Why do rivers flood?

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.

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.
Physical causes of flooding
Flooding occurs when a river’s dicharge exceeds the capacity of its channel to carry that discharge. the river overflows its banks. Flooding may be caused by a number of natural causes or physical factors:
· Excessive levels of precipitation occurring over a prolonged period of time. This eventually leads to saturation of the soil. When the water table reaches the ground surface, there is increased overland flow or runoff
· Intensive precipitation over a short period of time particularly when the ground surface is baked hard after a long period without rainfall. In such circumstances the infiltration capacity is such that the ground cannot soak up the rainfall quickly enough, so more water reaches the river than would normally be the case
· The melting of snow particularly when the subsoil is still frozen, so that infiltration capacity is reduced
· Climatic hazards such as cyclones in Bangladesh, hurricanes in the Gulf of Mexico or deep low-pressure weather systems in mid-latitudes bring abnormally large amounts of precipitation

The nature of the drainage basin has an influence on the likelihood of flooding. Some drainage basins are more likely to flood than others. Relief, vegetation, soil type and geology all have a part to play. In areas of the world vegetated by dense forest, interception and uptake by plants reduce the risk of flooding during time of heavy rainfall.
Key terms
Hazard: a ntural event that threatens life and property. A disaster is the realisation of the hazard. Flooding is an exmaple of a natural hazard

River management: River basins are subject to strategies desigined to prevent flooding and to ensure that there is an adequate supply of water

The Youtube video shows the flooding at Boscastle, Cornwall, August 2004

The impact of human activities on flooding
More people are living in towns and cities
Population growth and urbanisation has led to demand for land to build on – floodplains are flat and are food for housing
Concrete and tarmac, used for roads and pavements as they are impermeable, preciptation cannot infiltrate so gets into the river much more quickly
Less interception as trees and plant matter is removed so precipitation gets into the river much more quickly.
Often surface water is channelled directly into drains and sewers, so precipitation reaches the river much more quickly.
Bridges over rivers can constrict rivers, slow discharge and reduce the carrying capacity of the river.
In poor countries rapid deforetation has taken place.
Land is now used for framing, settlement and mining etc.
With no trees there is a greater risk of soil erosion as the preciptation is not intercepted.
Flood damage is greatest near the mouth of a river because wide,flat floodplains are most susceptible to damage. The volume of water is greatest here because many tributaires have joined the river.
River Management
The main aim of river management is to reduce the likelihood of flooding. However, in some circumstances it can actually increase the risk:
Bangladesh: flood embankments have built along some river channels. They are designed to increase river capacity but at times have prevented floodwater draining back into the rivers
Farakka Dam, India: Lots of rainfall, meant the lake behind the dam could burst. The floodgates of the dam were opened. This stopped the dam from bursting but it greater increased the discharge of the river in Bangladesh. This coincided with the normal floods and made the severity much worse
Climate change
Global warming has been blamed fr what some claim is an increasing frequency of flooding. There is evidence that average sea temperatures have risen and this rise has been blamed for the increasing frequency and sevrity of tropical revolving sotrms in the Caribbean. Such storm bring heavy rainfall and storm surges along the coastlines of countries lying in their path. In spring 2005, scientist reported that average sea temperatures were 3 degrees Celsuis above normal and predicted that the 2005 hurricane season in the Caribbean and southern states of the USA would be particularly savage. This proved to be the case. notable hurricanes included Katrina, which led to the flooding of New Orleans.

It is predicted that globalk warming will result in reduced rainfall in some areas, but in other, such as western Europem rainfall totals might increase. higher temperatures will result in increased evaporation over the seas and oceans, leading to greater precipitation. Such an increase will inevitably cuase more rivers to flood, particularly since most floodplains have become heavily urbanised over the last two centuries.

Global warming could lead to the melting of the polar ice caps. One major consequence of this would be a rise in sea level, so floodplains lying close to the present sea levels would be at risk from flooding. The major deltas of the world, such as those of the Nile, the Mississippi and the Ganges-Brahmaptura, would be at particular risk.