This lake and spillway were around for over 50 years. Its not a bad design. It still works. It has two fail safes. The spillway and the auxiliary spillway. Water runs down the chute in a controlled manner. In the primary spillway or an uncontrolled manner in the auxiliary. But if the rain + the excess water run down the hill along side the cement chute, the water erodes the land under the cement chute. That is what happened. That hole formed. It was not there or at least mostly not there before this past week. Water ran down the chute. And through leaks and overflowing the side rail it eroded the foundation of the chute. Once a small part of the chute fell the hole quickly enlarged.
The auxiliary chute shows the issue. Its the same land with grass and trees. And very quickly the land erodes so that the roads and trees and dirt go into the river below. In fifty years there has been several rainy days. There have even been 10 rainy days in a row. But they have always controlled the level of the lake without overflowing the spillway. This time they were caught with two much water and they broke their spillway. And they realized that their auxiliary spillway has two many structures that got wiped away.
We know the initial damage to the main spillway occurred where repair work was done previously. We also know the repaired area was not closely inspected in years subsequent. Much like a pothole, a very likely place to get a pothole is where an old pothole was repaired.
However, you are asserting grave operator error, the assertion being in a previous post that too much water was released down the main spillway, and the excess water overflowed the chute. This should be easy to prove: Records are kept of all such discharges.
A) What is (well, was) the maximum safe (ie., without overflowing the chute itself) discharge rate of the main spillway?
B) Was that rate exceeded before the disruption of flow became apparent?
It’s YOUR assertion...
MY best guesstimate is that the main spillway discharge rate (which is probably determined by a small group of engrs. and techs) was ok, but as the repaired area began to deteriorate, that flow was disrupted, which could have then caused overflow and then the accelerated failure you describe.
Keep in mind that at that point in the spillway, assuming something around 100,000 cfs flow, you have somewhere in the neighborhood of 1,000 - 2,000 megawatts of mechanical energy potential going by. It would not take much of a flow disruption to cause big trouble.