I don’t think that the emergency spillway was really ever meant for anything but an absolute emergency. The erosion, at least downhill from the spillway, was expected.
One thing that this short flow of water did do is provide some pertinent information as to what needed to be reinforced to allow emergency operations in the future. All that hardening will allow it to be used again, at least for a while.
As for the main spillway, I think this might be an inspection/maintenance issue more than a design issue. Or maybe both. It’s apparent that the upper half to 5/8 of the spillway is pretty solid, and appears to be built on solid bedrock. Ditto for the bottom section, which while it doesn’t serve much purpose at this time, does seem to be holding up. It’s that part where it broke - where the hole first appeared - that is questionable.
Seems to me that that section was not built on solid bedrock, and was perhaps not sufficiently built up and hardened to serve the support function. Drainage issues were brought up on another web site, that a couple of the sidewall drain pipes in the vicinity of the original break were not draining water prior to the failure. This would indicate that the area was being undermined by water.
I’m partly speculating here, and partly repeating what I’ve read from others, but it seems that that part of the main spillway was inadequate.
Here is one way to look at it. Look at the big picture: Look at it from “There are no excuses” standpoint - this is engineering. If you don’t know pre-performance data (ES erosion & fractured rock emplacement upon - “rotten rock”), you find out - beforehand**. If there were warning signs on the main spillway - you fix it (there were warning signs from multiple fixes & multiple inspections at the exact blowout area).
Compare this to the tremendous engineering of Trump International Hotel & Tower skyscraper in Chicago. It must meet its full & amazing engineering safety standards including all safety margins. They tested all of it thoroughly - including wind channeled tornadoes.
The Brooklyn bridge was designed to be six times the safety margin.
Now imagine the costs (original & now to “fix it”): Imagine a no-holds barred leader, such as Trump, coming into a meeting and asking... driving.. into the answers on why such a colossal failure. Imagine the answers that should be given to the Public in accountability on how they got there & to assure them that what is going to be done (fixes) will not meet such disastrous results.
I realize that there are a series of events that led to the current situation. However, in Engineering, and especially with Inspection & Maintenance warning signs, it is unconscionable to allow such huge “misses” to occur.
Scott Cahill, a dam expert, has been very good at word crafting the magnitude of this situation. Here is a quote from one of his recent writings:
Scott Cahill”
**For such a colossal performance Dam, you MUST know each and all of the failure conditions. Did you know that the ES was relegated to a larger size “secondary structure” since they couldn’t get the larger capacity ‘delta wing’ main spillway design to work? (they had rooster tails up to 100 feet high in model testing... they had vortices swirling in the main spillway, they had large waves sloshing over the tall spillway walls of the models...).
When you reduce the main spillway to a smaller non-turbulent laminar flow design (took three designs to arrive at this one), and then you make up for the much higher lost flood control numbers by placing a non-tested (hydrological model) ES with no real anchoring, and then rate it for such a monstrous capacity - with no concrete apron down the hillside... Using the reason as “its use will be infrequent” (actual wording in the design/construction archives). How would this hold up in an accountability standpoint?