Originally Posted by
Winemaker
True, but the spillway section was not earthen dam, it was reinforced concrete. The earthen dam section was the road section leading up to the spillway
AFAIK. Maybe I'm misunderstanding the design; the dam was built in 1956 - are there drawings of the dam structure? Was this a concrete spillway on top of an earthen dam? That is certainly not unheard of, the recent Oroville dam debacle in California is certainly an example of this.
Looking at the photo's of where the first breach was, which housed as multi story RC structure above it next to the breach, I'd say its very unlikely that that was all built above an earthern dam section. You can see the two retaining walls at the ends of the earthern dam section on aerial photo's with the sluice gates construction and control complex above built in between them.