Urbanization alters the shape of the storm hydrograph, reducing lag time and increasing peak discharge. This can be seen in the below diagram, it happens because there is less water reaching the channel by through flow and base flow and more by overland flow.
Water companies and local authorities will need to respond to these problems to prevent flooding in the river basin.
Due to such large demands for land in Bangladesh with a population of over 120 million, urbanization is inevitable and impossible to avoid, causing an increased risk of flooding, and as many people are choosing to build in the floodplain itself as there is no where else to live.
FIG 1
An example of where urbanization has affected the risk of flooding is in Canon’s Brooks basin in Harlow, UK. In 1951 the land uses in the river basin were mainly agricultural, grassland, woodland and gardens. Since then, due to urbanization 16.6% of the land has now become impermeable leading to an increase of surface run off by 59.4mm, an increase of over 30%. Although this was seen in Summer, research shows that as the soil is already saturated in Winter the conditions are approximately the same in a 1951 Winter as they are today with urbanization.
As well as urbanization playing a huge role in increasing flood risks another human factor that greatly affects the flood risk in deforestation. The removal of trees reduces interception and transpiration from the tree leaves, resulting in an increase in surface run off and soil erosion. The soil erosion may not sound like a big deal but, if it finds its way into the water course it gets aggraded into the river bed, reducing the channel capacity and increasing the likelihood of flooding.
In Nepal and Tibet an increasing in population has create a dramatic increase in deforestation. The forest used to play a major role in the hydrology of the upland drainage basins absorbing water from the ground, binding the soil particles and reducing the impact of rain droplets on the ground surface. Now without this protection, there is a reduction in interception, increased landslides and overland flow, which results in more flooding downstream in Bangladesh. The silt and soil is deposited in the river channels reducing the capacity of the rivers as it raises the river beds. Soil is said to be lost 400 times faster in deforested areas and is raising the Brahmaputra riverbed by 5cm every year.
In Bangladesh it is also believed that the building of the Farraka Dam in India in 1971 has raised the river bed of the Hooghly River, a tributary of the Ganges. In the dry season the river discharge is slower due to the dam and sedimentation is deposited on the river bed. This therefore increases the level of the riverbed and also increases the flooding risk to Bangladesh.
Global warming is also a human factor that has increased flooding in Bangladesh; higher sea levels mean that surface runoff on the flood plain takes longer to infiltration.
In Mississippi USA, mainly people believe that the flood defences built along the river have increased the risk of dangerous flooding. This is because the large levees surrounding the Mississippi will hold a large amount of water back in small floods but in exceptionally high floods it holds the water back and releases it all at once, when it breaks, or the water goes over top, this results in a massive wave of water sweeping across the flood plain flooding anything in its path. Many people that this sudden release of water will causes more dangerous floods than if their were no levees, as the water could spread across the flood plain and infiltrate into the land, depositing in sedimentation.
It is also said that shortening rivers can increase flood risks as the discharge gets down stream very quickly.
Although it may seem odd farmland is a major culprit of increasing the flood risk in the drainage basin. This is because firstly the land has been removed on trees, (basically deforestation on a very small scale), this decrease interception and increases surface runoff. This surface run off is aided greatly by the ploughed lines left in a field, which are an easy artificially channel for surface run off to use to get quickly to a river. Also due to heavy machinery much of the soil is heavily compacted, this prevents infiltration which leads to a greater amount of surface runoff.
Pollution in the flood basin can also cause an increase of flood risk. Pollution can kill off the trees in the basin via poisoning them with Sulphur Dioxide and other harmful gases; the removal of trees removes the interception which increases the surface runoff
Conclusion
There are many ways in which humans have interfered with the drainage basin causing an increase in flooding, including deforestation, building dams, urbanization, compacting ground, shortening rivers. There are also larger problems such as Global Warming which are affecting low lying flood proned land. There are different ways of dealing with such problems many people in MEDCs believe that we should leave the rivers to run a natural course, and leave the floodplains as we can afford to move to other places. This is not possible in LEDCs such as Bangladesh where there is such a high population that people are forced to live on the flood plain, especially since 90% of the country is floodplain. Unless all the population of Bangladesh is moved, which is totally impossible, and then there is no other option for the Bangladeshi government but to create flood defensives.
