Detailed flood data that relates to the real world is crucial for engineers when managing and designing assets and infrastructure that are resilient to flooding, now and in the future. Gavin Lewis, engineering lead at water risk intelligence firm Fathom, explains.
Flood hazard maps are a vital tool for engineers, planners, community officials and other risk professionals. However, depth data alone only tells us how high the water level is above the ground – an abstract measurement that does not tell us how the flood level relates to the real world.
Our clients in the engineering sector need to make more detailed engineering judgments around flood risk in a specific location or asset, which is why Fathom recently released its Water Surface Elevation (WSE) dataset.
WSE gives engineers an understanding of how flood levels relate to known asset elevations, and how this could change in the future against different climate change scenarios.
It helps engineers determine the flood resilience of existing or new assets and infrastructure, aids with the communication of flood heights across a flood plain, implement water level control programmes and determine what level of protection needs to be applied to achieve the required degree of resilience.
What is the difference between depth and water surface elevation?
Flood depth is the distance between the water’s surface and the ground. Water surface elevation (WSE) is the height of water above a fixed vertical datum (a specific vertical point on the Earth to which heights are referenced).
The diagram below shows how to calculate water surface elevation (C) by adding the Fathom flood inundation for each return period (A) to the base DEM (B) provided by Fathom
Why did we create the Water Surface Elevation dataset?
Fathom’s flood maps are based on FABDEM+ 2D terrain data, which combines thousands of data sources into one product to provide a comprehensive model of the Earth’s surface, with buildings and forests removed.
These flood maps are generated by taking this 2D terrain, adding river channels and then running through a hydrodynamic solver to release inundation depth maps for inland (pluvial and fluvial) and coastal flooding. Now we’re also releasing the exact version of base DEM used to generate the flood maps as a standalone product. By simply adding the base DEM to the flood depth, the WSE map can be resolved.

Water Surface Elevation in action: Early stage design
A typical use case for water surface elevation is in the early design stage. Let’s say an engineering company is designing a new linear asset such as a road or railtrack that needs to be resilient to the 100yr + climate change flood.
Using this dataset, the areas at risk from this flood can be quickly and consistently identified. Where areas at risk of flooding cannot be avoided, Fathom's WSE allows a design constraint to be put in place, to ensure the asset is designed above the level at risk of flooding. Placing these constraints early in the design helps reduce costs later on through complex and expensive redevelopment.
Water Surface Elevation in action: Flood and climate risk analysis
Existing linear infrastructure and interconnected assets can be rapidly screened for flood risk under both current and future flood conditions to identify problem areas. This involves applying the Fathom flood depths and the base DEM to determine the WSE across large areas or complete networks.
SWOT’s next: Unprecedented accuracy in water surface data
By providing WSE data – or the means to calculate it more easily – engineers can anchor flood depth information in the real world for a much more detailed view of flood risk.
And as for the next thing? The accuracy and real-world relevance of flood modeling and mapping is constantly being improved as scientists (including those who work at Fathom), continue to develop and update data, technology and methodologies.
There are also exciting advancements in satellite technology, such as SWOT (Surface Water and Ocean Topography), a new satellite that will survey the Earth’s oceans and inland water bodies at an unprecedented level of detail. Monitoring and measuring water elevation levels will reveal the previously unknown river channel bathymetry and lead to greater insights into coastal processes and the dynamics of floodplains and wetlands than we have ever had before.
Learn more about Water Surface Elevation data
The base DEM for Water Surface Elevation is available free of charge and can be provided together with Fathom flood maps and can be accessed via Fathom’s API, online or through the Jacobs Flood Modeller platform. If you’d like to know more, get in touch with Fathom’s engineering team at info@fathom.global

Author: Gavin Lewis, Head of Engineering at Fathom