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Old 12-11-2012, 23:56   #46
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Re: Tsunami- Head Out To Sea?

In my home harbour, the effect most often presents as the water emptying out, more than a major rise. It takes anything from twenty minutes up. In extreme cases, ships end up sitting in the mud.

It's about twelve miles to the 30m contour, but we're so far from any of the usual tsunami generating locations (Lyttelton NZ) that we generally get plenty of warning, so I wouldn't hesitate to head out to sea in the usual scenario.
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Old 13-11-2012, 00:03   #47
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Re: Tsunami- Head Out To Sea?

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Originally Posted by noelex 77 View Post
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.
Here's a graph using the formula. Clearly shows that getting to 100 ft will be a big advantage over your likely marina/mooring depth
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Old 13-11-2012, 00:08   #48
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Re: Tsunami- Head Out To Sea?

If you are in 20m depth but in the mouth of an inlet or off an area of low lying land when a decent sized tsunami strikes the current would be a concern as the enormous volume of water flowed like a river into the inlet or over the shallow land. Best head further out if in this scenario.
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Old 13-11-2012, 00:19   #49
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Re: Tsunami- Head Out To Sea?

Thanks that graph is helpful.
I presume the vertical axis is the ratio of the height of the tsunami wave and the horizontal axis is depth of water in feet.

10m=0.42
20m=0.35
30m=0.31
50m=0.29


I am still not sure what might be survivable with only minimum damage?

If we assume a large destructive tsunami. Is a 1/3 height wave like this survivable by a yacht?
The tsunami wave is very different to an ordinary ocean wave and is not very high, but has in incredible power. It's very difficult to translate this to the effect on a yacht.
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Old 13-11-2012, 00:57   #50
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Re: Tsunami- Head Out To Sea?

Another aspect to consider is the speed. In deep water a tsunami might have a wavelength of 200 miles, but as it can travel at 800 mph then you see waves every 15 minutes.

Even a wave that isn't all that large will be much more destructive at the speed a tsunami is traveling, and as DM noted above the volume of water being moved is also a factor.

I have no first hand experience, but seem to recall people underway at sea but in shallow bays were OK in the Boxing Day one in Indonesia. I think in around 100 ft you would notice it, but not be adversely affected. But as you get into more shallow water the effect very rapidly increases. At 50 ft depth it might depend on where you took the wave as to how much damage there was - pooped and possibly sunk if stern to wave?
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Old 13-11-2012, 01:09   #51
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Re: Tsunami- Head Out To Sea?

Its not really a wave as waves go,

Imagine two waves a mile apart, with no low point between them, It just a solid mass of water that keeps coming at the same level, Thats why it goes so far inland, It is continually pushed by this solid mass of water, and pushed up by the shallows of the beaches,

If you cant get out to sea, Get behind a big Island or land mass, It will diminsh the wave power, but Keep in mind, it will circle around the land mass to where you are,

Sail out to sea as soon as it stops coming in, before the crap starts flowing out with it, Dead bodies, trees, houses, the sea will be full of garbage, and you wont be able to motor in it,
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Old 13-11-2012, 01:30   #52
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Re: Tsunami- Head Out To Sea?

Mr B is right, its more like a tide that rises and falls very quickly surging over shallow ground. In Samoa it didn't break till it hit the reef and then came across the shallows as a roaring wall of water. I was there 3 days after and was told by eye witnesses.
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Old 14-11-2012, 21:09   #53
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Since our boat is our home we would get out (if time permits safely) as far as possible as fast as possible. We have an emergency response radio that alerts us immediately-we heard as soon as the Japanese earthquake occurred and I knew there would be a tsunami. Luckily, we had little effect in Marina del Rey,CA (there is a big breakwater and a dog leg turn before our boat).

We asked someone from NOAA at a boat show and they said 400 ft. Now I see why from the chart shown In previous post. I'm sure more shallow water is safe for small events but if its a biggie I'd want to be in deep water-deep as possible.
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Old 14-11-2012, 21:15   #54
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Re: Tsunami- Head Out To Sea?

The videos of the tsunami in Japan gave me an insight into the power of a tsunami. It just kept rolling, demolishing everything in its path. I had no idea they could be that powerful.
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Old 14-11-2012, 21:48   #55
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Re: Tsunami- Head Out To Sea?

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Since our boat is our home we would get out (if time permits safely) as far as possible as fast as possible. We have an emergency response radio that alerts us immediately-we heard as soon as the Japanese earthquake occurred and I knew there would be a tsunami. Luckily, we had little effect in Marina del Rey,CA (there is a big breakwater and a dog leg turn before our boat).

We asked someone from NOAA at a boat show and they said 400 ft. Now I see why from the chart shown In previous post. I'm sure more shallow water is safe for small events but if its a biggie I'd want to be in deep water-deep as possible.
Hola neighbor. I'm in B Basin-Tahiti Marina
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Old 14-11-2012, 21:51   #56
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Re: Tsunami- Head Out To Sea?

The NOAA advice is misleading and way too conservative. Based on the experiences of the boats in Thailand during the Indonesian Tsunami, 40 ft was a safe depth, not 400. My friends were anchored in 10 meters with 50 other boats, directly facing the epicenter. They noticed a strong surge, then watched the shoreline restaurants disappear. The damage occurred to boats in marinas and those anchored in less than 5 meters of water.
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Old 14-11-2012, 22:33   #57
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Re: Tsunami- Head Out To Sea?

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The NOAA advice is misleading and way too conservative. Based on the experiences of the boats in Thailand during the Indonesian Tsunami, 40 ft was a safe depth, not 400. My friends were anchored in 10 meters with 50 other boats, directly facing the epicenter. They noticed a strong surge, then watched the shoreline restaurants disappear. The damage occurred to boats in marinas and those anchored in less than 5 meters of water.
How far does the sea get sucked out? Always thought it was unusal that so many people died when they went out to the receeded beach to collect fish. Clearly the region had previous Tsunamis and they knew what it meant when the seas dissappeared from the beach...
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Old 14-11-2012, 22:49   #58
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Hola neighbor. I'm in B Basin-Tahiti Marina
It is a small world after all. We are in A -Tahiti Marina on Friendship- 3rd at the end of the dock. Channel side. Were you on A side a bit back?
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Old 14-11-2012, 22:58   #59
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Re: Tsunami- Head Out To Sea?

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It is a small world after all. We are in A -Tahiti Marina on Friendship- 3rd at the end of the dock. Channel side. Were you on A side a bit back?
Too funny!!!! Hello! It's Jeffry from Southern Comfort! Was 6 boats down from you last year helping to keep the sea lions off the dock. I helped John Terry sail Best Day Ever to Cabo last November. You offered to crew for me on a delivery sometime!
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Old 15-11-2012, 04:07   #60
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Re: Tsunami- Head Out To Sea?

This was a pretty terrifying read when I came across it a while back.

Its an account of the 2009 Samoa Tsunami by SV Learnavity.
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