# Why does shore water recede before a tsunami?
**URL:** https://boards.straightdope.com/t/why-does-shore-water-recede-before-a-tsunami/453471
**Category:** Factual Questions
**Created:** [June 19, 2008, 2:54pm UTC](https://boards.straightdope.com/t/why-does-shore-water-recede-before-a-tsunami/453471 "2008-06-19T14:54:35Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@SuaSponte](https://boards.straightdope.com/u/SuaSponte)
#### Post date: [June 19, 2008, 2:54pm UTC](https://boards.straightdope.com/t/why-does-shore-water-recede-before-a-tsunami/453471/1 "2008-06-19T14:54:35Z")
Every depiction and description of a tsunami I’ve come across has the water on the shoreline dramatically receding before the tsunami hits. But I can’t figure out why. Help, please.
Sua
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### Author:  [@Captain\_Amazing](https://boards.straightdope.com/u/Captain_Amazing)
#### Post date: [June 19, 2008, 3:00pm UTC](https://boards.straightdope.com/t/why-does-shore-water-recede-before-a-tsunami/453471/2 "2008-06-19T15:00:00Z")
It’s just the trough of the wave.
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### Author:  [@StinkyBurrito](https://boards.straightdope.com/u/StinkyBurrito)
#### Post date: [June 19, 2008, 3:01pm UTC](https://boards.straightdope.com/t/why-does-shore-water-recede-before-a-tsunami/453471/3 "2008-06-19T15:01:51Z")
A tsunami isn’t just one big wave, it is a series of many large waves and troughs between the waves. Water rushes in and it rushes out. The first part of the tsunami that reaches shore can either be the ‘water rushes in’, or the ‘water rushes out’ part of the wave cycle. So you get the phenomenom you describe about half the time. The other half of the time you get the rush of water first.
ETA - I think anyway.
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### Author:  [@SuaSponte](https://boards.straightdope.com/u/SuaSponte)
#### Post date: [June 19, 2008, 3:04pm UTC](https://boards.straightdope.com/t/why-does-shore-water-recede-before-a-tsunami/453471/4 "2008-06-19T15:04:17Z")
Got it, thanks.
Sua
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### Author:  [@Kevbo](https://boards.straightdope.com/u/Kevbo)
#### Post date: [June 19, 2008, 8:55pm UTC](https://boards.straightdope.com/t/why-does-shore-water-recede-before-a-tsunami/453471/5 "2008-06-19T20:55:38Z")
The ocean starts as flat. (relative to the tsunami). The Tsunami is a high crest of water…a series of them actually.
Ponder this question: Where does the water in the crest(s) come from?
The answer is that in order to create the crests, water has to be removed at the troughs. Part of the dynamics of wave motion is that water flows toward the crest from in front of it, creating the troughs. This counterflow is normally well below the surface. When land is reached, the land forces this counter flow to the surface. Compressed into less vertical space, the flows in both directions have to accelerate, causing the wave to break.