Flooding explained

Flood risk is a particular problem for the UK. The two primary causes are climate change, resulting in increased severity and intensity of rainfall, and new developments on floodplains, which are themselves at risk of flooding, and which increase the risk of flooding downstream.

It is estimated that:

  • 1.5% of the country is at risk from direct flooding from the sea
  • About 7% of the country is likely to flood at least once every 100 years from rivers
  • 1.7m homes and 130,000 commercial properties, worth more than £200 billion, are at risk from river or coastal flooding in England
  • Many more properties are also at risk from flash floods.

The effects of flooding, and managing flood risk, cost the UK around £2.2 billion each year: we currently spend around £800 million per annum on flood and coastal defences; and, even with the present flood defences in place, we experience an average of £1,400 million of damages.


The Causes of Flooding|

Climate Change and Increased Flood Risk|

The Effects of Flooding|

The Causes of Flooding

Tidal Flooding

Both sea and river defences may be overtopped or breached by a combination of low pressure weather systems and peak high tides. Storms with high wind speeds cause tall and powerful waves and low pressure fronts cause sea levels to rise above normal levels. High tide levels vary through the lunar and solar cycle and when superimposed upon other tidal variations exceptionally high tides result.

The onset of flooding from the sea and tidal rivers is often sudden and the extreme forces driving it present a significant danger to life. The east coast storm surge of 1953 claimed 307 people's lives in the UK and 1,835 in Holland. A similar storm surge tide in September 2007 came within a few centimetres of breaching a number of the UK's coastal defences.

It is often possible to forecast, with reasonable accuracy, this type of flooding due to the predictability of the tide and trackability of low pressure systems. The duration of this type of flooding is also limited by the cycle of the tides where drainage is available.

Fluvial Flooding

Flooding occurs in the floodplains of rivers when the capacity of water courses is exceeded as a result of rainfall or snow and ice melts within catchment areas further upstream. Blockages of water courses and flood channels or tide locking may also lead to ponding and rising water levels. River defences may then be overtopped due to increased water levels, or breached by large objects of debris carried at high water velocities. Flooding from rivers has in recent years been experienced in the Severn Valley, in Sheffield, in Hull from the river Humber in 2007 and Carlisle on the river Eden in 2006. The onset can be quite slow in some catchments with steadily rising water levels.

However, flash flooding can occur in steep catchments and is far more immediate. Flooding from rivers, particularly in recognized floodplains, can usually be predicted with good accuracy. However flash floods from sudden downpours such as those in Carlisle continue to challenge the capability of detection and forecasting systems. Water over about 250mm in depth may carry debris particularly in urban locations and can also be very cold. Even travelling at low speeds this can make it extremely hazardous to people caught in it.

Ground Water

Low lying areas sitting over aquifers may periodically flood as ground water levels rise. This type of flooding is often seasonal and therefore can be forecasted with good accuracy. It is often slow in its onset.

Pluvial Flooding

Surface water flooding is caused by rainwater run-off from urban and rural land with low absorbency. Increased intensity of development in urban areas has given rise to land with a larger proportion of non-permeable surfaces, a problem often exacerbated by overloaded and out-dated drainage infrastructure. These circumstances, combined with intense rainfall, can give rise to localised flooding.

This sort of flooding often occurs outside of recognised floodplains and because it is caused by quite localised weather conditions it is very difficult to forecast. Its onset can also be very rapid, and the level of flooding very severe. In the summer of 2007 much of the flooding experienced in Gloucestershire and Yorkshire was not directly caused by rivers but by surface water. Large volumes of rainfall early in the year saturated the ground and intense rainfall later caused both urban and rural areas to flood.

Flooding from Sewers

Flooding from sewers can occur where there are combined storm and foul sewers and their capacity is exceeded due to large amounts of surface water run-off in a short time. Poor cleaning and maintenance can lead to blockages that can also cause local flooding. This type of flooding is hard to predict, has significant sanitary consequences for those affected, and can occur very rapidly.

