A flood is a high flow of water which overtops the bank of the river. The primary cause of floods is mainly the result of external climatic forces whereas the secondary flood intensifying conditions tend to be drainage basin specific. Most floods in Britain are associated with deep depressions in autumn and winter, which are both long in duration and wide in aerial coverage. By contrast in India, up to 70% of the annual rainfall occurs in one hundred days in the summer south west monsoon. Elsewhere melting snow is responsible for widespread flooding.
Flood intensifying conditions include vegetation, soil type, rock type etc. the potential for damage by flood waters increases exponentially with velocity and speeds above 3m per second and can undermine the foundations of buildings. The physical stress is increased even more when rough, rapidly flowing water contains debris such as rocks and vegetation. Urbanisation can also increase the magnitude and frequency of floods in many ways, with the three most important being the creation of highly impermeable surfaces such as roads and roofs, drainage systems increasing the speed of run off and natural river channels often being constricted by bridge supports or riverside facilities, reducing their carrying capacity.
Deforestation increases flood run off and, because of increased deposition within the channel, decreases channel capacity. However there is little evidence to support any direct relationship between deforestation in the Himalayas and increased deposition of silt in parts of the lower Ganges-Brahmaputra. This is believed to be due to the combination of high monsoon rains in the Himalayas, steep slopes and the seismically unstable terrain which ensure that run off is rapid and sedimentation is high irrespective of the vegetation cover.
The causes for flooding are not solely natural, but the devastation is mostly as a result of human impact. Economic growth and population movements throughout the 20th century have caused many flood plains to be built on, which provided natural protection, and the impacts of urbanisation and deforestation have caused flooding to be more severe. The impact can be seen on the next page in the hydrograph which compares an urban area and a rural area. For the urban storm it can be seen the lag time is very small and there is a high peak discharge which is rapid. This is likely to increase the capacity that the river can hold, thus flooding occurs. The rural storm has a longer lag time and a lower peak discharge (of which the severity of discharge is low), thus flooding is less likely to occur and be less devastating.
Rapid snowmelt combined with rainfall can cause widespread flooding, especially in the interiors of North America and the Commonwealth of Independent States. These usually occur in late spring or early summer. In glaciated areas the flood hazard results from melting ice or the collapse of a dammed glacial melt water lake. These glacial outbursts occurred in the Alps before management was undertaken to reduce them and are a hazard today in the Andes. Melt water pockets may collect may collect over time and then suddenly burst causing rapid discharge forming a sudden flood wave. Outbursts can also occur as a result of volcanic activity causing ice melt. In Iceland these are called jökulhlaups. This can cause roads and bridges to be swept away and cause considerable turbulence in the sea if the flood waters spill over the coast and hit the sea bed. Flooding can also result form landslides and dam failures
Coastal floods can be caused by tropical cyclones (these cause high winds and can bring rainfall events of 100mm a day) and tsunamis. Storms in mid latitudes can also result in serious coastal flooding. The degree of flooding will depend upon the severity of the storm and the storm surge effect (these cause 90% of deaths), plus the level of the tide at the time of the event the risk of coastal flooding is increasing in many locations as a result of long-term regional-scale sinking of the land mass and eustatic sea- level rise (this is a very important cause of flooding if predicted sea level rises resulting from global warming do occur, and this effect can be seen in the UK. Southern Britain is subsiding at 1-3mm per year, and sea level is rising. This means that coastal flooding is the most severe natural hazard threat to the UK, and the devastation was seen in the sever floods of 1953 in eastern England, where a combination of factors created sever floods in which over 30 people were killed , over 800km² was flooded and the cost of damage exceeded £50 million.
The threat of coastal flooding is also present in Bangladesh which is mainly low lying flat land, with 80% of the countries land mass forming the Bengal delta. The funnel shaped Bay of Bengal means that tropical cyclones are funnelled up the bay bring storm surges, which in conjunction with rising sea levels, the addition of melt water from the Himalayas and monsoon rains causes regular devastation as seen in the picture above, where agricultural field and vegetation have been flooded and washed away.
Tsunamis are giant sea waves caused by large scale and sudden disturbances of the sea water. Most tsunamis are secondary hazards resulting from earthquakes, and nearly all tsunamis result from large and rapid vertical movements of the sea floor covering an area of many hundreds of square kilometres. Tsunamis are mostly generated at subduction-convergent plate boundaries, around the Pacific Ocean. 90% of damaging tsunamis occur in the pacific basin, and within the basin 33% of the tsunamis are generated in the deep sea trenches bordering Japan, the Aleutians and South America. The most active source area is the Japan Taiwan island arc (over 25% of the tsunamis occur here). The diagram below shows the formation of a tsunami, and how the surrounding environment will decide the severity of the impact. While tsunamis are generated by large scale disturbance of the ocean, not all earthquakes will result in tsunamis. Most result from earthquakes of magnitude 6.5 or more, with a focus depth of less than 50km. the larger the earthquake and the shallower the focus, the larger the tsunami. This relationship has been used to classify tsunamis by intensity and frequency. Tsunamis generated near to the land are potentially much more destructive, since there is little warning. Waves can travel at 80-950 km/hr in deep ocean water. This means that they can reach the coast in a matter of minutes.
Locally generated tsunamis are responsible for 99% of the deaths resulting from this hazard. Indeed, most deaths occur within 100km of the point of wave generation, which is why the Japanese islands are particularly vulnerable to these locally generated tsunamis. The largest tsunami ever recorded was locally generated from a rock slide in July 1958 in Alaska’s Lituya Bay. The confined shape of the fiord funnelled the water into a surge which reached an incredible maximum 540m in height. A boat and its two occupants were swept above the treetops and out to sea about 30m high over the spit of the bay.
Therefore the causes of hydrological hazards are mostly formed naturally, but human influences have also contributed to the hazards. For example location on floodplains for agriculture and location of towns and urbanisation has contributed to an increase in surface run-off which in turn has increased the severity of floods. The burning of fossil fuels has increased global warming, which will in turn cause sea levels to rise from Antarctica melting and the steric effect. The causes of these hazards are distinguished and early warning systems, forecasts has meant that the hazards can be predicted to occur and necessary action such as evacuation taken. The tendency for hazards such as tsunamis and monsoons to occur almost cyclically means that the people in the affected area are prepared.
The building of defences has also meant that every time a hazard occurs people are more prepared that the time before, and even LEDCs have salvaged means of defences, such as people in who live in the Brahmaputra delta, live in tall houses on stilts ready for flooding, and the main cities have been protected by sea walls to limit salinisation of crops and water supplies.
In some cases it is these defences that have helped cause hazards such as the flooding of Mississippi in 1993, where flood defences where obstructing the natural path of flow and thus preventing the quickest method of taking the excess water out to sea.
The natural causes will continue to produce hydrological hazards, but the human influence will determine the severity of the hazard, either reducing the risk of the hazard or increasing it.
