The impact of human activites on flooding

Flooding is a natural event. However, in recent times its effect have been exacerbated by human activity. Over the last two centuries urbanisation has resulted in an ever-increasing proportion of the worlds population living in towns and cities. This first occured in Europe and developed countries elsewhere, such as the USA. Since the middle of the 20th century, urbanisation has been an important feature in many less developed countries.

Coupled with natural population growth, urbanisation has led to an increasing demand for space to build housing and for other urban land uses. Floodplains were an obvious choice - thier flat land is suitable to build on and good communications are relatively easy to establish. However, floodplains, by thier very nature, are susceptible to flooding.

Human activity on the flat land sourrounding the river has added to the risk of flooding. Concrete and tarmac are used in urban areas for roads and pavements. Such surfaces are impermeable, so percipitation is unable to infiltrate slowly into the soil, as it would in a vegetated area. In addition to this, there is less interception from trees and uptake from plants is reduced. Overall, a higher proportion of the original rainafall makes it way into a river in a town or city.

To add to the flooding risk, surface water is channeled directly into drains and sewers in a urban area, so percipitation reaches the river quickly. This leads to a reduced lag time between peak rainfall and peak discharge.

Nautral river channels may become constricted by bridges, which can slow down discharge and reduce the carrying capacity of the river. In times of spate, debris can be deposited directly behind the supports holding up a bridge and exaggerate the effects of flooding. During the Boscastle floods in August 2004, huge amounts of debris blocked culverts upstream of the town, which had been constructed to allow water to drain quickly through the town.

In some (mainly less developed) countries rapid deforestation has taken place over recent decades. The rainforests of South America, Africa and Asia have been at particular risk as new land has been opened for farming, settlement and other uses. Other countries, for example Nepal in the Himalayas, have also suffered from deforestation - timber is a valuble resource, used for building and firewood.

Once trees have been removed there is a greater risk of soil erosion and sediments finds its way into rivers, obstructing them and addong to the flood risk. Trees intercept water and take it up through thier roots, so in deforested areas more water reaches the channel as runoff.

Flood damage is greater near the mouth of the river because wide, flat floodplains are most subceptible to damage. Here, the volume of water is at its greatest because manu tributaries have joined the river.

Bangledesh lies downstream from Nepal and most of the land is low-lying floodplain that is less than 1m above sea level, forming the delta of the rivers Brahmaputra, Meghna and Ganges. During the spring snowmelt occurs and once the heavy monsoon rains start in early summer there is a natural rise in the volume of water in rivers. In recent years, its has been claimed that flooding in Bangledesh has been more servere, partly as a consquence of deforestation in Nepal.

The main aim of river management is to reduce the likelihood of flooding. However in some circumstances it can actally increase the risk:

• In Bangledesh, embankments have been built along the river channels in some places. These are designed to increase the river capactiy, but at times have prevented flood water draining back into the rivers.
• The Farakka dam lies on the upper reaches of the River Ganges in northern India. In 1988, the Indian government allowed the flood gates of the dam to be opened during the rainy season, because the reservoir behind the dam was at risk of flooding. The saved the land sourrounding the dam but downtream in Bangledesh this was a different matter. The extra discharge in the river coincided with the normal flood excepted at that time of year and greatly increased the severity.

• The Mississippi river in the southern states of the USA is one of the most managed rivers in the world. Artificial embankments (levees) have been built along the lower reaches of the channel to protect the heavily settled floodplain. The city of new Orleans lies below sea level on the banks of the Mississippi and is at particular risk of flooding but is protected by levees and diversion channels, build by the government. In August 2005, devastating floods occured, submerging the city as the levees where breached. A storm serge, brought by Hurricane Katrina gushed up the river from the coast. This, coupled with the heavy rainfall brought by the storm caused the river to rise dramatically. Major damage to the embankments resulted as they were breached in several places.
• Some rivers in urban areas have been channelised. The involves lining the river channel with concrete and straightening it. Channelisation enables water to be directed through the urban area more rapidly. It may protect the immediate sourrounding area, but there is a greater risk of flooding downstream. This is because water is delieverd to downstream areas more rapidly than usual and the unmanaged river channel in these stretches is unable to cope with the rapid increase in discharge.

In recent years, global warming has been blamed for 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 increasing frequency and severity of tropical revolving storms in the Caribbean. Such storms bring heavy rainfall and storm surges along the coastlines of the countries lying in thier path. In spring 2005, scientists reported the average sea temperatures were 3 degress celcius 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 global warming will result in a reduced rainfall in some areas, but in others, such as western Europe, rainfall totals will increase. Higher temperatures will result in increased evaporation over the seas and oceans, leading to greater percipitaion. Such an increase will inevitably cause more rivers to flood, particularly since most floodplains have become heavily urbanised over the last two centuries.

Global warming has also been linked to an increased frequency of El Nino events. El Nino is caused by the reversal in ocean currents in the Pacific Ocean. Normally, a cold current flows up the coast of Peru, encouraging high-pressure weather conditions. In an El Nino year, low-pressure weather dominate, bringing increased rainfall and flooding to the west coast of South America.

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