Major Floods

There are several types of floods, each of which has a different cause. A flash flood, for example, may last only a few hours, but a phenomenon like the spring thaw can cause floods of longer duration.

A familiar example of a flood is the freshet, which occurs when melting snow and ice swell rivers in spring.

Floods can also occur during a temporary spring or winter thaw. If river ice breaks up, blocks of ice may be carried along by the current until they encounter an obstacle like a bridge abutment, an island, a meander, a shallow stretch or a layer of ice attached to the shore. The ice blocks then accumulate to form an ice jam, or temporary ice dam. As water backs up behind this dam, levels rise quickly and cause flooding.

Another type of flooding is the storm surge, also referred to as a flash flood. Brought about by violent storms, these floods are unexpected, can be dangerous and generally occur in small drainage basins. In the case of such flooding, rainfall is so heavy that the soil is unable to absorb the water quickly enough, and the result is flooding. Flash floods are localized phenomena.

In urban settings as well, heavy rainfall can cause rivers to overflow their banks. The presence of roads and buildings alters runoff rate and affects watershed drainage. When sudden, heavy rains occur, the runoff produced can exceed the transport capacity of the sewer system, causing flooded basements and highways.

Flooding also occurs in the Great Lakes and coastal areas as a result of the intense action of waves and wind. In addition, the combination of occasionally high tides and flow in estuaries can cause flooding.

Finally, an outburst flood is a type of flood that occurs where cirque glaciers are found in western Canada. This type of flood results when a lake that has been held back by a glacier or moraine suddenly drains, releasing tonnes of muddy, debris-laden water. Flooding occurs when the pressure exerted by the water in the lake breaks the dam formed by the moraine or melts a sufficient amount of the ice in the glacier.

Saguenay River Valley Flood, Quebec, 1996

Two hundred and ninety millimetres of rainfall fell in less than 36 hours and caused an estimated $1 billion in damages in the region and at least 10 deaths. Thousands of homes, roads and bridges were washed out and there were downed power lines. A total of 15 825 people had to leave their homes. At least 20 major bridges were heavily damaged. More than 50 towns and villages were inundated, or main roads destroyed and electricity cut off. Aluminum plants and paper mills slowed or closed for a month. At least 3200 industry workers were idled and three schools were closed. The Canadian Red Cross sent 1600 volunteers to set up a temp office and evacuate people. Overall, they assisted 16 000 people.

[D]
Click for larger version, 101 KB
The Jonquière power station dam (along Rivière aux Sables)

The Jonquière power station dam (along Rivière aux Sables) which was overtopped during the flood caused minor erosion of the left abutting valley side and an about 20 metres wide breach in the right wing of the dam.

[D]
Click for larger version, 100 KB
Chute-Garneau dam along the Rivière Chicoutimi

Chute-Garneau dam along Rivière Chicoutimi and the new channel eroded during the flood that bypassed the dam; viewed from the upstream of the dam.

[D]
Click for larger version, 99 KB
Suspended railway tracks

Railway tracks for 7.5 kilometres along Rivière à Mars suspended mid-air because of undermining by cutbank erosion of the valley side.

Descriptions for each of the major floods shown on the map are found in the document Major Floods.

• Major Avalanches Map | Read about this map
• Major Earthquakes Map | Read about this map
• Major Forest Fires Map | Read about this map
• Major Hailstorms Map | Read about this map
• Major Hurricanes that Affected Canada Map | Read about this map
• Major Landslides Map | Read about this map
• Major Tornadoes Map | Read about this map
• Major Volcanic Areas Map | Read about this map
• Tsunamis Map | Read about this map

• Environment
• Natural Hazards
• References and Links