Flood risk definitions

Hazard corresponds to the likelihood or probability of occurrence of a flood event of certain magnitude and characteristics (e.g. depth and velocity of water, duration of the flood and its load).

Exposure is defined as the people, property, systems, or other elements present in hazard zones that are thereby subject to potential damage.

Vulnerability corresponds to the characteristics and circumstances of a community, system or asset that make it susceptible to the damaging effects of flood hazard; it can be considered as a combination of susceptibility and value.

Urban pluvial flooding and climate change: London (UK), Rafina (Greece) and Coimbra (Portugal)

Susana Ochoa-Rodríguez, PhD student, Dr Karl Mapleston Smith, project manager; Maria Aivazoglou, PhD student; Rui Pina: PhD student; and Dr Ana Mijic, Grantham affiliated Lecturer in Urban Water Management all in the Civil and Environmental Engineering Department.

Urban pluvial (surface water) flooding – flooding in urban areas caused by intense and/or prolonged rainfall which overwhelms the capacity of the drainage system – is one of the principal hazards in modern towns and cities. This type of flooding often leads to major economic losses and devastating social and environmental impacts. 

Unlike other types of flooding, pluvial flooding is a direct, quick and localised consequence of rainfall. It often happens with little warning and in areas not obviously prone to flooding, making it hard to manage and predict. Whilst intense and/or prolonged rainfall events can also take place in rural areas, pluvial flooding is a predominantly urban phenomenon, as it is in urban areas that its effects are most pronounced and damaging.

The likelihood (i.e. hazard) of pluvial flooding occurring is higher in urban areas due to the high proportion of tarmacked and paved surfaces, which limit water infiltration and increase the amount of water running off the ground surface as well as it’s speed. This is exacerbated by the fact that natural drainage routes are often altered in cities, which results in reduced capacity for excess water. Further, the growing urban population and degree of urbanisation puts great pressure on the existing drainage systems, increasing the likelihood of them being overwhelmed. Moreover, the potential impacts of flooding are significantly greater in urban areas: a consequence of cities’ high density of population, properties and vital infrastructure.

Urban pluvial flood risk is expected to increase significantly in the future as a result of climate changes and demographic shifts: the former is likely to increase the magnitude and frequency of extreme storm events, the driving force of pluvial flooding, while the latter will increase exposure and hence, risk.

In order to develop new tools and strategies for better managing urban pluvial flood risk, researchers from Imperial College London have adopted, as case studies, three European cities at high risk of this type of flooding. These are: the Greater London Area in South East England; the Rafina catchment, a typical Mediterranean suburban area located in the Eastern Attica region in Greece; and the historic city of Coimbra, located in the centre of Portugal. In terms of flooding mechanisms and potential impacts, these three cities share the general features of urban pluvial flooding described above. However, in each of these areas there are specific factors which exacerbate or reduce flood risk in one way or another. 

Case studies

Rafina, Greece

Rafina, Greece

In addition to pluvial flooding, the town of Rafina in Greece also has problems with river flooding. As in London, river flooding exacerbates pluvial flooding because the capacity of storm water drainage systems is limited by the high water levels in the receiving rivers. In addition to this, the likelihood of urban pluvial flooding in Rafina is further increased by inadequate flood protection works: they are often either entirely absent or poorly maintained. Lastly, the current socio-economic situation in Greece has had the effect of increasing the country’s vulnerability to flooding.

Maria Aivazoglou explains her research on urban flooding in Rafina



Greater London

Greater London

In the Greater London area, in addition to the high proportion of paved areas, infiltration of water into the ground is further hindered by the presence of impermeable London clay, which leads to an increased amount of water running off the surface. Moreover, many areas of London are also susceptible to river (fluvial) flooding; in fact, the two types of flooding often occur simultaneously with fluvial flooding significantly exacerbating pluvial flooding.

Out of the three case studies, Greater London has the highest concentration of people, properties and vital infrastructure, which means that the impacts of flooding could be greater there than at the other two locations. Furthermore, London’s population is expected to increase significantly in the next few years, further increasing exposure, as well as increasing pressure on the already stretched urban drainage systems. Fortunately, major efforts have been made to improve the modelling, forecasting and management of pluvial flooding following the widespread UK floods in 2007. While many challenges still remain, significant progress has been made. In fact, Greater London and the UK in general have the most advanced urban pluvial flood modelling, forecasting and management tools and strategies of the three case studies.

Coimbra, Portugal

In the city of Coimbra, steep hills play a major role in exacerbating urban pluvial flood risk. The steep slopes make it difficult for rainfall runoff to enter the sewer system.  Hence, runoff remains on the surface, where it flows at high speed through streets and properties, until finally reaching natural or manmade sinks or ponding areas. This is particularly critical in the historic and commercial centre of the city, which is at the bottom of a hill. Major efforts are being made to improve the capacity of the inlet structures of the sewer system.

Potential impact of climate change on flood risk in the case studies

In Europe, global warming is expected to increase the intensity and frequency of extreme rainfall events, which are the driving force of urban pluvial flooding. In addition, pluvial flood hazard is likely to increase even further due to predicted warmer and wetter winters, with increasingly more frequent rain and less frequent snow. This, together with growing urbanisation and urban population, is likely to result in significantly increased urban pluvial flood risk.

It is hard to say which of the three case studies will present the greatest risk of urban pluvial flooding in the future. What is clear, however, is that there is already a high risk of flooding and moreover, this risk is likely to further increase as a result of climate and demographic change. It is esse ntial to expand efforts aimed at tackling each component of urban pluvial flood risk: namely, hazard, exposure, and vulnerability. Because of the similarities between the case studies, there is great scope for pooling the knowledge and experience acquired from each site to develop more sophisticated, as well as generic, approaches for better managing the risk of urban pluvial floo ding across diverse urban environments. Collaboration is crucial to the goal of mitigating and adapting to pluvial flood risk in towns and cities around the world.  

Pluvial (surface water) flooding in Coimbra, May 2006
Further information about the work being done at each case study can be found in the following project websites:

EU RainGain Project: http://www. raingain.eu/
EU FLIRE Project: http://www.flire.eu/