Floods can be one of nature's most destructive forces. The damage caused by the water itself is secondary to the crushing pressures of its currents. A torrent can topple buildings in what feels like a blink, often unpredictably.

How floods start, what areas are vulnerable

A house north of Morris, Man., is isolated by flood water on April 14, 2009. Areas north and south of Winnipeg saw high water levels that spring. (John Woods/Canadian Press)

Floods can be one of nature's most destructive forces. The damage caused by the water itself is secondary to the crushing pressures of its currents. A torrent can topple buildings in what feels like a blink, often unpredictably.

The gushing force of a flood is its most devastating characteristic. Where a surge of water is concerned, it actually isn't size that matters — water that's just 60 centimetres deep can move with enough thrust to carry away an average-sized vehicle. Just one-quarter of that amount can knock an adult off their feet.

Costly underestimations of Mother Nature result in almost half of all flood deaths — especially when people misjudge the force of water and overrate the modern automobile by trying to drive through flooded streets.

Scientists calculate likelihood and severity of flooding based on the shape, location, elevation and soil profile of land. By far, the most crucial issue is the amount of water the ground can absorb.

Why floods happen

Regions with very compact or clay-rich soil are more prone to flooding than untilled forested areas. Areas rich in bedrock will flood more readily than those with a loose soil composition. Porous ground means more space between soil particles. And, when water seeps, it seeps into spaces between particles.


If water from rain, melting snow or flooding doesn't drain quickly enough, rock and soil can become saturated with water.

If that earth sits on a slope it can become unstable and collapse under the combined weight, forming a slurry of soil, rock and mud that flows downhill.

Mudslides possess enough force and speed to destroy homes, even entire neighbourhoods.

In May 1971, heavy rains in St-Jean-Vianney, Que., caused the earth to collapse into a sinkhole. The mudslide that resulted killed 31 people and swept away 35 homes, a bus and several cars.

No matter the volume of rain that falls and in what direction it runs, water just needs a place to call home. If the ground has enough nooks and crannies to accommodate each drop of water, the land won't flood. But when all the spaces fill up, the ground turns the late coming droplets away, and they all collect on the surface. This happens more quickly when the ground is paved with asphalt than in a grassy meadow, but when capacity is reached the results are the same - water fills sewers, streambeds overflow, dikes give way and flooding occurs. 

Most of the time flooding happens after periods of intense rainfall. This is the case in some Asian countries such as Bangladesh and India, where the wet season is characterized by monsoons, and heavy rainfall causes major disasters.

Hurricanes are also a factor. Intense rainfall is part of the hurricane package, but not the only assault from the sea. The leading edge of a hurricane can cause massive waves (called "storm surges"), which bring about sudden and violent flooding. And, the eye of such a storm, long known for the calmest winds, can cause widespread coastal flooding (when the sea level rises and water overflows coastal banks). This happens because the eye of the storm is the area of lowest pressure, an area where sea level may rise by several metres.

A house resists a torrent of water during a major flood in downtown Chicoutimi, Que., in July 1996. (Jacques Boissinot/Canadian Press)

Coastal flooding can also be triggered by tsunamis - massive waves caused by shifts in the ocean floor.

When snow-covered land melts too quickly under the sun's rays, spring becomes a high-risk season.

Canada's flood zones

Many of Canada's rivers have basins susceptible to severe runoff. In fact, every landscape has a threshold that can, in theory, be reached. But some areas are more vulnerable than others.


Dams: A certain type of dam, called a dry dam, can be used to control flooding. The dam normally allows water to flow unimpeded, but holds back excess water during intense rainfall and releases it at a controlled rate.

But dams, a tool that civilization has adopted to mitigate (and sometimes harness) the effects of flooding, can cause what they have been designed to prevent. The architecture of a dam is meant to modify the flow of waterways by collecting water in a reservoir so that it becomes possible to increase or decrease the flow according to desired levels. But, when more water accumulates than engineers predict, the structure can burst under pressure releasing a wall of water across the land.

Dikes: A dike is a permanent or temporary wall along a river's bank to prevent flooding. The Netherlands has a complex series of permanent dikes called the Delta Works.

Levees: A levee is an artificial sloped embankment on a river, designed to contain rising floodwaters.

Coastal defences: Structures such as sea walls, jetties, breakwaters and artificial islands can protect land near the ocean from flooding.

On Manitoba's Red River, snowmelt water from the U.S. flows north through a wide, flat plain. The surrounding area, including several small communities and the city of Winnipeg, is an area at high risk for floods. A quarter of the country's most severe floods over the last two centuries occurred on the Red River floodplain.

The 2009 Red River Flood was the second-worst on record. A heavy rainstorm in early November 2008 combined with a high level of ground frost that prevented absorption were blamed for the flooding.

The worst flooding in the valley was in 1997, when more than 7,000 military personnel were called in to help evacuate 25,450 people to safety. About 1,000 homes were damaged.

The government of Manitoba's March 25, 2011, spring flood outlook again shows a high potential for spring flooding for much of the province, including around the Red, Souris, Pembina, Assiniboine, Winnipeg, Saskatchewan and Fisher rivers, as well as the Interlake region.

In 1996, the Saguenay region of Quebec was the site of the most costly flood in Canadian history. An intense rainstorm coupled with the insufficient storage capacity of local dams caused a flood that resulted in $1.5 billion worth of damages.

The Fraser Valley of British Columbia is another floodplain at significant risk. Local authorities have designed and built dikes to protect the area. The Lower Mainland and Fraser Valley faced a deluge in 2009 after heavy rain quickly melted large quantities of snow.

Areas of Alberta, Saskatchewan, New Brunswick, northern Ontario and the Northwest Territories have also experienced serious flooding in their pasts. Last year, New Brunswick Premier David Alward described the damage left by December flooding as "beyond imagination" after water reached the rooftops of homes following a storm surge.

Overall, more precise weather forecasting has improved flood prediction, and better building structures have minimized damage from some disasters. But, floods are a fact of physics and nature. We can prepare for them but we can never prevent them.

Categories of floods

Floods are categorized according to their likelihood of occurring over a period of time. A 100-year flood is the maximum level of floodwater that can be expected in an average 100-year period. It's also known as the one per cent flood, because there is a one per cent chance of it occurring in any one year.

Using other formulas and considering details such as average normal water levels, soil type, bedrock and groundwater profiles, scientists map out the extent of potential floods and the areas of inundation. These findings are an important part of environmental assessments for building permits and flood insurance. 

In addition to their destructive physical force, floods bring with them a more menacing chemical threat. If water levels are high enough, water seeps into houses and buildings, washing through laundry rooms, tool sheds, garages, chemical storage closets, septic tanks and garbage receptacles. Runoff carries chemicals, toxins and waste products that can filter through the water table for long periods of time. Flood victims must be extremely cautious about the water they drink, the food they eat, and how they bathe themselves.