What is holding the water?

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So when you have a big flood from something, you see pictures or reports saying X feet of water. Think like hurricane Katrina where it was up to peoples roofs. What is “holding” that water, so to speak? Why can’t it just spread out across the land or something?

In: Earth Science
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It is spread out across the land. There’s just a stupid amount of water and it takes time for it flow away. What you saw in Katrina was the Gulf of Mexico pushed up onto the land by the force of the winds.

Essentially it does spread until it hits land higher than the height of water and or the water isn’t being held back as much as it is still in the process of spreading out it just takes time for that volume of water to move any distance

It does spread out across the land. When there is an over abundance of water, the bodies of water and waterways rise significantly.

Many areas in the Gulf and East Coast are below or at sea level; therefore, when the sea levels or water levels in general rise, these areas are underwater.

The important thing to remember is that the ground isn’t flat – typically low around the coast, and rising in height as you move inland (though there are many exceptions to this).

So when you have a flood like Katrina or a big storm surge, the wind offshore pushed the water ahead of it, forcing it to move inland and uphill. At a certain point the hill got higher than the surface of the water, and that is where the flood ended.
So while it was two storeys deep in some areas, if you moved inland where the ground level is 3m higher the floor would only be one storey tall, and gradually reducing the further you travel inland/uphill.

You will also get flooding when there is too much rainfall or snowmelt further upstream – water collects in streams and rivers and flows downhill towards the sea, but if you have too much water flowing down the river than the river can cope with, it will burst its banks and flood the land around it. Generally this water will be moving water – after all it is still trying to travel downhill – until it is stopped by a blockage and slowly seeps into the ground, or rejoins the river where the river is better able to cope with the flows.