Since the ES weir was full nearly to the top with water and it’s unclear exactly how that water entered, what’s the point of keeping the weir? Did the water penetrate a seam? Did it enter from beneath the weir?
Why bother buttressing a weir defective in so many ways? It’s not clear how or even if it’s anchored, or what the condition of the rock beneath it is, and it leaks.
Why not build a new weir using RCC or other modern technology, and key & anchor it to competent rock? Get rid of the ambiguities and the defects. Remove the junk weir, inspect the rock, and build a decent weir.
I suspect the leaking seams were from various cracks in these concrete monoliths at the mid to upper height levels of the concrete. They were built with a 4ft shell layer of enriched concrete. Penetrating water in the "build" seam layers are likely where the linear cracks exist that allowed water to seep into the base concrete.
DWR has "blacked out" information on discussion revealing any of the Ogee Weir "flaws", other than a non-blacked out section discussing a way to drill & anchor the Weir to the footing rock (was just a passing "idea" that was abandoned). The discussion then evolved and became the new proposed concept design using massive buttressing with RCC.
At least they can control the integrity of the RCC Buttress construction, thus the envelope of the RCC will secure the "flaws" of shear & anchoring.
Their decisions are "triage" driven. It is the best choice given the time and circumstances.
I'm confident that there is discussion of improving the seal at the front side rock, called a "grout curtain". Their current drilling at the front of the ES will determine what they will need to do to seal off any "piping" channels under the Weir.
I will give the BOC credit. They are driving the issues & specifications to be met correctly.