drumboy said:

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Not a flood expert or hydraulic engineer, but I wonder if they take the momentum of the water into account along with the height of the bank that you're near. The water does flow faster through bayous during floods so I could see it pressing higher on banks on the outside of corners.

During Harvey, there were areas near Buffalo Bayou (from 6 to maybe inside the beltway) that flooded far away from the bayou while houses closer did not due to the speed of the release from the reservoir.

I used to gripe about being in the 100 year since our house didn't come close to flooding since Allison, but we got 18" inside during Harvey.

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Momentum/velocity was only taken into account in the downstream direction with the old maps, but was taken into account in all directions for these new maps. And that was one of the things that delayed the release of these maps by a few years.

The old maps had a cross section across the channel, and the elevation across that entire cross section was constant. If the engineer draws the cross sections in the right places, it actually works really well. The new maps don't rely on an engineer drawing a good cross section anymore and calculate elevations across a grid and the elevation at every grid cell is different. (note that Harris County did the advanced mapping for this project, not FEMA)

That being said, Houston is so flat that momentum around a curve is is responsible for erosion but has virtually zero effect on flood elevations.

That sounds like there has been significant changes to the models and software since I've touched any of it.

I spent several summers interning for a Hydrology/Hydraulics firm back in 99 and early 00's. (before I switchd from Civil to Geology) HEC-RAS & HMS were still just basic windows compatible GUI on the old Fortran code. Have they moved for a full 3D based model with finite element model?