Flood risk - the causes of flooding
Flooding occurs when a river bursts its banks and overflows onto the surrounding land. There are many factors which can cause a flood - often the natural landscape can influence flooding and human interactions can increase the risk.
Causes of flooding
- Prolonged rainfall - if it rains for a long time, the land around a river can become saturatedUnable to contain any more liquid. (it's holding as much water or moisture as can be absorbed). If there is more rainfall it cannot be soaked up, so it runs along the surface this is known as surface run-offThe water that runs over the surface of the land when the soil is unable to absorb it..
- Heavy rainfall - if there is heavy rainfall there is less chance of it being soaked up by the soil (infiltrationWater that soaks into the ground.) so it runs off into the river. The faster the water reaches the river, the more likely it will flood. Water travelling over the surface (surface runoff) will reach a river quicker than water travelling through the ground (through flowWhen rainfall or water flows through the land. and groundwater flow).
- Relief - a steep valley is more likely to flood than a flatter valley because the rainfall will run off into the river more quickly.
- Geology - permeable rockA type of rock which allows water to pass through its joints and cracks, eg limestone. allow water to pass through pores and cracks, whereas impermeable rockRock which does not allow water to pass through it, eg granite. do not. If a valley is made up of impermeable rocks, there is a higher chance of flooding as there is an increase in surface run-off.
- Vegetation - trees and plants absorb water, this is known as interceptionWhen trees and plants stop water from reaching the surface.. Lots of vegetation reduces flood risk. Sometimes people cut down trees (deforestationThe cutting down of trees and forests to allow a different land use.). This will increase the flood risk, as the water will not be intercepted and more will flow into the river.
- Urban land use - when an area surrounding a river is built on, it increases the amount of tarmac and concrete, which are impermeable surfaces. Drains and sewers take water directly and quickly to the river which increases the flood risk.
Effects of flooding
Floods can cause damage to homes and possessions as well as disruption to communications. However, flooding can also have more positive impacts. Flooding deposits fine siltFertile sand-sized particles found in the lower course of a river. Also known as alluvium. (alluvium) onto the floodplain, making it very fertile and excellent for farming.
low income country (LIC)Based on the World Bank's income classifications, a LIC has a gross national income (GNI per capita) of $1,045 or lower. (LICs) are usually affected more than high income country (HIC)A country with a gross national income per capita above US $12,735 (according to the World Bank) such as the Netherlands and the UK. (HICs) by the impacts of flooding. LICs often do not have the resources to prevent flooding or deal with its aftermath. More people rely on agriculture in LICs and fertile floodplains are attractive places to live and so have high population densities.