Identify and explain how human intervention in the drainage basin can increase the risk of flood

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David J Mothersole        Page         04/05/2007

Identify and explain how human intervention in the drainage basin can increase the risk of flood

David Mothersole 12BS


There are many human factors that can increase the risk of flooding. These include the deforestation of trees, urbanization, global warming and dam building. People also believe that building flood defences such as those along the Mississippi, USA, can also increase the risk of flooding, as can human intervention of a rivers course.

Flood risk is increased when there is more surface run off and fast surface water heading towards and into a river channel. Many physical factors can affect flood risk, but more common in the 20th and 21st century is the effect of humans on the land which is increasing the risk of flood. Deforestation by humans leads to less interception and more surface runoff etc.

Human causes of increased flood risk

The first and most obvious cause of a human intervention that has increased flooding in the drainage basin is the urbanization and building of large towns and cities on a river flood plain or in the river drainage basin. The problem is the size of the urban area increases it makes more of the land surface impermeable, and increases the amount of surface run off which goes directly into the river and greatly reduces the lag time of the river. These increases could be caused by the removal of topsoil for construction and compacting the ground with heavy machinery, building roads which increase the impermeable surface area. Also building drains and sewers which create a rapid means of transport for the water to the river channel and the straightening of rivers and lining them with concrete to enable building plans to succeed. The final reason can also lead the faster delivery of water downstream of the urban area increasing the flood risk in those areas.

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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 ...

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