Sand is an excellent foundation. All new houses are built on a sand base. For what its worth, I did the geometry for that new align. The elevation is based on hydraulic data and calculated to not overtop for a 50 year flood. Thank them that your road will likely never be under water.
Those curves wouldn't be underwater if Noah built a fukcing ark. I believe we had a 100 year flood in 07 and the old road was only underwater in between the curves and just east of the curve by the animal shelter. Even then, it was gone is a few hrs.
Don't get me wrong though. The road is much nicer with a large shoulder and more gradual curves but the elevation is major overkill.
Geometry? Does that mean you laid out the side roads to get on and off of 37?