Does that 100 year rule apply to undeveloped areas? My neighborhood is 20 years old and before it existed, it was wilderness in the middle of nowhere. I wonder how well flood data is tracked for that type of land.
I used to do permitting for construction of aerial and buried fiber networks.
Typically if you're doing some sort of thing where floodplain data is needed, you'll start by finding the dataset from USGS/FEMA/USGIS/etc. Import that in, you'll get a whole bunch of data that depending on package (i.e. if you are doing stuff in AutoCAD instead of working with real GIS software) you might have to shape to get more proper information; floodplain data looks like a bunch of contoured lines (often semiparallel to each other.) based on the survey, and have different embedded metadata accordingly.
So you'll do your drawing based on that, and ideally at some point, a Surveyor goes out and will Verify the map. They'll do this by probably re-pulling the original survey data, going out to the site, and basically making sure you didn't lie in your map.
This last part BTW is an important check&boundary, and IMO the more important one than municipal approval. A good professional surveyor knows that if he does the wrong (i.e. potentially harmful) thing, he could lose his license and livelyhood forever. Professional Seals are something that I have a lot of respect for.
FWIW, this is often the easier part of getting a permit as long as you are building sanely. The city is where politics, money, and process gets broken.
realistically they're measuring "big floods" and "bigger floods" in a way that is consistent, however the stated time frequency with which they supposedly occur is so extremely inaccurate that it does not rise to the level of 'science.'