Heavy rain will always have the potential to overwhelm the pumps on the stormwater tank for the tunnel. That's why there are the electronic signs all over, so it can be automatically closed temporarily. Of course, the question is "Why don't they design and build it so it can't do that?" The answer is, the costs would be astronomical. All roads are designed to flood in the heaviest rainfall events.
In this case, "flooding" means a couple of feet at most of water pooling at the low spot, which is very close to the west end of the tunnel.
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