They almost certainly have this data already. I live in Dresden, a city in East Germany that also has its fair share of floods from the Elbe river [1]. The city has an interactive map where you can display flood maps for various levels. It actually has an English version as well, so you guys can try it if you want: Go to [2], switch to English in the upper right corner if it does not autoselect English for you, enter "flood" in the search box and select one of the overlays like "at 900 cm at Dresden Gauge".

[1] The last "Jahrhunderthochwasser", floods of the century, were in 2002, 2006 and 2013, so yeah, that label does not stand up to scrutiny anymore.

[2] https://stadtplan.dresden.de/

To do what exactly? People aren't going to abandon their homes and build a new house somewhere else just because there is a flood risk. In the UK it's estimated 1 in 6 homes are at risk of flooding.

https://www.safesitefacilities.co.uk/knowledge-base/areas-mo...

It exists, like on the geoportal [0] of the Walloon Region (south of Belgium), but this time it's really unprecedented. I'm working for the Belgian railways and it's apocalyptic.

[0] https://geoportail.wallonie.be/catalogue/9c55d236-8d98-4964-...