Japan hit by 8.9 quake and following tsunami

[Richard Hakluyt]

Was their any expectation that their breakwaters would be effective against a wave of that magnitude?

The problem is that one would need something more like a dam than a mere wall. With the tsunami's long wavelength the water keeps on piling up when it encounters a breakwater, eventually spilling over, though the breakwater will have had a mitigating effect.

I was wondering this very thing myself: would a sea-wall that was only slightly lower than the tsunami do any good at all (assuming it held)? From the videos I get the impression that it would not really - any more than a dike slightly lower than sea-level would protect Holland.

I don't think a seawall would necessarily work even if it was higher than the tsunami 

It seems to me that the tsunami almost works like a temporary river...........so for a barrier to be effective it would have to be both very strong and much higher than the wave. I mean a normal 30-foot wave can drown the incautious.......but does not behave anything like a tsunami wave.

[Warspite]

If a tsunami wave 10m high with a wavelength of 1 km hit a seawall of 9m height, it would be better than nothing at all, because presumably then only 1m height of water for 1 km length is spilling over, rather than all 10m for the same wavelength.

[grumbler]

Sounds like a breakwater could actually make it worse, holding back a rather huge amount of water until it inevitably fails, imparting that much more momentum to the mass of water.

If you think about it, that's not the way physics works.  The sea wall or breakwater actually robs energy from the water by slowing it down (and transfers the energy to whatever anchors the sea wall).  Energy lost this way isn't recovered.

If there was some kind of continuous force behind the wall of water, the sea wall could act like a dam, robbing the water of some energy but accumulating even more energy so that the failure would allow for a more concentrated burst of water, but tsunamis are rather instantaneous events, not continuous ones.

[grumbler]

I was wondering this very thing myself: would a sea-wall that was only slightly lower than the tsunami do any good at all (assuming it held)? From the videos I get the impression that it would not really - any more than a dike slightly lower than sea-level would protect Holland.

As I mentioned to Berkut, the conditions are completely different in the case of continuous pressure and instantaneous pressure.  If the dike around Holland is six inches lower than the height of the sea at some extraordinary peak tide, it would be infinitely preferable to no dike at all, because you could hope to limit damage using pumps and drainage canals until the tidal peak had passed.

[DontSayBanana]

I don't think a seawall would necessarily work even if it was higher than the tsunami 

It seems to me that the tsunami almost works like a temporary river...........so for a barrier to be effective it would have to be both very strong and much higher than the wave. I mean a normal 30-foot wave can drown the incautious.......but does not behave anything like a tsunami wave.

This is an educated guess, but I don't think the tidal current alone is enough to rip apart something solid.  In plenty of videos, the seawall was shown to be holding up, with the water going over it, rather than crumbling.

In the case of the buildings, by the time the water has reached average window-sill level, there's already a substantial force of water pressure on the outside of external load-bearing walls, while there's only air inside.  After that, internal walls can't hold the structure up by themselves, so the base crumbles, and the tidal current sweeps the building away before it can crumble completely- after that, it's battered by impact damage from coming into contact with other obstacles in the water's path.  So it seems like it'd be pressure damage, followed by impact damage, that wipes the building off the map- the pressure alone might be enough to crack the seawall, but it doesn't seem like it's enough to take it out completely.

Again, this is an educated guess.  I could be totally off.

[Malthus]

I was wondering this very thing myself: would a sea-wall that was only slightly lower than the tsunami do any good at all (assuming it held)? From the videos I get the impression that it would not really - any more than a dike slightly lower than sea-level would protect Holland.

As I mentioned to Berkut, the conditions are completely different in the case of continuous pressure and instantaneous pressure.  If the dike around Holland is six inches lower than the height of the sea at some extraordinary peak tide, it would be infinitely preferable to no dike at all, because you could hope to limit damage using pumps and drainage canals until the tidal peak had passed.

From what I've seen, tsunamis appear to act neither as instantaneous nor continuous events, but rather as something inbetween - they appear to last an appreciable time, sort of like a temporary shift in sealevel that lasts a half-hour or so. The wave when in hits shore may have a wavelength of 20 km or so, at least according to wiki:

While everyday wind waves have a wavelength (from crest to crest) of about 100 metres (330 ft) and a height of roughly 2 metres (6.6 ft), a tsunami in the deep ocean has a wavelength of about 200 kilometres (120 mi). Such a wave travels at well over 800 kilometres per hour (500 mph), but owing to the enormous wavelength the wave oscillation at any given point takes 20 or 30 minutes to complete a cycle and has an amplitude of only about 1 metre (3.3 ft).[19] This makes tsunamis difficult to detect over deep water. Ships rarely notice their passage.

