Why floods occur

Flooding of land and buildings causes enormous damage in many countries across the world, incurring vast expense as well as causing loss of life. Floods that occurred in England in 2007 led to 180,000 insurance claims, against which 3 billion pounds sterling was paid out (according to the review by Sir Michael Pitt). The 2010 floods in Pakistan affected 20 million people and there were at least 2,000 deaths. In Australia in 2010-11, according to the BBC, the area affected was as large as that of France and Germany combined.

However, many people are affected by flooding on a much more local scale. For the individual, the number of people affected may be of less importance than the fact that they themselves have fallen victim. A flood can be just as traumatic if it is only one house that is affected, should that house happen to be one’s own.

It is therefore important to know why some areas are more prone to flooding than others. Even if floods cannot be prevented, it is possible to design buildings so that they are flood-resistant and the damage that is caused when flooding happens is easier to put right.

Flooding is, put simply, too much water in the wrong place. It can come from rivers, from the sea, from sudden downpours, from bursting dams, from rising groundwater, or even from pipes bursting within the property.

What interests scientists is the possibility or predicting when and where flooding will occur, and producing figures that demonstrate the level of flood risk at any one place. These figures will help planners to decide the best and worst places to build new houses (for example), and allow insurers to set premium levels that are realistic for the risks involved.

River flooding

Rivers, especially in temperate regions such as Europe, are Nature’s way of conveying excess water to the sea. When more rain falls in inland areas, the rivers get swollen with the excess and may not always be able to contain the extra load. This is a perfectly natural phenomenon and floodplains border many rivers in their middle and lower reaches. Winter flooding helps to fertilise these plains by spreading silt across them, and farmers in many countries down the centuries have come to rely on this flooding for their future crops. 

The floodplains are also able to act as safety valves for the normal load of excess water, partly because, over thousands of years, they have built a layer of gravel which is able to act as a sponge, soaking up much of the excess and releasing it slowly during drier spells.

However, pressure on land for building has led planners to regard floodplains as prime sites for new housing estates adjoining towns that were originally built on the higher ground slightly further away from the rivers. This has had the effect of disrupting the natural drainage pattern in many places, and not only have the new estates been threatened by flooding, so have other riverside communities further downstream because the rivers are forced to carry more water further down their courses, due to the gravel beds no longer being available as reservoirs. 

Sea flooding

Traditional sea defences have protected many coastal communities from flooding, but these are expensive to maintain and breaches are becoming more frequent. In some areas of eastern England, for example, it has been decided to allow sea walls to be breached and for farmland to revert to salt marsh, especially where the land was originally reclaimed from the sea, often centuries ago.

One reason for increased coastal flooding in south-eastern England is “isostatic recovery”. This has been going on for many thousands of years, ever since the ice sheets that once covered northern Britain melted at the end of the last Ice Age. The enormous weight of the ice pushed the land down, possibly by thousands of feet. With the ice no longer there, north-western Britain has been rising, but one consequence of this has been that south-eastern Britain has been sinking. The process is still continuing today.

Climatic changes, for whatever cause, are also leading to higher sea levels and more (and more violent) storms, which put extra pressure on sea defences as well as causing greater erosion of the low soft cliffs and shingle banks in this area. As seawater reaches further inland, the rivers are not able to send as much water to the sea, and flooding of low-lying areas is the result. 

A storm surge occurs when a strong wind blows onshore under cyclonic conditions. Particular problems arise when a surge coincides with a high tide, especially a spring tide at the new or full moon. At least these conditions are easy to forecast and warnings can be given.

Groundwater flooding

Groundwater flooding occurs when water levels in the ground rise above surface elevations. Chalk, which is a permeable rock, forms aquifers as the chalk stores groundwater and allows it to flow. The chalk areas show some of the largest seasonal variations in groundwater level, and are the most extensive sources of groundwater flooding. Within England, chalk downland is found in a swathe running from Eastern Yorkshire and Lincolnshire through Norfolk south to the Chilterns and the downs of Berkshire, Hampshire, Wiltshire, Dorset, Kent and Sussex. 

Land and buildings on lower slopes in these areas are normally safe from flooding when the water table reaches the surface at a point lower down the slope. However, at times of prolonged heavy rainfall the water table will rise and could break the surface much higher up the slope. The water will then run across the surface and through any properties that might be in its way.

Groundwater flooding may take weeks or months to dissipate because it moves much more slowly than surface water in streams.

Surface flooding

This occurs when sudden rainfall cannot disperse quickly enough, possibly because the ground is already saturated. The practice in urban areas of turning lawns into hard standing for cars has made a significant difference in the impact of surface flooding, with the water often entering houses or forming pools. Flash floods can be highly dangerous, for example if they cause vehicles to aquaplane on flooded road surfaces.

Localised flooding

This can happen for a number of reasons, including burst pipes within properties, possibly after they have been frozen due to inadequate insulation, water mains bursting in the street, or drains and culverts becoming blocked.

Flooding can be the unintended consequence of activities such as mining or the digging of deep foundations. Anything that disrupts the natural flow of water could cause it to go where it is not wanted.

Sometimes water can “pond”, which means that an obstruction such as a wall or mudbank could prevent it from flowing away and a temporary pond is created. There is a danger that the barrier could fail suddenly due to the weight of water becoming too great, which would in turn lead to the sudden release of a quantity of water that has to go somewhere.

How many properties are at risk of flooding in the UK?

According to a 2004 report by the Office of Science and Technology, some 2.1 million homes in the UK are in areas at risk from river and sea flooding. 48.5% of these properties are at risk of flooding from the sea, 48% from rivers and 3.5% from both. The Environment Agency states that more than five million people in England Wales live or work in properties that are at risk of sea or river flooding. Apart from those risks, sewer and drainage systems play a significant role in the problem of flooding in the UK. It is estimated (by the National Audit Office) that around 6,000 properties are flooded internally each year by sewage.

Insurers and others look to the scientists for predictions as to whether conditions are set to worsen in future. Clearly, if rainfall increases significantly, or if there are more violent weather events during which large amounts of rain fall over a short period of time, the problem will get worse. The scientific consensus, at least in the United Kingdom, is that climate change will indeed lead to more problems. The solutions, in terms of better flood defences, building design, and decisions on where new building should take place, are in the province of engineers and politicians.

© John Welford