[KC Burke]

Of course what we are talking about is not mesh in either case but pre-tied and (here) tied-in-place mats of reinforcing bar.

I believe the earlier picture may have been for vertical wall placements or non spillway slabs. Hence the lack of epoxy. It was being pre-tied flat and stacked on cribbage 2 x 4 s for separation at the time of later crane pick.

Epoxy shows up a lot in roads and bridges due to salts used on roads. Here it’s use is just for long life durability.

[EarthResearcher333]

Mesh as in "Pattern" i.e. the preformed wire tied sections in the Upper Main spillway. Simpler for discussion using a common term then to add more words to make a distinction from wire mesh.

There is one consideration for the epoxy coated reinforcement steel bar on top; that is for an extra measure of protection from any small surface cracks that may transmit moisture. The upper surface of the concrete in the lower spillway will see flow velocities of 90+ feet per second (surface cavitation risk if surfaces not kept optimal).

It's also possible a "decision" was made to upgrade to this epoxy coated steel (for the upper layer) after debating the pro's/con's in the early engineering phase. I do believe the extremely corroded rebar observed in the drain cracked slabs may have influenced this decision too (if so, that could have created a huge order change in materials/fabrication turn around time).