Flooding: The basics!

Causes of Flooding
Most floods occur as a result of moderate to large scale rainfall events. They are unpredictable events.
Possible causes of floods:
Rainfall: either single intense events, or prolonged less-intense events
Snow and icemelt - may also link in with volcanic eruptions melting ice volumes. (Mt St Helens)
Ice jam floods: on certain rivers which freeze over during the winter, and then thaw in the nival flood
Landslides: e.g Vaiont Dam disaster
Estuaries / tidal conditions e.g 1953 East Coast Storm surge flooding (see elsewhere on the site.)
Effects of Flooding
Flooding causes both Direct and Indirect losses, which can be classed as both Primary (immediate) and Secondary (longer term...)
The Direct losses occur immediately after the event as a result of water making physical contact with people and their property.
Effects include:
Loss of life
Disease caused by contamination of water supplies
Famine caused by damage to crops
Stress: physical and mental health problems
Damage to infrastructure: roads and communications
Rebuilding costs
Loss in the value of properties, and increase in insurance premiums
Indirect effects include:
Disruption to traffic which costs business money, and disrupts transport
Effect of reduced spending power in the local area as people lose money, jobs etc.
Loss in tourist spending in the area (although Prague has actually increased its level of tourism following floods of 2002)
Survivors feel vulnerable, and also have difficulty getting insurance for their properties (postcode 'blacklists' being drawn up)
People leaving the area
Less investment in the area

Floods can also occur because of:
Inundation of the floodplain
Water table rising to the surface
Rainwater flooding in dry conditions e.g rainfall causes floods in the deserts, creating features called wadis. People drown in the desert every year!
Sheetwash flooding in tropical areas
Urban flooding affecting stormwater drains

What factors make floods worse?
Drainage basin factors: size, shape.
Slopes: gradient of slopes in drainage basin, relief.
Climate
Soil type
Underlying geology
Vegetation cover
Land use: management issues: deforestation or reafforestation.
Human influences on storage capacity