<p>Short answer: Tsunami models use the shallow-wave mathematical approach, because their wavelength is usually much larger than the relevant water depth determining their propagation.</p>

<p>Long answer:
Tsunamis are often called 'tidal waves' to highlight the idea that their characteristic time response is closer to tydes than to the standard wind waves we are used to see in a beach. The full spectrum of water wave periods is shown in this figure (tsunamis being classified as long-period waves, together with waves caused by meteorological events):
<a href="https://i.sstatic.net/NzQn4.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/NzQn4.png" alt="enter image description here"></a>Classification of the spectrum of ocean waves according to wave period. Redrawn from Figure 1 in: Walter H. Munk (1950) "Origin and generation of waves". Proceedings 1st International Conference on Coastal Engineering, Long Beach, California. ASCE, pp. 1–4.</p>

<p>This image shows that the wave frequency depends on the cause behind the perturbation of the water surface. </p>

<p>Oceanographers generally treat ocean waves as free surface waves in an ideal fluid [<a href="http://ms.mcmaster.ca/craig/PASI-VI-2_JDDE1133.pdf" rel="nofollow noreferrer">2</a>]. </p>

<p>Mathematically, tsunamis are surface gravity waves approached as <strong>shallow-water waves</strong> (see chapter 4.1.5 in <a href="https://engineering.dartmouth.edu/~d30345d/books/EFM/chap4.pdf" rel="nofollow noreferrer">here</a>), meaning that the wavelength <strong>λ > 20H</strong> (H being the water depth). The involved shallow-water-wave mathematical approximations oppose to the deep-water waves (λ &lt; H, see section 4.1.4 in the same [ref <a href="https://engineering.dartmouth.edu/~d30345d/books/EFM/chap4.pdf" rel="nofollow noreferrer">3</a>]). </p>

<p>Whereas for shallow waves the wave frequency and the wave speed are (ref. <a href="https://engineering.dartmouth.edu/~d30345d/books/EFM/chap4.pdf" rel="nofollow noreferrer">3</a>):</p>

<p>$ω = \sqrt {g k 2H}$    with $k = 2\pi / \lambda$</p>

<p>$c = \sqrt {g H}$</p>

<p>for deep-water waves, instead:  </p>

<p>$ω = \sqrt {g k}$ </p>

<p>$c = \sqrt {\frac{g \lambda}{2\pi}}$</p>

<p>As the wave approaches the shore and H decreases, the wave velocity decreases and energy is preserved by increasing the wave amplitude.  </p>

<p>Detailed information here: <a href="https://engineering.dartmouth.edu/~d30345d/books/EFM/chap4.pdf" rel="nofollow noreferrer">3</a>.</p>

CC BY-SA 3.0
2016-11-28T08:21:54.507Z
I think it would be beneficial if you could address the question about the temporal evolution of the tsunami.
Isopycnal Oscillation
CC BY-SA 3.0
2016-11-29T17:13:34.090Z
i tried to improve it
DrGC
CC BY-SA 4.0
2023-10-14T15:51:52.143Z
The link after "Detailed information here" appears to be broken.
Mark Foskey
CC BY-SA 3.0
2016-11-27T18:01:07.650Z
2017-02-28T04:24:49.480Z
2017-02-28T04:24:49.480Z
DrGC
<p>I'm interested in knowing which type of wave a tsunami is, in a strictly mathematical sense.  </p>

<p>I'm gonna cite from <a href="https://en.wikipedia.org/wiki/Tsunami" rel="noreferrer">Wikipedia</a> for this:</p>

<blockquote>
  <p>As the tsunami approaches the coast and the waters become shallow, wave shoaling compresses the wave and its speed decreases below 80 kilometres per hour (50 mph). Its wavelength diminishes to less than 20 kilometres (12 mi) and its amplitude grows enormously – in accord with Green's law.</p>
</blockquote>

<p>and also</p>

<blockquote>
  <p>Except for the very largest tsunamis, the approaching wave does not break, but rather appears like a fast-moving tidal bore.</p>
</blockquote>

