If I understand your general use case correctly, you should be able to do this in FME. Sounds like you want to use a DEM to find out where flooded areas are likely to occur based on elevation above nearby water. Typically I've seen 2 common use cases: flooding above a static water body such as a lake or ocean, and river flooding. With static water bodies you just need to push down the DEM by the flood amount and then calculate which areas are under water. For example, you can use RasterSingularCellValueCalculator, subtract flood level, in the case of sea level all areas under 0 elevation are potentially flooded. You might want to buffer the water bodies to limit how far inland flooding can occur.

For rivers its significantly more complicated but doable. I have a workflow that drapes the river onto the DEM to make it follow the slope in 3D. Then I buffer it, turn it into a DEM surface, and subtract it from the DTM (multiply by -1 and then RasterMosaicker - sum). Any areas below 0 are flooded. For an example where this was used see the Winnipeg example from last year's FME UC: 'Flood and Landslide Impact Components for the OGC 2021 Disaster Pilot using FME" https://engage.safe.com/presentations/flood-landslide-impact-components-ogc-2021-disaster-pilot-using-fme/

Once you have the flooded area grid, you can then classify it with a RasterCellValueReplacer and then convert it to polygons using RasterToPolygonCoercer. After that you should be able to style it and write it out to Cesium.

If this looks interesting we can probably get the workspaces and input data to you to try it out.

Just chiming in to let you know we have a Building Climate Resilience webinar scheduled for July 6th. I think Dean will be covering some flood-related material, perhaps this would interest you! Best, Kailin.