I don’t get how houses/buildings can go under water?
In: 8
Not if there’s a lot of it. You’re underestimating how much water there actually is when it floods.
Usually floods don’t happen because rain water can be absorbed by the ground as it falls. Floods happen when the ground can not absorb the water that is coming in. So water gathers above ground and becomes a moving wall of pain.
Remember, it’s not the rushing water that will kill you (unless you’re pulled under) it’s the debris in the water you’re smashing into (broken branches, cars, rocks, broken glass, ect) that will do deadly puncture and impact damage.
They usually happen when there is too much water for the rivers to handle. Happens often due to spring thaw of snow or heavy raining.
Maybe the river can handle 10m3 of water per second but due to heavy raining the flow is doubled. Suddenly the riverbed is just too small and the excess water needs to go somewhere – outside of the riverbed into the fields and houses.
It does.
We just force it into small convenient channels.
Which get bigger when large volumes of water get dumped up stream.
I have three quick experiments for you:
1) Grab a bowl. Fill it with water. Congratulations, you just have a “flooded” bowl.
2) Grab a bottle of soft drink (the plastic ones) and fill it with water. Then try to empty it. It doesn’t flow all the way out at once, because the hole doesn’t allow for that much water movement, thus it is a slow flow. You have a “flooded” bottle.
3) Grab a strainer/colander, and try to put the water from the bowl in #1 (to save water, I’m not an environmental monster) into it. You’ll see that even if the water doesn’t flow through the strainer in one swift motion, it definitely doesn’t pool up significantly.
With those experiments, it should become pretty obvious that floods happen when water flows in faster than it can flow out.
Usually city floods happen because concrete and asphalt are a lot more waterproof than a park or forest’s soil. That is because soil and sand act as a natural colander for the water to pour down through them. But even they have a limit, which is why you can have swamps, bogs and other water-logged soils and sands. This would be the difference between #1 and #3 up there.
Now cities do have sewers, which should take care of the excess water, except that if the downpours/water coming into the city come in faster than it can be moved out, you have a situation like the soft drink bottle, where it will get out, but it will take time. It will take even more time if the place where the sewers are draining the water to are also being flooded, and you must then wait for it to drain, before the water can drain properly. (This one can be simulated by putting the tip of the soft drink bottle underwater as it drains: If it keeps going, you’ll see it go much, ***much*** slower.)
So, in short, while the water should (and will) technically flow all the way out at some point, the speed at which it comes in, versus the speed at which it goes out are what cause the flooding.
Lots of variables here so I’ll try to separate parts I can best explain
1.Soil
Depending where you live, the type of soil you have varies, for example the soil where I live is very clay-like, so as a result during heavy rainfall small flooding can occur. Nothing major just small puddles in my yard. Though if you let’s say live in a valley, then the water has nowhere to go. Can’t go into the soil since it’s too thick, and if can’t go downhill so it’s stuck where it is and builds up which leads to flooding.
2. Sea level
Basic explanation of sea level is picture a flat line and that’s sea level or just 0, if you are above sea level, this means you are more elevated and less likely to experience flooding.
So for these two parts let’s make a little example, New Orleans often gets hit hard with tropical storms and causes extreme flooding. New Orleans is only 6′ above sea level, so off the bat not that high up, to add on New Orleans also has thick clay like soil so the water doesn’t seep into the ground it stays above. So since New Orleans is low the water can’t travel up hill, and the water can’t go into the ground it’s going to stay where it is and increase in depth.
A basic experiment of this is just getting a few containers and put them a lil bit higher then eachother, as if they were steps on a stair case. Then attacht a tube at the bottom of each except for the last one and have the tube go into the top of the previous container. If you fill the top container with water eventually all the water is going to make it’s way down, so the top container theoretically will have little to no water, while the bottom one will be full. *edit* This is just to explain water traveling down hill
Now for let’s say a river overflowing. Rivers are lower then the land it’s around, but if the water is equal in height around it, it’s going to spill over in simple terms. and keep spilling till it can’t anymore.
Imagine a bath or sink/basin.
If you turn the tap on, and the water is flowing in faster than the drain allows it to drain out, it will fill up. If you leave it like that for a while, eventually it will overflow. That’s a flood.
When the incoming water is sustained for a while and overwhelms the ability to drain (into the soil, downstream, etc.) then it will start to build up. If that continues for hours or days, eventually the river, etc. will overflow.
Rivers tend to occur on rocks etc. that *don’t* absorb water (are not porous) because otherwise they wouldn’t form a river in the first place, the water would just drain away. Natural rivers tend to form on non-porous rock, which means that the water can’t drain into the landscape and can only go “downstream”. But there’s a limit at which the surrounding soil/rock can contain it, and places that get flooded tend to have non-porous underpinnings and/or are lower than the river (so when it “spills over” from the non-porous rock that normally contains it, it goes downhill to the areas that get flooded instead of going downstream).
Plus, if you have a serious storm, the sheer volume of water means the river can be handling something like 5-10 times the normal amount of water in it… so it wants to “become” wider and deeper anyway and floods floodplains – those places near a river which are naturally where the water will flow if the river overflows.
If you multiply the size of water in the river, it’s not going to flow downstream in the same way at all… it’s going to find every little nook, cranny, come over the top of the riverbanks, and find a new way down to the sea… through the town if that’s the way gravity takes it.
It’s like having a gutter drain that the water normally streams down, then putting ten times the amount of water down it. It’s not going to flow down the gutter, it’s going to spill out over the sides and go anywhere it likes.
In addition to everything else already said, floodings on land mostly happen not only with one river, but with a whole ecosystem. When there was severe flooding in Germany last year, the problem weren’t the big rivers. The flodding happend in the smaller rivers. The region is a bit mountainous, so the rivers are in the valleys with relatively small place. The rivers are meandering. When it rained hard for days, the waterlevel in the rivers kept rising. the streams swelled so much, they completly swept over the meanders, crushing the houses between them. and when wild water with higher water levels and debris, these will hit bridges.
Floods happen when drainage systems and rivers can’t handle the amount of water trying to drain away. The problem is even worse when you live in a flat region like the Gulf Coast. There just isn’t anywhere lower for the water to go.
Hurricanes dump huge amounts of water. When a hurricane approaches land it is pushing a massive amount of water in front of it called storm surge. This water flows over land and immediately fills up the rivers. Sometimes it is so much water that the river will flow backwards. Then as the hurricane makes landfall all the rain it brings with it has nowhere to go. Take Hurricane Harvey as an example. Harvey stalled out over Houston and dump an entire years worth of rain in 3 days. No system of drainage is built to handle that much water. And as the hurricane moves on it dumps even more rain further up river which still can’t drain away because the river are still full.
I recommend a YouTube channel called Pratical Engineering. He has several videos explaining how drainage systems and water handling works and what ways they can fail. And he explains everything in a way that’s easy to understand and he builds nice models to demonstrate. Here a playlist of flood control systems. https://youtube.com/playlist?list=PLTZM4MrZKfW-_GFGXeWYgQ5zfC29Om1Np
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.
Latest Answers