W-7

Floods occur when a river bursts its banks if it is carrying so much water that it cannot be confined to its usual course. Flooding is not a normal condition for the river, but is seen as an extreme situation due to high levels of flow. The extent to which the river exceeds the flow that can be contained in its banks determines the severity of the flood and is sometimes related to how often flooding occurs.
 * __Why do rivers flood?__**

Usually bigger floods occur less often and less severe flood events occur more frequently. Floods are common events. Problems and issues arise when people are affected. Building on floodplains results in property being damaged and lives being lost in what becomes a hazard. media type="youtube" key="k88kppkphfY" height="315" width="560"
 * __Causes of floods__**

There are a variety of causes. They can be split into physical and human factors: Physical factors
 * Heavy rain
 * Rapidly melting snowfall

Human factors
 * Building, particularly on floodplains, increases the area of impermeable surfaces – reducing infiltration and increasing surface run-off
 * Destruction of natural environments, e.g. by deforestation, increases the rate and amount of run-off

Location of recent flood events: Use Exam paper resources


 * Key terms**


 * Floods**: these occur when a river carries so much water that it cannot be contained by its banks and so it overflows onto surrounding land – its floodplain
 * Hazard**: an event that occurs where people’s lives and property are threatened and deaths and/or damage result
 * Soil erosion:** the removal of the layer of soil above the rock where plants grow