiles and ensure they extend at least six feet away from the building to direct water to safe discharge points 

• Sump Pumps and Backwater Valves: Install these devices with battery backups to prevent basement flooding from sewer backups 

• Water-Resistant Materials: Use these materials for below-ground construction to minimize water damage 

• Landscaping: Plant erosion-resistant plants and maintain swales to manage runoff effectively 

  
  

Mitigating Overland Flooding 

In the event of potential flooding, there are steps you can take to mitigate damage: 

• Emergency Plans: Develop an emergency plan that includes evacuation routes and communication strategies for family members 

• Sandbags: Stockpile sandbags in advance to create barriers against incoming water 

• Elevate Valuables: Move valuable items and electronics to higher floors or elevate them off the ground in basements 

• Shut Down Utilities: Turn off electricity if water threatens outlets, and ensure gas-powered equipment is used in ventilated areas 

• Insurance Coverage: Consider purchasing overland flood insurance coverage, which is often separate from standard policies but essential for high-risk areas 

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 *SOURCE: https://climateinstitute.ca/news/fact-sheet-climate-change-and-flooding/