What Causes Floods?
Heavy rainfall causes floods more than any other phenomenon. However, there are a number of other causes of floods, especially in coastal areas. They can result from a hurricane, a tsunami, storm surge or a high tide coinciding with higher than normal river levels. Dam failure will result in flooding of the downstream area, even in dry weather conditions.
Other factors which may contribute to flooding include:
• the ability of an area to cope with runoff
• a wet season preceding to an exceptionally heavy rainfall
• ground cover
• tidal influences
• run-off from a deep snow melting
• over-saturated soil, when the ground can’t hold anymore water.
• frozen soil
• high river, stream or reservoir levels caused by unusually large amounts of rain
• ice jams in rivers
• urbanization, or lots of buildings and parking lots
There are two basic types of floods. In a regular river flood, water slowly climbs over the edges of a river. The more dangerous type, a flash flood, occurs when a wall of water quickly sweeps over an area. Almost three-quarters of the approximately 92 deaths from floods each year are due to flash floods.
Here are the factors that help determine whether a flood is minor or major:
- Melting deep snow can produce excessive amounts of water. Deep snow melting rarely causes floods though. A combination of quickly rising temperatures and a heavy downpour of rain can create severe flooding conditions.
- Ice dams happen when ice breaks into chunks that are moved along by currents and dams are formed when they encounter obstacles like bridges or curves in the river. When water levels rise behind the ice dams it causes floods up-river from the dams. Sometimes the dams break causing a rapid movement of water down-river which can also result in flooding.
- The ground can’t absorb water when there is frozen soil. Melting snow or heavy rain causes floods when water can’t penetrate the ground.
- Melting snow and rain cannot be absorbed by saturated ground. Water runoff flows into rivers and streams and when coupled with heavy rains and/or rapidly melting snow flooding can result.
- High water levels in rivers are a clear warning sign that flooding can occur with a sudden influx of water from rain or melting snow. Water levels need to be monitored if the river can overflow its banks and cause floods.
- Overflowing reservoirs can result in flooding. Reservoirs are large basins intended to hold water for drinking and irrigation. They are also useful to fill when river levels are high to alleviate river flooding by reducing the depth of water downstream. The reservoir can only be filled so much though so when it is past its capacity flooding may occur down river.
- Heavy rain is the one ingredient that causes floods ahead of all other factors. When there have been extensive periods of rain many of the conditions listed above can be created such as saturated ground soil, overflowing reservoirs and high river levels.