How does flooding actually happen? Can’t the water just keep moving

Question

I don’t get how houses/buildings can go under water?

In: 8

Answer 1

What is important to understand is that water has viscosity and big mass of water takes a long while to accelerate.

Take a bottle and tip it so it starts to flow, you notice that when you tip it, it doesn’t start to flow instantly nor does it leave the bottle right away.

This is because water has mass, mass takes a while to accelerate. You can imagine mass of water, like a flooded area, to be made of many small points of water. Each of these points has to accelerate to start moving and and the mass of water wants to resist movement of these points.

If you have ever had to deal with acetone or something like that which has really low viscosity. You see that it flow very quickly and everywhere, and thorough everything.

So if you have a valley from which water can only leave through an opening, the flow of water is limited. It is based on the size of the opening and pressure behind. On an open plane the limitation is the viscosity, terrain (water has friction against surface it is flowing on top of).

If you want to imagine how water flows, you could imagine a stack of paper sheets. Start pushing it from the top so it starts to spread. You might notice that the top layer moves more while the lowest sheet might not move at all, and each layer of paper moves slow. At some pont the sheet droops over the edge and touches the table and stops moving, but the ones above it keep sliding. This is how water spreads but in 3D.

Water keeps moving as fast as it can to any direction it can and has force to go in to. It’ll go up down, where ever as long as something is forcing it to move. Whether it be pressure or gravity.

Also it is important to understand that the a mass of water can be pushed. For example strong winds can slowly accerelate and push sea, layer by layer (just like the stack of paper) towards the ground. Adding more and more force, enough to competing against the force of gravity. The same way you can blow on a hot cup of tea and make it splash, or use a garden hose to push water uphill. Or even push away water by using pressurewasher… with water.