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Riding out a tsunami

adam

Member III
This information might just come in handy someday so I have to ask.

Tsunamis in the middle of an ocean are harmless to a boat. They only become dangerous as reach the shallower water near the shore.

http://en.wikipedia.org/wiki/Tsunami#Characteristics

But from a boater's perspective the question becomes "how shallow is shallow" enough to be dangerous?

And a tsunami really is more like a very strong, and very quickly rising tide than a typical big wave. It's going to drag a boat along. But how many miles would a tsunami drag a sailboat?

If you're 3 miles from shore and the tsunami drags you 2 miles, you'll probably be fine, right?
 

AleksT

Member III
Except you aren't going to know in advance how far the Tsunami will push you. The predictions are good but they are not 100% accurate. In Marina del Rey they predicted arrival at 8:30 am but the big surges did not happen till around 9:30 or 10 am. More important would be knowledge of the sea floor shape. You should get into the deepest water you can.
 

adam

Member III
Of course one should head out to the deepest water you can.

My question is more along the lines of if you know a tsunami is on it's way, how do you calculate (approximate) how much time you need to get to safety?

Using Santa Cruz as an example:

http://maps.google.com/maps?f=q&sou...569,-122.012787&spn=0.54407,0.861053&t=h&z=10

Do you need to make it 10 miles (~2 hrs), past the edge of the continental shelf, in order to save yourself and your boat?

Or if you make it 5 miles out (1 hour), still in relatively shallow water, will the tsunami tide only carry you part way back to shore?

Except you aren't going to know in advance how far the Tsunami will push you. The predictions are good but they are not 100% accurate. In Marina del Rey they predicted arrival at 8:30 am but the big surges did not happen till around 9:30 or 10 am. More important would be knowledge of the sea floor shape. You should get into the deepest water you can.
 

AleksT

Member III
I don't have a chart for the Santa Cruz area and that is what you should be looking at to come up with a tsunami escape plan.
Looking at a chart for Los Angeles if I traveled about 6 or 7 miles out of Marina del Rey bearing about 215 I would be in about 180-200 feet depth. If my bearing was 220 I would be in about 600 feet of water.

So if i was having a pleasant day sail just outside the Marina and heard that a tsunami was about an hour away 220 is the direction I would head. Mainly because in an hour I possibly would not be able to get back to my slip tie up my boat then drive to a safe area.

Here is another concrete example. During the biggest surges in Marina Del Rey (a little over three feet crest to trough) there was a boat motoring in the center of the marina. (depth is about 20 feet) And he was doing just fine. I personally think his tactic was ill concieved.
 

treilley

Sustaining Partner
I recall the tsunami in malaysia boats anchored in the harbor faired very well.

I think the shape of the inlet also has some relevance.

A tsunami is not a moving wave of water until it reaches shore or near shore. It is a wave of energy. The water molecules in Japan did not travel to california, only the energy did. Being 3 miles offshore I would think you would only feel the rise and fall of the waves. At some point that energy needs to disipate and that is when it picks up the water and moves it. We get 2 10ft. waves here every day but it takes 6 hours for them to arrive. A tsunami is moving much faster and the energy needs to disipate very quikly or the water will run up onto shore.

Wave theory dictates that the water molecules in a moving wave travel in a circle, not a straight line when viewed from the side and generally return the their starting location after the wave has passed.

Everything I have ever read about tidal waves and tsunamis is never try to outrun them. Head into them. They are smaller the further from shore you are.

Some facinating reading in the book "The Wave". It tells the story of a 1,740 ft. wave that hit Alaska in the 50s. It also talks about Cresent City, CA which it defines as the worst place on the west coast for a tsunami as it is perfectly shaped to cause the most damage.
 

Mark F

Contributing Partner
Blogs Author
Hi Adam,

Stating the obvious, it depends how big the tsunami is. I hear in Japan the initial surge was a 30 ft cresting wave many miles out to sea.

Here in Santa Cruz some people left the harbor. One of them that left said that he and his wife got out "a few miles" and could see large swell coming through. They ended up not being able to return and went to Moss Landing (+/-10 miles). Good thing they got out of their slip as their dock was destroyed.

A slip neighbor told me the trough to crest difference was 6.5 ft and that was happening in 2 minutes 20 seconds!

The Harbor is still closed, if you try to move your boat it's a felony and $25000 fine. We are hearing the Harbor may open on Sunday.
 
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adam

Member III
A tsunami is not a moving wave of water until it reaches shore or near shore. It is a wave of energy. The water molecules in Japan did not travel to california, only the energy did. Being 3 miles offshore I would think you would only feel the rise and fall of the waves.

