Shrinkage cracks and edge curling occurs in large slab segments supported by subgrade to one degree or another as a large slab cures and hardens. Cure cover retards the curing and helps avoid large cracks but does not eliminate the shrinkage forces.
Large amounts of horizontal reinforcement bond with the concrete as it cures and the deformations of the reinforcement surfaces make the concrete with reinforcement work as a composite material with tensions of shrinkage adding to the forces on the slab.
Now, you take that shrinking plate that is curing and shrinking on that sub-grade and you stitch-anchor it down to supposedly help keep it on the hillside and supposedly help it resist disassembly forces should it segment -- this is what was done with the anchoring bars. However, that pins the segments where it is trying to shrink horizontally at right angles to such anchorage.
The problem we are seeing of some shrinkage cracking is a factor of that sort of design attempt. The factor that would limit this occurrence is to further limit the dimensions of the slab segments and increase the frequency of jointing which has its own severe life-cycle problems of cleaning, and caulking which never last.
I would have preferred the thickness of the slab being sufficient to hold it in place as opposed to that anchorage system -- a difficult issue on this degree of slope -- but, I am just an old contractor -- this is a design issue.
Not a civil engineer, but I have to wonder if water in the cracks will shorten the life of the spillway. Freezing, corrosion of any rebar it reaches, etc.
To prevent or reduce this, perhaps some sort of sealant should be applied to the surface after the concrete has completely cured. It would have to be completely dry when applied, so I doubt it could be done until after the rainy season.
"Adjusting the concrete mixture could reduce the number of cracks, Petersen said, but experts need to be careful not to reduce the concrete's strength."
Apparently, FERC is the one that discovered these cracks and then asked DWR to explain it. This begs the question of "why didn't DWR find it first?".
The concrete in question is the special mix "erosion resistant" concrete. By Peterson discussing an option of "adjusting the concrete mixture" reveals that there is some "tuning" that may have been in the design bedrock anchoring/settling assumptions.
OR - there were "unknown" "unknowns" regarding the geologic bedrock settling. Either way, it is truly a PR mess. The mayor is angry. The citizens are now alarmed. And it's possible that DWR may have "fumbled" this from a PR/communications standpoint.
Best to be open and up front early on, just as how the Corps handled the collapsed gate failure at Folsum Dam.