Water isn't passing over the dam, it's passing over a much shorter wall to the side of the dam with unprotected earth around and below it. Large volumes of water tend to wash away dirt and could potentially undermine this side wall. So there is a potential for release of a very large volume of water, without compromising the main dam itself, because the erosion cuts a new exit channel for the lake.
People have been saying that if the emergency spillway fails, it will release the top 30' (?) of the lake. What determines that height? Is there bedrock or something else expected to stop erosion 30' below the level of the emergency spillway or something?
You can see that they blasted into the rock to make the spillway.
However, the idea that erosion will undercut the emergency spillway and cause it fail implies that it's not built directly on the rock. This suggests that there could be more than 30' of draining.
If you look at the photos, some on Wikipedia, you can see the spillways are around a bend in the river from the main dam and pass over a high bedrock canyon wall.
If reverse Superman flew up and dropped a mountain in the spillways so the dam could be overtopped, the dam would begin to erode.
Whichever part of the dam was lowest would see the fastest flowing water and would erode the most quickly. As a chasm was cut down through the dam, the water would flow faster and faster through it - if the height of the water above the bottom of the cut is 1 foot, only the top foot of water is "pushing" the water over; if the cut is 10 feet deep, now the top 10 feet of water is "pushing" water out the hole, and it will be pushed much faster.
This will turn into a feedback loop, and the water will rapidly cut a channel through to the bottom of the dam. As water flows through it, faster and faster, it will weaken the sides of the chasm, and more of the dam will collapse. This will let more of the water flow through, but slower, eventually limiting further erosion.
This is essentially the same thing happening in the spillways right now, but slower, because the spillways are designed to resist erosion. Once the vicious cycle gets started, though, the end result is the same.
The dam is 922', but the emergency spillway is 901 feet and very, very wide. It's designed to make the dam impossible to overtop. Even the worst case scenario this year would not lead to overtopping the dam.
The question is about why water flowing over the dam would damage it. Clearly they have engineered the spillway to prevent that, so there must be a reason, which is why the person asked.
If someone asks you what would happen to the people inside of a submarine if they let it fill with water, you don't respond by saying they design submarines to prevent them from filling up with water.
