In message , at
12:14:51 on Tue, 6 Aug 2019, Robin remarked:

Once the slabs lifted there would be massive erosion very quickly.
That's why you have spillways: if the water overtops anywhere else the
earth embankments fail v v quickly.

From the link I posted earlier:

'Chris Binnie, Visiting Professor, University of Exeter, Water Engineer
specialising in dams and water resources development and Fellow of the
Academy of Engineering, said:

"What would appear to have happened is that the dam was originally
built with a small spillway on the hillside beside the dam. The design
standards for spillways then increased, i.e. it became a requirement
for the dam to be able to pass a larger flood flow. To provide that, a
large central spillway was constructed on the dam surface using slabs
laid on the downstream face of the earth embankment. This has the risk
that, if the slabs crack, then high velocity water can get underneath
the slabs and erode the underlying earthfill and the slabs then settle,
and failure of the slabs takes place. This is what appears to have
happened here."'

But let's not forget that without the additional spillway, the dam would
probably have collapsed anyway.
--
Roland Perry