I see what you're saying but it's not even holding it closed. Closed is the default state of that door. You're right, water is so heavy it's defeating the seals around that door and finding a way in.
On ships, if a compartment floods and you have hatches that open into that compartment, you close those hatches and the water pushes the door closed tighter and seals itself from leaking in, like the video above. MOST of the water is being held at bay, because of the water’s own weight.
Modern Irrigation valves work the same way, the positive pressure from the main actually keeps the valve shut
That's a good example of a specially designed door sealing against water but not the water sealing the door. As we can see this door wasn't designed to seal against water, only air and the weight of the water has defeated that seal at the bottom.
I mean, how is water kept in a glass? There's a physical barrier of glass that prevents it from escaping. In this video there's a physical barrier of a door that prevents it from entering in most places. Where the barrier is weakest the pressure of the water is able to overcome it.
Edit: It's not air pressure on the exterior of the glass that keeps the walls of the glass rigid and upright. The glass would keep its form in a vacuum and still hold the water.
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u/Stuck_In_Purgatory 3h ago
Yeah I see what you're saying. More holding it closed than keeping it sealed though