Leveraging geospatial technologies in response to flooding

Tewkesbury Borough has a rich history and the town itself is known for the Battle of Tewkesbury, the Abbey and lots of other beautiful historic buildings – even the oldest pub in Gloucestershire!

When I mention living in Tewkesbury to someone who isn’t local however, they quite often say “that’s the place that floods!”. They’re not wrong – Tewkesbury town is situated right between both the Severn and Avon rivers, and flooding here has been a common occurrence for hundreds of years, with records dating all the way back to the 15th century.

We’re continuously reviewing and looking at how we can improve our approach to emergencies, especially the more frequent occurrence of flooding – no flooding event is ever quite the same, although the affected areas in the borough normally are.

We are a small yet far-reaching borough council, our boundary covers a massive 160 sq. miles, so making sense of localised data is critical in understanding our communities and how we can best support them. We invested in a dedicated GIS role in late 2023, and this has brought us the necessary skills and knowledge to surface insights. In this blog post I’ll focus more on our use of GIS technology and spatial analysis

In our more significant recent flooding event of January 2024, we trialled making use of some of the new geospatial tools powered by the ArcGIS platform. There’s so much data out there – we wanted to try to project which properties are likely to be affected so we have something we could use for flooding events in the future.

Whilst not always entirely accurate, the Environment Agency maintains a number of measuring stations along the Severn and Avon rivers, which provide predictive figures for how high the river levels may rise. These models are based on previous events and recorded levels further upstream – giving us a good indication of what we may see further downstream. The Environment Agency also provides geospatial data showing predicted flooding extents for ‘once in a thousand’ and ‘once in a hundred’ year floods.

But how does that help us? …

Well, we took the satellite imagery taken by the Sentinel-2 satellite orbiting high above Earth, and this makes frequent passes giving imagery every few days. Thankfully, we had a clear day when the satellite made its pass, and this gave us a good idea of the flooding extent – as we could clearly see the areas that had been affected.

The Environment Agency has datasets that contain elevation data of land across the UK. We made the (relatively safe) assumption that water will find level, and then we cross referenced the flooding extent from our satellite imagery with the borough’s elevation data. We then projected this same height data across the borough – and the result was a predictive model of where the flooding should have occurred.

It was surprisingly accurate and with some minor changes, it reflected what we were actually observing across the borough. You can see this below, compared to the EA’s flood zone 2 and 3 boundaries.

The image below shows how closely our predictive model matches with what we observed – this being the St. Marys Lane car park, Tewkesbury (facing North East).

Being a borough authority, we deal with planning applications and are the custodians for local land and property data – meaning we know ‘what, when and where’ for all buildings and land across the borough. With this dataset we were able to take our flooding extent and surface all residential properties that are within the flooding boundary. We were then able to cross-reference this list of properties with the datasets we knew contained vulnerable residents or households, allowing us to get our teams to proactively contact them and make sure they were getting the support they needed, including sandbags and welfare. This has also allowed us to be more strategic and data driven with where we place our community sandbags, making sure they are accessible to the areas that need them the most.

For the future, we are looking to gain reference points by using the measuring stations’ data and relating this with imagery taken across the borough. This will allow us to link, for example, what ‘4.6 meters’ at The Mythe measuring station really looks like across Tewkesbury town.

The data we’re logging will help build both historical references and help us to respond more effectively to flooding in the future.