I listened to a recent radio report on this subject, a gentleman from some infrastructure crisis management organization claimed ha that e spill way paved section was covered with only 4” of concrete seamed slabs, indicating that the poor design was bound to failure.
The emergency spillway was probably originally meant for the extreme storm event, a 50-100 year storm event. It wasn’t meant for three decades of neglect leading to failure of of the primary spillway features.