Flooding is the most widespread of all natural hazards, often arising from adverse meteorological conditions such as: – intense or prolonged rainfall in river catchments – storm surges at the coast and in estuaries – storm-generated waves at the coast. The main factors which need to be considered regarding flooding include technical, social and governmental aspects. This ROCK aims to provide a concise description of these factors on a worldwide basis but with a more specific European and UK focus.

Published July 2011

Flooding is the most widespread of all natural hazards, often arising from adverse meteorological conditions such as:

• intense or prolonged rainfall in river catchments
• storm surges at the coast and in estuaries
• storm-generated waves at the coast.

Flooding may be triggered by a series of other natural hazards. For example, earthquakes may cause tsunamis. They also cause landslides which may block river valleys and impound water. These landslide deposits later breach resulting in flooding downstream. Another source of flooding is the failure of water management infrastructure such as dams and raised flood protection embankments.

Our perception of floods and how to react to them has changed over time. Originally, floods were regarded as acts of God and society accepted the vagaries of nature. With technological development in the 18th, 19th and 20th centuries the concept developed of man attempting to overcome or control nature, an approach or attitude which continued until very recently. During this period the dominant philosophy was one of taming floods, flood defence and flood prevention. Today, with the emergence of sustainability as a dominant driver of international policy and human activity, there is a move towards social responsibility and the development of general policies for flood risk management.

Flood risk management can be viewed as a continuing cycle of activities with prevention and protection at the fore in normal times. When a flood is imminent or in progress, the attention moves to flood warnings and emergency management responses. After the flood, there is a period of recovery, relief and review to learn lessons before the next flood occurs. Flood risk management recognises that the reduction of flood damage needs active engagement with the public at large so that when a flood comes, individuals and businesses are prepared and can act appropriately. This approach aims to create greater resilience within communities.

Flood risk can be analysed through a systematic consideration of:

• the sources of the floodwater
• the pathways by which the water moves over the land surface
• the exposure of people, property, businesses and the environment to the floodwaters
• the consequences of inundation on everything exposed to the floodwater.

In essence, flood risk analysis provides a static evaluation of the risk, measured through a summation or an integration of flood probability and an evaluation of the consequences. This evaluation of the risk can be for either current hydrometeorological and socio-economic conditions or for some future scenario. Flood risk can be modified by altering the character of the source, pathway, receptor or consequence through a variety of measures.

Flood risk management is achieved through a portfolio of measures including the construction of traditional flood protection schemes, the use of policies to restrict inappropriate development on floodplains, the installation of flood warning systems and the testing of action plans to protect the population.

Internationally, policies and practices in flood risk management are evolving in response to many drivers including:

• consideration of potential climatic changes in terms of precipitation and sea levels
• increasing potential for damage to the growing world infrastructure
• decreasing acceptance of flooding by communities
• competing demands on public expenditure
• ageing of existing flood defence infrastructure.

The main factors which need to be considered regarding flooding include technical, social and governmental aspects. This ROCK aims to provide a concise description of these factors on a worldwide basis but with a more specific European and UK focus.