Top 5 Components Of A Good Flood Map

In conversation with Insurance Specialist, Jill Boulton

Flood mapping is a complex process, with lots of different elements to consider in order to create the most predictive map, and not all flood maps are made equal. It requires a combination of hydrology know-how, map-checking and validation skills, a regular update plan, and most importantly a good team to help clients make the most effective use of the data.

Over the last few months, JBA has been sharing the latest update of its UK Flood Map which includes an update of the English and Welsh coasts, as well as updates to our Canada Flood Map.

In celebration of these milestones, we sat down with Insurance Specialist, Jill Boulton, to get her view on what makes a good flood map.

1. Get The Basics Right: Before mapping begins

The importance of a good bare earth map

A good flood map always starts with a good quality bare earth map or DTM (Digital Terrain Model), also referred to as a height or elevation map. When used to create flood maps, the type and quality of elevation data has a greater impact on the output than with other perils like wind or hurricane, for example, due to the localised nature of the hazard. 

There are various types of elevation data to choose from (read more on the subject here) but at JBA we select the best elevation data available by reviewing all the options and choosing the one that is most appropriate for use at the time of development. And when we make our regular updates, we review the suitability and quality of the elevation data in case new data are available.

Check the detail

The next important element is to check and edit the elevation maps, adjust and remove man-made features, and ensure we have the best representation of a truly ‘bare earth’ map.

With the largest group of flood risk specialists under one roof, this attention to detail at the very start of the process is just one of our unique selling points and makes a real difference to the quality of our flood maps

Rain falls across all areas so when we review a height map, we have to scour the entire map to ensure it truly represents bare earth. For example, we look for impediments to water flow such as roads or buildings that might have been missed during the processing of the DTM, which now need to be removed from the map to ensure a truer representation of flow paths. 

We also check that any remains of man-made features are adapted to allow water to flow realistically during our flood simulations. Bridges built over roads and rivers, for example, will be treated as a barrier to waterflow on a height map so they need to be edited to ‘allow’ the water to flow under the bridge as it would in reality.

Finally, we check for ‘over-removal’ where buildings may have been removed but the bare earth map may have been changed as a result. If this is not corrected it might create ponds of water on the flood map that would not actually occur. To support this in-depth review, we’ve embedded state of the art machine learning processes into our flood map development.

Select the right resolution

We select the most appropriate resolution of DTM for our flood map development, which might involve resampling the data in-house.

Our global flood mapping has been available at 30m resolution since 2014 and for the UK, continental Europe, US, Canada and Australia we also have 5m resolution maps.

You can see the difference between 5m and 30m resolution in the diagram below taken from our blog.

(Above) Difference between 5m and 30m resolution on a flood map.

2. Work The Magic: Create the flood map

When we have the elevation maps in order, we can then start pulling all the flood data together.

First up is land-use data. As you can imagine, the behaviour of water as it flows across the ground is different in urban, suburban and rural settings. In order to understand the nature of the land surface we use a variety of different data sources. 

Once we identify the land-use across each catchment we then apply different processes to understand how the rainfall behaves once it hits the ground. For example, we apply the Mannings N parameter which indicates the roughness of the land, because friction has an important influence on flow rates.

We consider hydrological processes too, to understand exactly where the run-off goes. For example, rain falling on concrete or a paved surface will flow rapidly, with almost all the water contributing to surface runoff. Rain falling onto a field of crops will not flow as freely, due to the friction of the ground, and a significant amount of the rainfall will soak into the soil or be lost to evaporation, therefore reducing the amount of water contributing to surface runoff.

Once we have applied our science to the land-use data we add more hydrological input in the form of our rainfall estimates, river flows and sea-level information (in the UK we also include groundwater data). This data allows us to identify the separate potential impacts of surface water (pluvial), river (fluvial), and coastal flooding in the finished flood map and gives an extremely comprehensive view of flood.

Having different flood types in our flood mapping helps users to better understand their potential exposure to different types of flooding, and the potential damage. Coastal flooding for example involves salt water and the saline is corrosive; river water is often dirty and flooding can take a while to recede; surface water flooding might be perhaps cleaner, and potentially won’t last as long but is highly localised. These different aspects have different indications in terms of insurance losses or resilience building.

With all the data collated we set the 2D hydraulic flood model (JFLOW®) running to generate our flood map.

For over a decade now we have had the world’s largest GPU-based 2D hydraulic modelling grid. This enables us to produce maps of the quality required for effective and predictive flood modelling, essential for insurance and financial sectors.

3. Add The Extras: More risk insights

With the flood maps now established we also offer additional layers that can be applied. Most popular are the defence layers which indicate the position and effectiveness of flood defences and can be turned on or off. Those who are more risk averse frequently view our flood maps with defence layers turned off for an understanding of the ‘worst case scenario’.

Additionally, for the UK we have Internal Drainage Board Maps. Internal Drainage Boards (IDBs) are organisations that manage drainage systems on arable and farmland to prevent or reduce flooding. JBA shares maps with details of IDBs that are effective and likely to reduce the risk of flooding.

We offer flood maps for the entire world, which can include data such as storm surge, as with the latest Canada flood map and our recently launched 5m resolution US map. When we model precipitation globally, we have to consider rainfall driven both by tropical cyclones and extra-tropical cyclones, as well as accounting for snowmelt where appropriate, to make sure we’re capturing all main drivers of flooding.

4. Good Customer Service: A key component

Our maps are relatively easy to apply but a solid customer service is key to supporting clients to get the most out of the data that we provide.

Flood is a complex peril. Users cannot just be expected to understand a flood map at first sight. That is why JBA always offers client training and regular ongoing support. Direct contact with a technical person at the end of a telephone (or MS Teams call in recent times) is very helpful when questions arise.

We collaborate with our clients so we can be part of their team, help both parties learn and offer support with any issues or questions that arise. There is inherent uncertainty in modelling and no flood map can be 100% correct but we are always on hand to help with any anomalies and resolve any issues. 

5. Best Available Data

JBA updates its flood maps regularly, steered by client feedback and the need to focus on risk hotspots. The frequency of updates ensures that we are always using the best available data and the best technology for the most informed, effective and predictive flood map. We are a pioneering company, and adopting new ideas and innovations in methodology is a key focus. This has led us to a few mapping firsts:

• From 2008, we were the first to include surface water (pluvial) flooding as standard in our UK flood data
• In 2014, we released the first flood maps on a global scale at 30m resolution, including surface water flooding
• We were also the first-to-market with a consistently modelled national-scale flood map of Canada, which is now in its fifth generation
• More recently, we were first to market with EU and US flood maps at 5m resolution nationwide

And of course, our flood maps underpin our global probabilistic catastrophe modelling and our suite of climate change data too.

We're committed to continuously updating our processes and data to share with our clients. Our 2021 UK and Republic of Ireland flood map updates will be ready later this year.

For more information on any of our global flood data, please get in touch using the form below.