Why do tsunamis (20~ft) cause more damage and flooding than the waves that surfers surf (world record 86ft)?
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A wave is just a wave. Even large ones only contain so much water. They’re caused when wind blows the water, causing part of it to rise up and then crash back down onto the surface.
A tsunami isn’t caused by wind, they’re usually caused by earthquakes. Whereas wind pushes a little water along the surface, something that causes a tsunami pushes a *lot* of water along under the surface. As the ocean floor rises up to become land, that same volume of water is pushed up and out, so tsunamis can become quite large. The world record is over 1,700 feet high. In either case, even small tsunamis tend to have a *lot* more water in them then even very large waves, and they have the momentum to keep going inland that waves tend not to have.
Surf waves tend to be, well, out in the surf. They also usually don’t come close to 86 feet tall. Tsunamis can also get much taller than 20 feet.
There aren’t many buildings or garbage cans out in the surf, so besides other surfers there isn’t much to damage or hit your head on.
Tsunamis, however, are tall *on land*. They are in a place where they can do damage.
Imagine you have a ramp
Now you make 20 Snowballs and place them in a line so that they can roll down the ramp next to each other.
Now you make 2000 Snowballs and place 20 next to each other but 100 Snowballs deep.
Can you imagine the difference, the snowballs have all the same size (heigh)
A surf wave crashes where it normally does – on a surf break, in the ocean a bit out from shore. The wave moves towards land and then as the shore approaches, the bottom of the wave slows, the top gets bigger, and then it breaks.
The Tsunami doesn’t break and crash on the beach. It’s a much bigger wall of water that just keeps pushing inland. It’s more like ‘the ocean got 20 feet taller for a half hour’ and so if there is a beach town that is 10 feet above sea level, suddenly that town is 10 feet below water for a half hour or so. And water is heavy, so it is these waves just crashing into the side of a house, into cars, into patio furniture, etc. Pushing debris inland which in turn crashes into other houses…
The question is energy. Wave height is one part of it, but so is wavelength – tsunami can have wavelengths of several hundred kilometers.
its not just height. surfing waves tend to be tall, but fairly short in length that crash and dissipate once they hit shallow enough water.
A tsunami wave may not be very tall, but its humongous in length. when it hits the shoreline, it doesn’t stop, its got so much water and energy behind it that it bulldozes through everything in its path.
If you look at some videos of tsunamis, out of context it looks like an entire river decided to change course and plow through a city. you don’t see “waves” because its all one giant wave, its a continuous rushing of water inland for up to an hour before the wave loses enough energy to recede.
Simplest answer: **surface waves only move water up and down. Tsunamis move water inland.**
“Waves are created by energy passing through water, causing it to move in a circular motion. However, water does not actually travel in waves. Waves transmit energy, not water” [*—NOAA.gov*](https://oceanservice.noaa.gov/facts/wavesinocean.html#:~:text=on%20the%20horizon.-,Waves%20are%20created%20by%20energy%20passing%20through%20water%2C%20causing%20it,across%20an%20entire%20ocean%20basin.)
A fun experiment is to place something that floats on the water. You will see the thing move up and down but it generally returns to the same place as each wave passes.
With a tsunami, you have a huge amount of water moving, not just waves.
The height doesn’t matter as much as how much water the wave carries.
A normal surface wave is… Just waviness of the surface. It can be tall but it’s only as wide as it looks, so it only carries as much water as you can immediately see.
A tsunami is a gigantic mass of water pushed by a geological event. Even if it doesn’t look tall, it’s because it covers such a large area that it just kind of becomes “the surface”. It is really deep, though. It’s only noticeable when it is forced up by the shore and rushes inland.
Just compare videos of how much water retreats before a tsunami (the sea can retreat hundreds of meters to kilometers) vs. a normal wave (sub-meter to a few meters) to get an idea of the difference in scale.
As you may know any object has inertia. If it flies in one direction it will try to keep flying in that direction.
A large way has some volume of water, hits the surface of a beach and dissiptes its energy and the. Flows back into the ocean. It‘s just a surface level phenomenon.
A tsunami is far more. It‘s not just a wave. Its all the water underneath that wave that is moving along with it. Theres probably cubic kilometers of water moving towards a coast. And when it hits the coast, all that moving mass keeps going.
Waves are short, several hundred feet long. Tsunamis are long, they can stretch for miles along a coastline. Because of the huge mass of water behind them, tsunamis carry much more energy with them and can cause damage far inland.
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