Surface water flooding:
A growing risk in Europe

Surface water flooding (otherwise known as pluvial flooding or cloudburst) is caused by extreme rainfall (precipitation) and occurs when the ground or existing drainage systems are unable to absorb the amount of water falling at the time of the event. The water flows over the ground to settle or 'pond' in low-lying terrain, where it temporarily accumulates. Surface water flooding is sporadic, short lived, hard to predict and can happen anywhere, often far away from river floodplains and in unexpected locations.

Surface water flooding in Europe - why should you consider it?

The number of devastating flood events in Europe has more than doubled over the past 30 years, with a proportionally higher increase in the frequency of events caused by surface water flooding (EASAC, 2018). Projections suggest this trend will continue, with effects exacerbated by climate change (Alfieri et al., 2018). The annual reports of the German Insurance Association indicate that 2018 was the third year in a row where flood damages were dominated by extreme precipitation events of very short duration, affecting small areas across the country (GDV, 2017; GDV, 2018). Although there was no particular event that would generally be considered significant by the insurance industry, the accumulated damages per annum are of concern.

What do JBA's Europe surface water flood maps show?

As river and surface water floods have different impacts on risk, our experienced teams separately model these two flood types. Rainfall is applied in a computer model creating high resolution maps that indicate whether each 5m by 5m terrain cell could be affected by surface water flooding due to overland water run-off and ponding. The 5m resolution maps show the annual probability of surface water flooding occurring in each cell, expressed as 6 different return periods (20 years, 50 years, 100 years, 200 years, 500 years and 1,500 years). For example, a return period of 20 years means the annual probability of surface water flooding occurring at a location is 1/20 or 5% per annum. The maximum flood depth for each return period is also specified per cell.

Pictured: JBA 100-year return period flood map for Münster, showing river flood (blue) and surface water flood (purple). The map captures surface water flooding that occurs away from rivers that may otherwise be missed when assessing just river flooding.

How can surface water flood maps complement existing flood data?

Surface water maps can be used in conjunction with existing flood data to provide a fuller picture of the risk, providing additional information on areas at risk of flooding beyond river floodplains. This gives re/insurers more insight into the flood risk, extending beyond the primary concern of river (fluvial) flooding. JBA's Germany Flood Map, for example, is the first to specifically address surface water flood at high resolution, modelling both surface water and river flooding consistently on a country-wide basis across all federal states. This provides an opportunity to include surface water flooding as an additional risk in flood assessment and risk management processes.

Lessons learned from using surface water flood maps in the UK

First released more than a decade ago, our surface water flood maps for the UK (extract pictured) have become standard in flood risk analysis for insurers, as well as property search companies, asset managers and financial institutions in the UK.

For example, JBA's maps have helped UK insurers to account for the potential impacts of surface water flooding in daily underwriting by:

  • Identifying and setting the surface water element in pricing decisions, including automated underwriting processes
  • Understanding the extent to which surface water flooding may have contributed to flood losses separately to river flooding, which has an impact on claims costs
  • Identifying how many properties in an insurance portfolio are potentially exposed to this flood type

At JBA, we believe it's increasingly important for insurers to understand the potential impacts that both river and surface water flooding may have on their risks. As a result, JBA's surface water flood maps for Europe are complemented by river flood maps, incorporating small and major rivers, with a separate dataset showing areas benefiting from river flood defences for a comprehensive source of flood risk information.

To find out more about our Europe flood maps and to request free sample data, get in touch or email our International Business Development Lead, Erik Hoeppner.

5m flood maps for Belgium, France, Germany, Italy, Luxembourg, Liechtenstein, Switzerland and the Czech Republic are now available at 5m resolution. Get in touch for more information.

References

Alfieri, L. Dottori, F.; Betts, R.; Salamon, P.; Feyen, L. 2018. Multi-Model Projections of River Flood Risk in Europe under Global Warming. Climate. 6 (1), 6.

European Academics Science Advisory Council (EASAC), 2018. Extreme weather events in Europe. [online] Germany: Schaefer Druck und Verlag GmbH. Available at: https://easac.eu/fileadmin/PDF_s/reports_statements/Extreme_Weather/EASAC_Extreme_Weather_2018_web_23March.pdf [25 June 2019]

Gesamtverband der Deutschen Versicherungswirtschaft e. V. 2017. Naturgefahrenreport 2017. [online] Available at: https://www.gdv.de/de/zahlen-und-fakten/publikationen/naturgefahrenreport [25 June 2019]

Gesamtverband der Deutschen Versicherungswirtschaft e. V. 2018. Naturgefahrenbilanz 2018. [online] Available at: https://www.gdv.de/de/medien/aktuell/versicherer-leisten-2-7-milliarden-euro-fuer-stuerme-und-starkregen-42732. [25 June 2019]