Representing uncertainty in flood maps

Flood maps are a vital tool in managing exposure to flood risk and JBA has been developing market-leading global flood maps for over a decade. Using the latest available data, bespoke methods, and validation against real-world flood events, the flood maps provide an indicative view of which areas and properties are susceptible to flooding, and are updated regularly for a current view of the hazard.

As with any scientific modelling, however, there is inherent uncertainty within flood mapping. This uncertainty arises from many factors, including limitations in data sources, difficulties in capturing complicated real-life scenarios, and an ever-changing climate.

This was shown in the July 2021 floods in London, where some properties experienced flooding which was not captured in flood mapping, resulting in a number of claims.

As a result, it’s important to consider the areas that fall outside of the flood maps – which may still have some risk of flooding – during risk management decisions, and understand the tools available to help account for this uncertainty.

Uncertainty in flood mapping

Flooding in a real-life scenario often has many complex and unforeseen details that cannot be captured in flood maps.

For example, garden walls, steps, burst water mains or blocked drains can all play an unexpected role in the way flood water flows across the ground and interacts with properties, and may cause flooding that wasn’t predicted.

Every flood event is different, with many factors influencing flooding in real-time that are difficult to capture in a simulated hydraulic model.

Similarly, the creation of flood maps relies on external inputs for the hydraulic models, including elevation data (representing the terrain) and hydrology data (information on river flows, rainfall data and other environmental considerations for estimating water amounts).

There is some discrepancy in the quality of data available to use for mapping purposes. In the UK and Republic of Ireland, for example, this data is usually of high quality, although different technologies and methodologies used by third parties to capture the data can still result in different levels of reliability.

As a result, there are some limitations to flood map outputs, which are sometimes referred to as uncertainties.

Unflood maps

In the UK and Ireland, JBA has created a tool to help organisations understand and account for this uncertainty.

JBA’s Unflood maps are created from our high-resolution 5m UK and Ireland flood maps and represent the areas not captured within the river, coastal and surface water flood maps. (JBA does model other flood types such as groundwater, but these are not considered in this map).

The Unflood maps indicate a property’s height above, and distance from, the nearest area of flooding.

As illustrated in the figure (below), the house on the left is much higher above our modelled flood water level compared to the house on the right, and therefore would be less likely to flood, despite being a similar distance away from the flood water.

A fuller view of the risk

The Unflood maps help to distinguish between all of the areas that fall outside the flood maps and can be used to locate areas and properties that may still be susceptible to flooding under more uncertain scenarios.

The data is banded into three classifications where band 1 (equal to or less than 0.5m) is closest in elevation to the nearest flooded area and may be more likely to carry a risk of flooding. Areas over 10m (band 3) above the nearest area of flooding can be considered as less of a risk to flooding.

The image (above) shows how close the property (orange dot) is to the nearest flooding (white), and also its height above this flooding (pale green representing band 1 through to dark green representing band 3).

With the property being located close to flooding and in band 1, the building could still be at risk.

By using the Unflood maps alongside the UK and Ireland flood maps, organisations can ensure their risk selection and pricing strategies better account for uncertainty.

Start to future proof your business

As our climate changes, it’s possible that some areas may experience more flooding in the future.

The Unflood maps help to show low-lying areas that are already close to modelled flooding and, alongside JBA’s wider Climate Change Analytics, can provide an indicative view of which properties may be at a higher risk of flooding in the future.

If you'd like to learn more about how our global flood mapping, Unflood maps, and climate change data can help your risk management practices, please get in touch with the team using the form below.

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