As the tsunami approaches the coast and the waters become shallow, wave shoaling compresses the wave and its velocity slows below 80 kilometres per hour (50 mph). Its wavelength diminishes to less than 20 kilometres (12 mi) and its amplitude grows enormously. Since the wave still has the same very long period, the tsunami may take minutes to reach full height. Except for the very largest tsunamis, the approaching wave does not break (like a surf break), but rather appears like a fast moving tidal bore.[20] Open bays and coastlines adjacent to very deep water may shape the tsunami further into a step-like wave with a steep-breaking front.

Thus, when the wave hits a sea-wall that is lower than the wave, it floods over it and this flood moves inland - the presure of water crushing/pushing everything in its path. 

I don't really know whether having a sea-wall lower than the wave helps. The Japanese cities mostly had sea-walls, and got flattened anyway, but what is unknown (to me) is whether the damage would have been worse if they didn't have sea walls at all.

[Warspite]

I don't really know whether having a sea-wall lower than the wave helps. The Japanese cities mostly had sea-walls, and got flattened anyway, but what is unknown (to me) is whether the damage would have been worse if they didn't have sea walls at all.

Well imagine if a 9 metre wave with a 120 km wavelength hits a seawall that does not fail, but that is only 8.50 metre high. An intuitive visualisation would suggest the damage would be lessened. But perhaps this gets the fluid physics wrong.

[Slargos]

I don't really know whether having a sea-wall lower than the wave helps. The Japanese cities mostly had sea-walls, and got flattened anyway, but what is unknown (to me) is whether the damage would have been worse if they didn't have sea walls at all.

Well imagine if a 9 metre wave with a 120 km wavelength hits a seawall that does not fail, but that is only 8.50 metre high. An intuitive visualisation would suggest the damage would be lessened. But perhaps this gets the fluid physics wrong.

I would think that intuitively, the total amount of energy of the wave that hits the shore would be lessened by whatever is soaked up by the wall, but the water will eventually pile over the wall in a more or less continuous motion with little loss of energy once it has climbed the wall. This given the constant push of the back of the wave.

But then, I would also think that our intuitions are not very similar.

[Warspite]

I don't really know whether having a sea-wall lower than the wave helps. The Japanese cities mostly had sea-walls, and got flattened anyway, but what is unknown (to me) is whether the damage would have been worse if they didn't have sea walls at all.

Well imagine if a 9 metre wave with a 120 km wavelength hits a seawall that does not fail, but that is only 8.50 metre high. An intuitive visualisation would suggest the damage would be lessened. But perhaps this gets the fluid physics wrong.

I would think that intuitively, the total amount of energy of the wave that hits the shore would be lessened by whatever is soaked up by the wall, but the water will eventually pile over the wall in a more or less continuous motion with little loss of energy once it has climbed the wall. This given the constant push of the back of the wave.

Indeed, but are we then talking about a 10cm spill vs a 10m flow?

[Slargos]

I don't really know whether having a sea-wall lower than the wave helps. The Japanese cities mostly had sea-walls, and got flattened anyway, but what is unknown (to me) is whether the damage would have been worse if they didn't have sea walls at all.

Well imagine if a 9 metre wave with a 120 km wavelength hits a seawall that does not fail, but that is only 8.50 metre high. An intuitive visualisation would suggest the damage would be lessened. But perhaps this gets the fluid physics wrong.

I would think that intuitively, the total amount of energy of the wave that hits the shore would be lessened by whatever is soaked up by the wall, but the water will eventually pile over the wall in a more or less continuous motion with little loss of energy once it has climbed the wall. This given the constant push of the back of the wave.

Indeed, but are we then talking about a 10cm spill vs a 10m flow?

Most of our intuitive experience with the behaviour of overflow will naturally be from a sink or a bathtub, but even so I have to wonder what good a wall of anything but biblical proportions will do against a wave of water 200km deep.

[Malthus]

It strikes me that us guys speculating on this issue is about as useful as a group of neanderthals arguing about long division - we sorta have an idea about the topic, but no concrete notion of the math (and in this case physics) tools necessary to actually solve it. 

[Slargos]

It strikes me that us guys speculating on this issue is about as useful as a group of neanderthals arguing about long division - we sorta have an idea about the topic, but no concrete notion of the math (and in this case physics) tools necessary to actually solve it. 

In this hypothetical scenario, do you think the jew invents long division in order to create a system of economy that allows him to parasite off the hard work of the other neanderthals?

Regardless, your protest would stand in 95% of Languish threads, but why open that can when you know it contains worms? 

[Malthus]

It strikes me that us guys speculating on this issue is about as useful as a group of neanderthals arguing about long division - we sorta have an idea about the topic, but no concrete notion of the math (and in this case physics) tools necessary to actually solve it. 

