In 2011 FWR published a review (ROCK FR/R0015) entitled ’Floods, alleviation, protection, response and risk management’, that was wide ranging and provided a concise description of the main factors which need to be considered regarding flooding. The purpose of this current review is to focus more closely on flood mitigation, including resistance and resilience, accepting that absolute protection against all future flood events cannot be guaranteed.

Published January 2018

In 2011 FWR produced a review (ROCK, FR/R0015) entitled Floods: alleviation, protection, response and risk management. This ROCK was wide-ranging and covered the issues described above. The purpose of the current ROCK is to focus more closely on flood mitigation, including resistance and resilience, accepting that absolute protection against all future flood events cannot be guaranteed.

Flooding is the most widespread natural hazard, 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.

In many countries, little of the coastline or land area has escaped human influence, with increasing pressures over generations from settlement, agriculture, industry and commerce. The human influences include water and flood management activities which control the extent and frequency of floods and the drainage of water from the land. 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 prevailing 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 alleviation 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 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.

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.