Hey Harvey, sorry for the delay responding. I've been busy working extra time on the emergency response and now the restoration design to get this working for next rainy season. I'm also teaching a college engineering course and wrapping up side jobs that I took on before this happened. For the record, I never worked a day on this dam until this happened.
I read the PBS story you linked to and its not entirely correct, but mostly correct. There is a lot of crappy rock on that hillside mixed in with the good rock. Many of us predicted severe erosion of the hillside if the emergency spillway were to spill. The main spillway chute was excavated down to mostly good rock, but construction quality could have been better, and the design of the concrete slab could also have been better. Things were done differently back then and more should have been done to bring things up to modern standards. More details will come out in the forensic report.
Its so obvious looking back at a dam failure or bridge collapse, but not so easy to convince people to spend lots of money up front to prevent disasters like this. That's the state of our infrastructure today. We've been living off the investments of previous generations without even keeping up with maintenance let alone rebuilding things. Expect to see more of these unnecessary failures as time goes on. We can pay now, pay later, or just let things crumble. Its all politics.