Tsunami- Head Out To Sea?

[roverhi]

From experience, if you can get into at least a 100' feet of water before the first wave is predicted to hit you won't even know there was a Tsunami. Staying in a harbor is irresponsible if getting to not very deep water is possible. A tsunami is not a wave as in the Poseidon Adventure till it reaches shallow water. The tsunami is more like a very rapid rise/lowering of tide. Kind of resembles a travelling lump in the ocean.

Running for high ground and expecting the insurance to pay for it is irresponsible. One of the reasons insurance is so expensive.

In that same vein, the only picture I saw of a sailboat driven onto rocks by Sandy had the partially furled headsail still on. Probably another mindless "leave it to insurance" type who didn't even take minimal precautions to protect their boat.

[DumnMad]

Once the wave hits shallow water it changes from swell with water in a circular motion to a river-like flow at a rate of knots.

/Users/nickbarber/Pictures/iPhoto Library/Masters/2011/10/11/20111011-113757/Samoa AfterTsunami 048.JPG

[Astrid]

It's all a matter of time and geography. In Saturday's case, I would have had an hour's sailing to exit Skidegate Inlet and another 40nm through shallow parts of Hecate before I was in deep enough water (12 fathoms)to breath a little easier. From that point the bottom drops off rapidly and another half hour or so would have me in deeper water still. However the earthquake was only 30 miles away, so that would have not left me enough time. With the Japan quake and tsunami, I had adequate warning to get out to sea, but not this time. Fortunately there was little tsunami effect this time.

[Sir Rondo Normal]

From what I'm reading here, it sounds like my best bet would indeed be to exit hardy bay as quickly as possible. We could be in 500 (or more) feet of water in a matter of 1/2 an hour, including the time it take to get out of the marina BUT what worries me are all the small islets and rocks at the entrance to Queen Charlotte Sound. The water through there is only a few hundred feet deep so a tsunami would have to rise up and speed up through that area before meeting us in the deeper and more open waters of the sound.

[DavefromNZ]

There are a number of SMS/email Tsunami alert systems. I have subscribed to this one for the last few years Tsunami Alarm System - tsunami warnings on your mobile - tsunami prediction

It is very good and about USD30 pa. I usually get a message within 20 mins of a tsunami possibility and the info comes from the various govt authorities especially Hawaii.

The downsides are you need to be in SMS contact and it does warn for all worldwide alerts. However I usually only get woken once or twice a year.

[Sir Rondo Normal]

I started following embc (emergency management British Columbia) on Twitter so that I could be tweeted alerts. What a joke. Their response time to the 7.7m quake discussed here was 54 min and their response time (after much criticism about the 54 min response) was 59 min for the 6+m quake off Port Alice a few days ago.
I have since stopped following embc and signed up for sms alerts from the American centre in Alaska.
It's shameful when you can't rely on your own gov't

[Mr B]

That big tsunami in 2002 That wrecked Indonesia and Thailand, The boats that were a mile offshore, didnt even know a tidal wave had hit till they went back to shore and found the damage that had been created,

One boat skipper found bodys in the water floating out to sea after the event,

Govts dont like to announce things as it causes panic,

[DumnMad]

If you are on land, the shaking makes it difficult to walk and it lasts a minute or more, run for high ground. The wave could be only 10 minutes away. Official Tsunami warnings take a while to be calculated and are only OK if the wave origin is thousands of miles away.

[Astrid]

I have since stopped following embc and signed up for sms alerts from the American centre in Alaska.

Fortunately, in my community, the mayor sent initiated the tsunami warning minutes after the quake instead of waiting for official word.

If you are on land, the shaking makes it difficult to walk and it lasts a minute or more, run for high ground. The wave could be only 10 minutes away.

That is very true. If the quake is local and if a tsunami is generated, you may not have time to get out to deep water, in which case it is best to head for high ground. If the quake is a long ways off, make a break for deep water.

[Sir Rondo Normal]

Originally Posted by Astrid

Fortunately, in my community, the mayor sent initiated the tsunami warning minutes after the quake instead of waiting for official word.

There were a number of communities on Van Isl that didn't wait for the official word. Ours wasn't one.
When Krusty Cluck (Premier) was questioned about the less than satisfactory response time, she replied, "No one was injured, no one was killed, the system worked.". What an arrogant idiot she is. The system did not work, there was no tsunami. That's a big difference!

[Astrid]

We were somewhat lucky in that we had only recently had a practice tsunami drill and everyone had a good idea of what to do.

[Morrissey]

The physics is well known. Here is some info that covers it:


A tsunami is basically a shallow-water wave, even in deep seas. This means its velocity is v=√gH, where H is the water depth and g is the gravitational acceleration.
The energy of the tsunami scales as the square of its amplitude A, and thus the energy flux S goes as S~A^2√H. Conservation of energy then implies that the wave amplitude depends on the sea depth as

A~H^−1/4

a result known as Green's law.

So, an earthquake volcanic eruption or landslide in deep water causes a problem as the tsunami moves fast and gets large amplitude in shallow water.

What to do? Depends. If you are in the PNW then you probably wont have time to do much at all when, and note its 'when' and not 'if', a major earthquake occurs in the Cascadia Seimic Gap.

http://seismo.berkeley.edu/~rallen/e...ctures/L15.pdf

NEPTUNE Canada cost $100 m, and will give enough warning for the key utilities to be automatically shut off, to prevent fires. Its automatic because there may not be enough time for human action....
http://www.neptunecanada.ca/about-ne...-tectonics.dot

But if there IS time, then go to deeper water, even if its just a bit deeper. As the stories above indicate and the formula allow you to calculate, a few hundred ft will be fine provided you aren't getting refraction, behind a choke point etc.

[Pelagic]

Originally Posted by Astrid

It's all a matter of time and geography. In Saturday's case, I would have had an hour's sailing to exit Skidegate Inlet and another 40nm through shallow parts of Hecate before I was in deep enough water (12 fathoms)to breath a little easier. From that point the bottom drops off rapidly and another half hour or so would have me in deeper water still. However the earthquake was only 30 miles away, so that would have not left me enough time. With the Japan quake and tsunami, I had adequate warning to get out to sea, but not this time. Fortunately there was little tsunami effect this time.

+1 There is no hard and fast rule...just a judgement call

[Richard5]

Insequent, thank you for posting that. All I can add is that a wave is a "shallow water" wave when the water depth is 1/2 the wave length. That the wave length of tsunamis is measured in miles, a tsunami will be in shallow water in all waters except perhaps deep island arc trenches. Whether the wave spills or not is a function of several factors as outlined in the material Insequent linked.

How the waves propagate is consequent to various factors chiefly to do with bottom topography. However, a headland or narrowing inlet can cause increased surge. The north coast of British Columbia presents an abundance of potential hazardous due to various coastal landforms and varying depths.

While there do exist guidelines to aid how one should navigate to safety, having an intimate knowledge of the area is of great value.

[noelex 77]

Originally Posted by Insequent

But if there IS time, then go to deeper water, even if its just a bit deeper. As the stories above indicate and the formula allow you to calculate, a few hundred ft will be fine provided you aren't getting refraction, behind a choke point etc.

Can you quantify the water depth any more.

My impression is that in water as shallow as 20m (60feet) a "typical" tsunami wave would be unlikely to damage a cruising yacht, particularly if the wave was met head on.
Is this reasonable?

I realise deeper water will provide much more protection, to the extent the tsunami would not even be noticed, but even a very rough idea of the minimum depth would be helpful.

Anchored only a few miles from Santorini, as I am at the moment, is a stark reminder of the power that can unleashed.