<p>While the first property strongly hints at a tsunami being a shallow water wave, the latter fact, namely that they usually don't break would rather suggest for the tsunami being a <a href="https://en.wikipedia.org/wiki/Soliton" rel="noreferrer">soliton-solution</a> of a Korteveeg-de-Vries-type equation.<br>
I thought the latter property would not be inherent to shallow water waves although <a href="https://en.wikipedia.org/wiki/Shallow_water_equations" rel="noreferrer">Wikipedia, again</a> seems to argue differently, however only with a picture.</p>

<p>So maybe someone can clarify:  </p>

<ul>
<li>Which equations govern the surface profile of a tsunami? Has this model been tested against satellite measurments?</li>
</ul>

CC BY-SA 3.0
2016-11-25T23:22:40.720Z
There are two main phases of the Tsunami. While traveling in the open ocean the tsunami will behave like a shallow-water wave. However, because it is so long, as it shoals it will behave more like a bore rather than a shallow-water wave.
Isopycnal Oscillation
CC BY-SA 3.0
2016-11-26T14:05:35.367Z
@IsopycnalOscillation: Well I can see as much from wikipedia. That doesn't answer the question however.
AtmosphericPrisonEscape
CC BY-SA 3.0
2016-11-26T21:15:41.670Z
The equation to model a shallow water wave is simply a sinusoidal function $\eta = \eta_0 \sin(k x - \omega t)$, where $\eta$ is the surface water displacement, $\eta_0$ is the wave amplitude, $k$ is the wavenumber and $\omega$ is the wave frequency. $x$ and $t$ are the horizontal direction and time, respectively. When the wave is a bore the Froude number is very large (typically larger than 1) so you most likely need numerical modeling.
Isopycnal Oscillation
CC BY-SA 3.0
2016-11-28T15:58:51.390Z
I don't see any sense in this reasoning in the question: "that they usually don't break would rather suggest for the tsunami being a soliton-solution of a Korteveeg-de-Vries-type equation". The breaking of a wave depends on the transition from deep to shallow conditions. A tsunami is not a soliton.
DrGC
CC BY-SA 3.0
2016-11-28T21:31:35.170Z
@DrGC: Then give me the exact set of governing equations and we're done here. I don't see any sense in ranting here.
AtmosphericPrisonEscape
CC BY-SA 3.0
2016-11-28T21:47:34.063Z
I'm not asking about pictures. I know the shallow water theory very well, but no model is perfect. That's why I'm asking for the best model we can get, and if available, satellite measurments to back it up.
AtmosphericPrisonEscape
CC BY-SA 3.0
2016-11-29T17:11:48.077Z
no ranting, just feedback.
DrGC
CC BY-SA 3.0
2016-11-29T17:12:45.390Z
and the full formulation is in link I used several times in my answer. http://ms.mcmaster.ca/craig/PASI-VI-2_JDDE1133.pdf cheers
DrGC
CC BY-SA 3.0
2016-11-29T17:39:20.767Z
@DrGC: Ah, I didn't look into your reference, as I assumed it is some undergrad treatment of the shallow water system. Thank you, I'll read it and come back to you.
AtmosphericPrisonEscape
<tsunami><solitary-waves>
Tsunami - Mathematical classification
AtmosphericPrisonEscape
# Tsunami - Mathematical classification

(Asked by: username_0 on 2016-11-22)

I'm interested in knowing which type of wave a tsunami is, in a strictly mathematical sense.

I'm gonna cite from [Wikipedia](https://en.wikipedia.org/wiki/Tsunami) for this:

> As the tsunami approaches the coast and the waters become shallow, wave shoaling compresses the wave and its speed decreases below 80 kilometres per hour (50 mph). Its wavelength diminishes to less than 20 kilometres (12 mi) and its amplitude grows enormously – in accord with Green's law.

and also

> Except for the very largest tsunamis, the approaching wave does not break, but rather appears like a fast-moving tidal bore.

While the first property strongly hints at a tsunami being a shallow water wave, the latter fact, namely that they usually don't break would rather suggest for the tsunami being a [soliton-solution](https://en.wikipedia.org/wiki/Soliton) of a Korteveeg-de-Vries-type equation.  
I thought the latter property would not be inherent to shallow water waves although [Wikipedia, again](https://en.wikipedia.org/wiki/Shallow_water_equations) seems to argue differently, however only with a picture.

So maybe someone can clarify:

* Which equations govern the surface profile of a tsunami? Has this model been tested against satellite measurments?