In this hypothetical scenario, do you think the jew invents long division in order to create a system of economy that allows him to parasite off the hard work of the other neanderthals?

Regardless, your protest would stand in 95% of Languish threads, but why open that can when you know it contains worms? 

Heh, point; but in this case I do think there is a "right answer", but not one we are likely to get - except by accident. 

[Richard Hakluyt]

There is an elementary introduction in this short paper :

http://hal.archives-ouvertes.fr/docs/00/11/36/12/PDF/dias.pdf

I believe you are correct though malthus, fluid mechanics is a difficult field and I recall physicist friends moaning about it in their undergraduate days 

[Josquius]

Hmmm... that raises some questions with me then, as the Sendai plain has been hit with very large tsunamis in the past (there was one in the middle ages that IIRC was so destructive that it helped the Japanese win a war against the Ainu that still inhabited the area) and there had been predictions for some time that the area was overdue for another massive quake.

From what I read, the Japanese invested massively in breakwaters.  A coastal city just earned a place in the Guiness book of records for having the most elaborate and deepest breakwaters in the world.  The system was completely ineffective in this crisis and was destroyed. 

The nuclear plants were also protected by breakwaters that were not up to the task.  That's why the Japanese were caught off-guard.  They thought they had enough protection and the cooling system would remain intact. 

Way to ignore my post

http://www.japanprobe.com/2011/03/14/worlds-deepest-breakwater-failed-to-stop-tsunami/?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+JapanProbe+%28Japan+Probe%29

Several months ago, an award ceremony was held in the town of Kamaishi in Iwate prefecture. An official from the Guiness Book of World Records had come to proclaim that the Kamaishi Tsunami Protection Breakwater (63-meters deep) the title of world's deepest breakwater. The event was described in a blog post:

    The 1,960 m (6,430 ft 5 in) long breakwater was constructed at the mouth of Kamaishi bay to prevent from the threats of tsunami disaster from Pacific sea side. It reduces the bay-mouth opening to decrease tsunami run-up height as well as wind waves and swells. The construction work started in 1978 and it took amazing 31 years until it finally completed in March 2009.

    To celebrate the much anticipated completion as well as the Guinness World Records achievement, Kamaishi city held the ceremony on the sightseeing boat, Hamayuri. I was honoured to be invited to present the certificate to the city mayor.

    It was moving to hear from the people involved how much hard work had been spent for the construction. The citizens of Kamaishi are proud of their breakwater which protects their lives. It surely is an incredible world record!

Unfortunately, the record-breaking breakwater proved to be incapable of stopping the massive tsunami that hit the town on Friday:

    ...waves swelled over the barrier, engulfing buildings and cars and smashing everything in its path to smithereens in a matter of minutes.

    The images from Japan's Pacific coastline have been a scary reminder of nature's power. Kamaishi thought it had built just the thing to keep the forces of nature at bay.

    The concrete breakwater, nearly 207 feet deep, was designed to blunt an incoming tsunami. Its construction marked the culmination of decades of research on wave dynamics and dissipation. It stretches 6,430 feet and was completed in March 2009 after more than three decades of construction.

Videos of the tsunami destroying Kamaishi:




Norimitsu Onishi of the New York Times has written an article that is extremely critical of Japan's use of sea walls:

    The risks of dependence on sea walls is most evident in the crisis at the Daiichi and Daini nuclear power plants, both situated along the coast close to the earthquake zone. The tsunami that followed the quake washed over walls that were supposed to protect the plants.

    Peter Yanev, one of the world's best-known consultants on designing nuclear plants to withstand earthquakes, pointed out that the plants' diesel generators were situated in a low spot on the assumption that the walls were high enough. That turned out to be a fatal miscalculation.

    Critics have long argued that the construction of sea walls is a hubristic effort to control nature as well as the kind of wasteful public works project that successive Japanese governments used to reward politically connected companies. Supporters have said the sea walls increased the odds of survival in a quake-prone country.

    In Kamaishi, 14-foot waves surmounted the sea wall — the world's largest, erected a few years ago in the city's harbor at a length of 1.2 miles and a cost of $1.5 billion — and eventually submerged the city center.

    "This is going to force us to rethink our strategy," said Yoshiaki Kawata, a specialist on disaster management at Kansai University in Osaka and the director of a disaster prevention center in Kobe. "This kind of hardware just isn't effective."

Onishi is infamous for skewed and misleading articles he penned back when he served as the main Tokyo correspondent for the paper, so I'm not sure what to make of his article. It does seem obvious that sea walls failed to protect several towns from Friday's tsunami, but is it because the whole concept of sea walls is flawed, or because they were not built to stop waves from a magnitude 9.0 earthquake?