The importance of knowing the tides, and more

[eneumeier]

Before you launch:

Is the tide supposed to be going out, or in?

How high, or low?

How fast are the tidal currents supposed to be?

What are your bailout options at different tide levels?

Recognizing that the tide was not doing what it was supposed to be doing and reacting correctly to what they observed saved these folks:

http://news.bellinghamherald.com/stories/2...es/224950.shtml

Liz N.

[glad]

Very interesting story, Liz. Thanks for the post. Still it's a slightly scary decision to go out where it could have been huge breaking waves depending on the sea depth. I guess it's better than being slammed into something hard on the terra firma.

What do you other folks think?

[eneumeier]

They apparently knew the water was deep farther out and headed to it, i.e., they knew enough to realize deep water was the safest place to be.

Liz N.

[djlewis]

Scary? Maybe. But if you also know the depths, you know where even a very large wave is likely to break. What's the formula? The depth must be less than some fraction of the wave height to cause a break.

Actually, while searching for that answer (a wave breaks in water that is 4/3 its height), I found this site http://www.geosc.psu.edu/~sowers/waves.html

which says this...

Tsunami

A Tsunami (word originates from the Japanese words for a harbor and wave) results from a vertical displacement of earth in the ocean. The most common form of displacement is along faults and generally accompanies some severe earthquakes accounting for the label of a seismic sea wave = Tsunami. These waves have extremely long wavelengths (<200km) and are therefore shallow water waves (average depth of ocean = 5km which is much less than 200km/2 = 100km). They travel at speeds approaching 200 meters per second. These waves are nearly imperceptible in open ocean as their heights are only 60cm. However, as the Tsunami approaches land, the height can grow to about 30m. The waves are extremely destructive. Fortunately, a warning system was put into place in 1948 to alert countries which are about to be hit by a tsunami.

(Note the irony about the warning system.)

So, the guy really knew his stuff to go out to sea, namely that a tsunami is a very shallow wave and won't break in open ocean. Of course, ~now~ we all know this, and will know what to do in case of a North Atlantic Tsunami.

Not that it matters, but I wonder what the story is for a super-tsunami as some folks are saying might come out of the Canary Islands someday.

--David.

[djlewis]

Right... but according to that article, a tsunami is a ~very~ shallow wave, like 60cm, that is, about 2 feet. I had heard 3 feet for this one. So it is not going to break anywhere in open ocean except on a shallow shoal. All the energy is in the length of the wave.

--David.

PS: There's other interesting stuff in that article, like the first decent explanation I've seen of why there is a high tide on the side of earth opposite the moon. Answer: the earth and moon are really like two balls on the ends of a rod, rotating as a unit around a pivot point on the rod; so the oposite side tidal bulge is due to the centrifugal force from that rotation. Neat!

[djlewis]

Hmmm... maybe I'm wrong. This article says to seek 50m depth. Perhaps that's a safety margin, and maybe it's because in less depth, the wave height will still start to increase, even though there's no actual break. Wonder what's the function that maps wave characteristics (open water height, period, length) and depth into height in shallower water? My intuition still says a shallow wave, no matter how long in period and length, will not start to set up unless the water is quite shallow, well under 50m. But I'm not sure how accurate intuition is here.

Anyway... gotta get this off my mind and back to (paying) work ;-))

--David.

http://www.science.sakhalin.ru/Tsunami/

TSUNAMI SAFETY RULES

1. Most of tsunami are generated by earthquakes occurred under the ocean floor, more often at peripheral parts of the Pacific. Potentially dangerous parts of coast are the low lying ones situated along the beach in gulfs and bays with heights less than 15 m above sea level when distant tsunamis are observed and less than 30 m in case of a local tsunami occurring.

2. A tsunami is not a single wave, but a series of waves. Therefore, stay out of dangerous zone until all the waves are passed or an "ALL CLEAR" is issued . A danger of tsunami may exist for a few hours.

3. Pay attention to distant tsunami warnings. There were 61 human losses and a few hundreds injured in Hilo (Hawaii) in 1960, although tsunami warning was issued 10 hours before the initial wave arrival.

4. Any undersea earthquake occurred near coastline may generate a local tsunami. If you felt such quake, leave the beach immediately. More than 230 persons were lost in Japan in 1983, May when tsunami struck NW-coast of Honshu, although everyone could feel the earthquake and should be careful when received a warning.

5. Approaching tsunamis are sometimes heralded by noticeable rise or fall of coastal waters. Such sign always must be a warning.

6. Never come down to the sea to have a look at opened bottom or tsunami. Tsunamis can move faster than a person can run! It may be too late!

7. Taking in account the first tsunami signs, mentioned above, keep calm and quickly move to higher ground (from 30 up to 40 m above the sea level) away from the coast. Tsunamis can travel up rivers that lead to the ocean, so you must come up a slope and not along a riverside. If there isn't a higher ground you need come away from coastline on distance from 2 up to 3 kilometers.

8. If you are on a boat or ship and there is tsunami warning, move your vessel to deeper water ( at least 50 m).

9. When no major waves have been observed for two hours after the strong quake it may be assumed that the tsunami threat is over.