Many many thanks for your excellent analyses and pictures!!!!
In this picture, look near the two white rolls (blueprints?) I noticed a dark squiggly line going from them towards the camera location, angled to the left foreground of the picture.
A crack?
Definitely. ER333 has established, at least to my satisfaction, that those longitudinal cracks are almost certainly located where the underdrain slabs are located. The vcp pipe has displaced part of the slab, causing it to be thinner and weaker at those locations.
Highlighted that long crack in the url FRpost below (see 2nd image in post) in blue. Also noted were the wide seams from "pull forces" in the wall and the spillway slab (highlighted blue). The wandering crack extends to just short of the wall. The angle is common to the under drain "herringbone" pattern cracking observed up & down the slabs in the mid-to-lower spillway.
Workers have sealed this long slab crack with Elastomeric sealant. The more interesting question is "what does the pull forces" seam widening in the wall and the adjacent spillway slab indicate? In the tilting scaffolding image it shows they have filled the main spillway slab notable "gap" with what looks to be a concrete patching compound. I suspect the scaffolding is intended for repair of the wall gap. The image reveals the width of this gap. This wall and slab section is just 1 up from the last collapsed slab & wall section of the blowout. For this size of a gap to appear -if neither present before the spillway failure- would indicate great forces could have been applied to the slab anchor bars and to the numerous & deep anchor rods of the spillway wall.
Engineers discussing a long crack in the Upper Main Spillway..
Scaffolding in preparing to fix seam gap in sidewall. Note the same width gap filled with grey patching compound in main slab seam in foreground.