[abb]

If that wall panel is close to the failure point, the gap could be from the entire spillway shifting downslope slightly, after losing the support of its adjoining panel.

[EarthResearcher333]

"If that wall panel is close to the failure point, the gap could be from the entire spillway shifting downslope slightly.

I agree. I was searching photo databases to see if there is a tell-tale shift of the slab alignment joint next to this one to get another data point.

[EarthResearcher333]

Rotation of Broken section of Main Spillway placing stress on slab anchor bars..

Hi abb, Looks like it is a rotation movement. The 12" dia drain break behind the spillway wall undoubtably contributed to erosion of the wall anchoring. This wall has substantial mass. I have seen images of load transfer bars holes in sidewall to slab joints. These parallel load transfer bars could bind from a rotational component - rather than "slip" in the normal designed lateral expansion. However, I have not seen load transfer bar evidence on this wall side of the spillway. Only beveled mating joints (you can see this type of beveled joint in this image).

In any case, there seems to be a linking force between the mass of the sidewall and the spillway slab as the force to pull/rotate a slab would be substantial considering the slab anchor bars into rock (they drilled NX holes and then grout sealed the thick rebar anchor bars into the rock).