The distance doesn't matter, it's the depth. And a couple of miles may be beyond the continental shelf and 1000s of feet deep, or it could only be before that dropoff point and only be 100s of feet deep.

So, I'm trying to get a picture of what a tsunami would be like in 200 feet of water, or 50 feet of water.

Everything I have ever read about tidal waves and tsunamis is never try to outrun them. Head into them. They are smaller the further from shore you are.

I think that's certainly true within reason. But given 3 hours warning, that may be enough time to dock, and make it up a mountain before the tsunami hits.

I asked about Santa Cruz because it's a rather simple scenario. And, I want to keep is as a general question because I might someday be somewhere else on a boat when a tsunami hits.

But I'm based out of Alameda. And depending on where I am, I could be a half-day away from deep water. So, it's a slightly more complex question with 3 options.

a) Try to make it to deep water offshore.
b) Ride it out in the somewhat shallow Bay (50' depth)) assuming that you won't be dragged more than a couple of miles.
c) Rush to dock, and then head up a big hill.

P.S. I'm not trying to fear-monger here or try to pretend that this is a likely scenario any day soon, I just like being prepared with knowledge in case things go wrong.
 

treilley

Sustaining Partner
If you are in 50ft. of water I doubt you will be dragged far if you are not in a restricted channel. You are still more likely to just go up and down and a bit of surge and you should return to your original position. Everything I have seen/read indicates that you are fairly good if you are in an area that is generally twice as deep as the expected wave and not in a restricted channel. It does not break until you are in depths that approach the size of the wave or it runs up on shore and trips over itself.

A tsunami moves along the bottom of the ocean floor. This is why it dissipates by the time it crosses an ocean. It does not ride along the top like wind driven waves. So when it passes under you it simply pushes the water column up.

Santa cruz and other areas were hit hard because of the surge that the wave generated as a result of rapidly rising water in restricted areas.

Check out some of the videos on Youtube from California beaches. They show water rise and wash up further on the beaches but a couple hundred feet out is no problem.
 
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Sven

Seglare
Next time we head out

This is the second tsunami we've had while in Cabrillo marina. The Chili quake had a lot more impact in the marina, probably sending us up and down 2-3 feet (4-6 feet peak to trough) every 10-20 minutes. This one only had a 1-2 foot up/down.

Still, if there is ever another one I will not wait in the slip. If the leading trough drops enough for long enough we could hit bottom at low tide and that is no good. Likewise, a crest during high tide could start damaging the docks or the water might come over the breakwater and then all bets are off.

In retrospect I feel it was stupid to just stay on the boat working.



-Sven
 

Blue Chip

Member III
Adam...depends where you are on the Bay and how much ime you have...and how brave you are.
Check the depth in the Bay...places at the end of the Oakland Estuary are well over 60 feet...around Alcatraz well over 100 and near the GG Bridge...over 200 feet. I think if you picked the right spot and crossed your fingers you might survive a pretty healthy "bump"
Don't know if I want to test my theory, but IF I had too I'd be checking the chart and heading for the "downwind" side of deep water so the energy has time to disapate before it got to me.
Probably a theory that looks good on paper..but???
 

adam

Member III
Blue Chip,

On a sailboat, I think there are a _whole bunch_ of theories you don't want to have to test, but it sure is good to be informed. :)

If I'm out on my boat someday, and the call comes over the radio "tsunami expected in 2 hours", I feel I'm now much better informed about what to do.
 

exoduse35

Sustaining Member
The real key to survival here is :not to run into anything and don't run aground. You only need to be out as far as the surge can push in. In an area like a harbor the water is funneled in and has a large push, on the lee side of an island there is almost no push at all. Even being just offshore of a cliff shore is relatively safe as the water will not push onshore. If you look at all the news reports, all the boats were floating fine till the hit something or ran aground a mile or so Max inland. You only deed to get out of the harbor and a mile or two out then all will be good
Edd
 

Dan Morehouse

Member III
A Coast Guard video shot from a helicopter at Crescent City showed a fishing boat trying to high-tail it out of the larger outer harbor during one period of a surge receding. He was being swept about, and could be seen gunning his engine to dodge "oncoming" pilings. He managed to miss all the obstructions and exit the harbor, and you could see that once outside, he was going to make it.

The worst places were narrow channels, shallow water, and dead ends...like marinas! The boats that got smashed on the west coast were practically all in narrow channels, marinas, or otherwise tied up to land based structures. Just getting your boat out of the immediate harbor may not have helped you in Japan, though. So, for a big quake on the other side of the ocean...get out of the harbor to open water. For a big quake on this side of the ocean...get the %^!#% to higher ground!

Dan Morehouse
1981 E-38 "Next Exit"
 
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