**Tags:** tsunami, solitary-waves

> **Comments:**
> **username_1**: There are two main phases of the Tsunami. While traveling in the open ocean the tsunami will behave like a shallow-water wave. However, because it is so long, as it shoals it will behave more like a bore rather than a shallow-water wave.  
> **username_0**: @username_1: Well I can see as much from wikipedia. That doesn't answer the question however.  
> **username_1**: The equation to model a shallow water wave is simply a sinusoidal function $\eta = \eta_0 \sin(k x - \omega t)$, where $\eta$ is the surface water displacement, $\eta_0$ is the wave amplitude, $k$ is the wavenumber and $\omega$ is the wave frequency. $x$ and $t$ are the horizontal direction and time, respectively. When the wave is a bore the Froude number is very large (typically larger than 1) so you most likely need numerical modeling.  
> **username_2**: I don't see any sense in this reasoning in the question: "that they usually don't break would rather suggest for the tsunami being a soliton-solution of a Korteveeg-de-Vries-type equation". The breaking of a wave depends on the transition from deep to shallow conditions. A tsunami is not a soliton.  
> **username_0**: @username_2: Then give me the exact set of governing equations and we're done here. I don't see any sense in ranting here.  
> **username_0**: I'm not asking about pictures. I know the shallow water theory very well, but no model is perfect. That's why I'm asking for the best model we can get, and if available, satellite measurments to back it up.  
> **username_2**: no ranting, just feedback.  
> **username_2**: and the full formulation is in link I used several times in my answer. http://ms.mcmaster.ca/craig/PASI-VI-2_JDDE1133.pdf cheers  
> **username_0**: @username_2: Ah, I didn't look into your reference, as I assumed it is some undergrad treatment of the shallow water system. Thank you, I'll read it and come back to you.  

---

## 1 answers

### Answer by username_2 on 2016-11-27. Score: 8 | Accepted answer

Short answer: Tsunami models use the shallow-wave mathematical approach, because their wavelength is usually much larger than the relevant water depth determining their propagation.

Long answer:
Tsunamis are often called 'tidal waves' to highlight the idea that their characteristic time response is closer to tydes than to the standard wind waves we are used to see in a beach. The full spectrum of water wave periods is shown in this figure (tsunamis being classified as long-period waves, together with waves caused by meteorological events):
[![enter image description here](https://i.sstatic.net/NzQn4.png)](https://i.sstatic.net/NzQn4.png)Classification of the spectrum of ocean waves according to wave period. Redrawn from Figure 1 in: Walter H. Munk (1950) "Origin and generation of waves". Proceedings 1st International Conference on Coastal Engineering, Long Beach, California. ASCE, pp. 1–4.

This image shows that the wave frequency depends on the cause behind the perturbation of the water surface.

Oceanographers generally treat ocean waves as free surface waves in an ideal fluid [[2](http://ms.mcmaster.ca/craig/PASI-VI-2_JDDE1133.pdf)].

Mathematically, tsunamis are surface gravity waves approached as **shallow-water waves** (see chapter 4.1.5 in [here](https://engineering.dartmouth.edu/~d30345d/books/EFM/chap4.pdf)), meaning that the wavelength **λ > 20H** (H being the water depth). The involved shallow-water-wave mathematical approximations oppose to the deep-water waves (λ < H, see section 4.1.4 in the same [ref [3](https://engineering.dartmouth.edu/~d30345d/books/EFM/chap4.pdf)]).

Whereas for shallow waves the wave frequency and the wave speed are (ref. [3](https://engineering.dartmouth.edu/~d30345d/books/EFM/chap4.pdf)):

$ω = \sqrt {g k 2H}$ with $k = 2\pi / \lambda$

$c = \sqrt {g H}$

for deep-water waves, instead:

$ω = \sqrt {g k}$

$c = \sqrt {\frac{g \lambda}{2\pi}}$

As the wave approaches the shore and H decreases, the wave velocity decreases and energy is preserved by increasing the wave amplitude.

Detailed information here: [3](https://engineering.dartmouth.edu/~d30345d/books/EFM/chap4.pdf).

> **Comments:**
> **username_1**: I think it would be beneficial if you could address the question about the temporal evolution of the tsunami.  
> **username_2**: i tried to improve it  
> **username_3**: The link after "Detailed information here" appears to be broken.  

---

