Risks from flooding owe more to physical factors than human factors

Flooding is a natural occurrence which takes place when a river reaches its capacity and overflows its banks, or in coastal regions, when the tidal energy is sufficient to result in the water exceeding the elevation of the land, or coastal defences. It presents a major risk to humans, as it can damage property, disrupt electrical systems and other infrastructure, and result in disease or a lack of clean water. These risks owe more to physical factors as they ultimately result in an increase of water into the system thus triggering a flood event to occur. These factors vary depending on if the flooding is inland or coastal, but they include heavy rainfall, storm surges, and snow melt. However, humans can exacerbate the hazard in a number of ways, such as through increasing the amount of impermeable surfaces, building in floodplains and a high population density in flood risk areas. One of the primary causes of river flooding is prolonged and heavy rainfall. This adds water to the catchment and therefore increases the run-off into the river, as the ground become saturated and unable to absorb more water, resulting in overland flow.

The heavier the rainfall, the shorter the lag-time, meaning that the river can quickly reach its peak capacity. The consequence of this is the overflow of river banks. An example of where rainfall was a significant factor in contributing to flooding was in the 2007 flood of the River Severn, Gloucester. Here, 135mm of precipitation fell in 16 hours. Similarly, the Boscastle flood of 2004 was largely due to the presence of heavy rainfall, as a result of the convergence of two air masses over warm land forming cumulonimbus clouds, which are associated with extreme weather. The physical factor of heavy rainfall is important as it becomes a source of extra water, and it can be argued that without this factor a flood is unlikely to take place. Other physical factors can also add water to the system. One example of this which is particularly prominent in cold environments is snow melt. This means that a large amount of additional water flows to the river in a short period of time. The Yangtze River frequently experiences flooding, and one of the main causes of this is rapid snowmelt, and Bangladesh suffers from snow melt in the Himalayas.

It can also be argued however, that this problem is being intensified by global warming, which increases the period when snow melt occurs. When looking at coastal flooding, the ultimate physical cause is waves which exceed the normal tidal range. This can be caused by storm surges and strong winds. Major geological events such as earthquakes and volcanic eruptions also trigger flooding as a result of vast displacements of water. This can cause tsunami’s which cause widespread flooding as the huge amount of water is unable to recede back to the sea. For example, the Asian boxing day tsunami of 2004 rendered whole islands uninhabitable due to them being overrun by floodwaters, when the sea floor moved up by 3m in an earthquake of magnitude 9. Although a rare event, this illustrates how catastrophic flooding can be entirely as a result of physical factors. Essential to allowing a flood to become a risk is the placement of human settlements. In the case of coastal flooding, the fact that coastal regions are favoured for development due to aesthetic appeal and tourism opportunities, means that people are in a vulnerable area to flooding.

An example of where this was a problem was during the 2007 Cyclone Sidr, in Bangladesh where a storm surge resulted in the inundation of the southwest coast. The impact of this event was substantial, with 1.5 million houses damaged and a total economic cost of US$450 million. It can be argued that the flood would not have presented such as risk if the area was not so densely populated. By way of example, 40 million out of Bangladesh’s total population of 154 live in coastal areas, meaning that population density is considerably high. The placement of settlements is also an essential human factor in river flooding. For example, developing on a floodplain means that more people are put at risk. Developing in a river valley also presents a risk, as the steep gradient of the slopes around the settlement mean that water reaches the river quickly. This was the case in the Boscastle flood of 2004, where the shape of the valley meant that water was concentrated into the relatively small settlement.

With human development also comes the factor of impermeable surfaces, which can exacerbate flooding but is unable to cause the flood directly. It means that water is unable to percolate the soil and therefore runs off the land towards the river. In conclusion, physical factors are undoubtedly the ultimate cause of flooding, both coastal and inland. Despite this, it cannot be denied that humans often result in the flood becoming a risk. If humans did not develop in flood prone areas, the overflowing of river banks would not present a hazard to anyone. As is illustrated by Boscastle, both human and physical factors converge to cause a problem. Here, deforestation, impermeable surfaces and building in the river valley were coupled with heavy rainfall and steep valley sides to create an issue. Therefore, whereas the risks do, on balance, owe more to physical factors, humans play a major role in the damage a flood can do.