Allow for movement. Consider the depth of the water table and your area's likely (or unlikely) flooding when deciding how high the patios should be.

Ask your architect/designer. Galveston has issues with soil subsidence (as does much of the Gulf coast) and an experienced designer in that area will know best how to deal with the issue. Subsidence is quite different from flooding (although that may be a factor as well) and is unfortunately irreversible. Sinking, cracking and breakage are all a concern regardless of the materials used.

No flooding here.