There are two spillways, the main spillway and the emergency spillway.
The main spillway is the concrete spillway, and it developed a hole in late January. Due to very real concerns that it would deteriorate, the decision was made to use the emergency spillway until the concrete spillway would be fixed.
The emergency spillway is basically a concrete weir--when the lake level reaches a certain height, it spills out over concrete and runs down the hillside. This is considered an emergency spillway, and erosion is usually expected for such spillways (the regulatory definition of an emergency spillway is basically "it can be used once and then it needs a long downtime while it's repaired).
When the emergency spillway was in use, what happened was the very same erosion that was underpinning the main spillway was also occurring near the concrete weir. If left unchecked, the fear was that it would start eroding the concrete weir leading to potential collapse of the concrete weir. This prompted the decision to evacuate as much water as possible using the damaged main spillway which greatly exacerbated the damage (as the pictures show, a large section of the spillway now fails to exist, and it looks like most of the lower spillway would need to be torn up instead of being unused).
I'm not a geologist, and I certainly don't have detailed nature of the local geology. But my suspicion is that the prolonged drought followed by the intense wet season destabilized the slope (by weakening the organic stabilization of the slope and then loading it heavily with water), so spilling any dam water anywhere on it would lead to massive erosion (including undermining the concrete structures). The emergency spillway is supposed to be able to carry far more water loads. I don't think the severe-drought-then-extreme-rain scenario was envisioned during the dam construction (although it really should be after this event), so I don't know if the spillway would have performed as badly in "expected" conditions.
Also not a geologist, and not familiar with local geology. Packing so much water in a very small channel really helps water erode anything. If i remember correctly, it's thought Niagara falls moved a few miles in a day. That seems terrifying to me, it's a long drop down. Waterfalls slowly work their way upstream, obviously. Less obviously, a little weakness lets more water though, compounding the abrasive effects. Which allows more water, which cuts away the land faster.
So yeah, i don't think the dryness had anything to do with destabilizing anything. Water is just really good at cutting. The dam created a huge reserve of water, so lots is available to cut.
The main spillway is the concrete spillway, and it developed a hole in late January. Due to very real concerns that it would deteriorate, the decision was made to use the emergency spillway until the concrete spillway would be fixed.
The emergency spillway is basically a concrete weir--when the lake level reaches a certain height, it spills out over concrete and runs down the hillside. This is considered an emergency spillway, and erosion is usually expected for such spillways (the regulatory definition of an emergency spillway is basically "it can be used once and then it needs a long downtime while it's repaired).
When the emergency spillway was in use, what happened was the very same erosion that was underpinning the main spillway was also occurring near the concrete weir. If left unchecked, the fear was that it would start eroding the concrete weir leading to potential collapse of the concrete weir. This prompted the decision to evacuate as much water as possible using the damaged main spillway which greatly exacerbated the damage (as the pictures show, a large section of the spillway now fails to exist, and it looks like most of the lower spillway would need to be torn up instead of being unused).
I'm not a geologist, and I certainly don't have detailed nature of the local geology. But my suspicion is that the prolonged drought followed by the intense wet season destabilized the slope (by weakening the organic stabilization of the slope and then loading it heavily with water), so spilling any dam water anywhere on it would lead to massive erosion (including undermining the concrete structures). The emergency spillway is supposed to be able to carry far more water loads. I don't think the severe-drought-then-extreme-rain scenario was envisioned during the dam construction (although it really should be after this event), so I don't know if the spillway would have performed as badly in "expected" conditions.