Flooding from Man Made Infrastructure

Canals, reservoirs and other manmade structures can fail causing flooding to areas downstream. Industrial activities, water mains and pumping stations can also give rise to flooding due to failure.

Climate Change and Increased Flood Risk

The potential impacts of climate change during the next 30 to 80 years have been estimated by UKCIP, the United Kingdom Climate Impacts Programme. UKCIP| 02 (2002) provided a range of climate change scenarios recommended for use by Government for the assessment of climate change impacts. In the context of assessing flood risk the key results are:

  • the UK climate will become warmer;
  • winters will become wetter and summers may become drier everywhere;
  • heavy winter rain and snow will become more frequent;
  • relative sea level will continue to rise around most of the UK shoreline; and
  • extreme sea levels will be experienced more frequently.

Climate change is therefore likely to increase flood risk significantly and progressively for all areas of the UK over time. At particularly increased risk will be low-lying coastal areas, as sea levels rise, and areas across the UK not currently prone to fluvial or tidal flooding as more intense rainfall leads to significantly higher risk of flooding from surface run-off and overwhelmed drainage systems.

To find out more about climate change, its effects, and ways of combating it visit our climate change pages, here|.

The Effects of Flooding

The effects of flooding from the sources outlined above are felt by various 'receptors'. These include, people, buildings, infrastructure, agriculture, open recreational space and the natural world.

In extreme cases flooding may cause a loss of life. However, the social and emotional costs from flooding can also be significant and are often widespread and indiscriminate in flooded areas. These costs include: displacement from homes, the loss of personal valuables and the ongoing fear and insecurity caused by the experience. Potable water supplies may be lost or contaminated in a flood and this can have immediate health effects upon people and animals.

The economy can also be severely affected by flooding. Businesses may lose stock, patronage, data and productivity, and disruption to utilities and transport infrastructure can have knock-on effects to a wider area. Tourism, farming and livestock can equally be affected. The Association of British Insurers has estimated the cost of the July 2007 flooding, in insurance claims alone, at over £3billion.

The built environment may be damaged or destroyed as a result of flooding with high repair costs and long periods required for reinstatement. The public realm is often badly affected through damage and the deposit of potentially large quantities of debris. Land contamination may also be transported and spread during flooding.

Vital infrastructure may also be damaged or disrupted. Electricity and gas supplies can be interrupted to individual properties but also to wider communities if sub stations and transformers themselves are flooded. Road links, railways, canals etc. may be blocked causing disruption to the wider transport network, and accessibility severely disrupted for local inhabitants, especially amongst those considered most vulnerable.

A knock-on effect of the loss of electricity experienced in the 2007 summer floods was the loss of communications networks. Telephones, radios, televisions and the internet are all increasingly reliant upon mains power and without a robust means of conveying information to householders, rescue and clean up operations may be hampered.

The Pitt Review: Lessons Learnt from the 2007 Floods describes in detail the specific events that took place in June and July and the impact of the floods on individuals, businesses and the wider community and economy. It contains a number of recommendations for how flooding can be better handled in future, including recommending that design solutions to flood risk are more widely considered and applied across a range of developments.


Factors which Determine the Effects of Flooding:

  • The level of predictability – this affects the timing, accuracy and communication of warnings given before a flood.
  • The rate of onset of the flood – how quickly the water arrives and the speed at which it rises will govern the opportunity for people to prepare and respond effectively for a flood.
  • The speed and depth of the water – this dictates the level of exposure of people and property to a flood. It is difficult to stand or wade through even relatively shallow water that is moving. Flood water often carries debris, including trees, and water over 1m in depth can carry objects the size of cars. Fast flowing water can apply devastating force to property and other receptors.
  • The duration of the flood - this is another important factor in determining the extent of its impact, particularly on individuals and affected communities.


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