The Rising Tide: Flooding in Canada

Floods are the most frequently occurring and the most costly type of natural disaster in Canada. Despite their prevalence, companies often fail to thoroughly consider flood exposure when building their enterprise risk management practices. This oversight may prove to be a devastating mistake, as flooding causes approximately $1.5 billion in damage to Canadian households, property, and infrastructure each year.

In this article — first in a three-part series on flooding in Canada — we outline the main types of flooding and mitigation techniques.

Source: Figure 1 Large flood losses (>100M) from 2012 to 2021 in 2021 Canadian Dollars. Source: IBC Facts Book 2022

Types of flood exposures

Flooding is defined as an overflowing of water onto dryland; however, not all flood types are created equal. Below, we have outlined the most common types:

Riverine flooding

Also known as fluvial flooding, riverine flooding occurs when water overflows its banks from a river, stream, or creek, as it did during the 2013 southern Alberta floods. It is typically caused by heavy rainfall and can be coupled with snow or ice melt, which is also called freshet. There is usually substantial warning time prior to riverine flooding, a few days to a few weeks, based on weather forecasts. Flooding can last a few days to a month, depending on watershed size.

An extreme example of riverine flooding, such as what happened at Fort McMurray in 2020, is caused by ice jams, where ice blocks the river flow and causes flooding upstream of the ice jam. It can also cause downstream flooding when the ice dam fails. This type of riverine flooding is much less predictable and frequently more destructive.

Rainfall flooding

Also known as pluvial flooding, rainfall flooding occurs when a storm causes flooding that is independent of an overflowing body of water. Factors such as precipitation amount and timing, ground and soil conditions, and natural and/or engineered drainage infrastructure greatly influence the possibility of pluvial flooding. Rainfall flooding is less predictable than other flood types and has a much shorter warning time. When flooding begins within six hours of heavy rainfall, and often within three hours, it can result in flash flooding. Even though these storms may only last a few hours, they are capable of leaving behind catastrophic damage, as was the case in Toronto in 2013.

Coastal flooding

Coastal flooding occurs when a volume of water over and above the predicted astronomical tide is pushed towards the shore by strong winds. While it is typically associated with oceans, coastal flooding can also happen along lake shorelines. Coastal flooding is influenced by many factors, such as storm intensity, speed, size, and angle of approach, in addition to coastal features, such as bays and estuaries, and the width and slope of the continental shelf. Coastal flooding has a longer warning time than rainfall flooding, typically at least a few days, based on storm/tide forecasts. It usually lasts a few hours to no more than a day after a storm passes.

An extreme example of coastal flooding is caused by tsunamis, and sea level rise could exacerbate coastal flooding in the coming decades.

Exacerbating factors

Although the types of flooding outlined above may occur on their own, it is also possible for them to happen concurrently, aggravating their effects. For instance, the severity of freshet flooding depends largely on the speed at which snow and ice melts. On top of the risk of pluvial flooding, rainfall has the ability to dramatically speed up the melting process, which can quickly cause rivers and streams to overflow. Similarly, debris or ice accumulations along natural or manmade bottlenecks such as bridges or culverts can block the natural flow of water, resulting in higher flood levels.

Ground and soil conditions, as well as urbanization can also play a role. If the ground is frozen, the meltwaters cannot easily penetrate the frozen ground, instead becoming runoff and flowing directly into nearby bodies of water. Urban areas with many impervious surfaces like concrete and asphalt are dependent on well-engineered drainage systems to avoid excess runoff. Heavy storms can overload them, leaving the rainwater with nowhere to go.

Building in protection

According to Public Safety Canada, every $1 invested in flood mitigation measures could potentially save you $7 to $10 in damages. Further, having appropriate measures in place could lessen the amount of time your operations is interrupted due to the flood.

The most reliable flood protection strategies are permanent physical measures that will require minimal human intervention. These strategies may include:

Installing engineered flood protection measures (for example, floodwalls, flood doors and windows, and backflow preventers)
Sealing above and below grade openings and applying waterproof coating to exterior walls
Raising equipment and relocating equipment to higher elevations
Using flood-resistant materials wherever possible

Temporary flood protection devices — like portable berms or sandbags — may help, but they come with trade-offs. Although typically more affordable and less visually disruptive than permanent measures, these devices need human intervention for deployment, and their feasibility depends on the nature of the flood. For a flood with little warning, there might not be enough time to deploy temporary flood devices. Additionally, relying on human intervention makes them susceptible to human error, which can lead to bodily harm on top of flood damage.

Make a plan

In order to accurately estimate damage potential and determine appropriate mitigation techniques, it is crucial to understand the flood scenarios that are most likely to affect your premises and which areas are the most vulnerable. Working with professional risk advisors can help you assess your current flood risk and provide advice on flood mitigation. They can help you create action plans for before, during, and after the flood that are specific to your facility’s needs. More generally, a flood mitigation plan should consider the following actions:

Before the flood occurs:
- Regularly maintain flood protection equipment, and check it when a flood is imminent
- Periodically conduct drills to practice emergency response during a flood
- Ensure gutters, downspouts, and drains are kept clear
When the flood is imminent:
- Shut down process and equipment
- Raise and relocate valuables
- Deploy flood protection measures
Once the flood is over:
- Prioritize clean-up for critical areas
- Get help from sub-contractors for restoration
- Activate business continuity plan

In an insurance market with a changing outlook on catastrophe risks, it is imperative to stay ahead and mitigate the risk of flooding before it happens. For more information on how working with experienced risk consultants can help you better understand your organization’s flood risks and inform your mitigation strategies, contact your Marsh representative.

With floods becoming more severe with climate change, we will explore the strategies of climate adaptation against flooding in our next article.

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