I’m sure we have all seen the horrific footage of the Japanese Tsunami pouring over sea-walls and going on to devistate cities. In some cases, those cities had truly massive sea walls - which did not save them, as the tsunami was simply too large.
My question is this: if the sea wall is marginally lower than the height of the tsunami, would it offer some protection? Or is it more like a dike that is too low - the sea floods in and it does very little good?
Say the sea wall was 8 metres high and the tsunami was 10. Would the wall do any good, minimizing the amount of water that gets in, or must the wall be higher than the tsunami to have protective effect?
It would certainly slow the flow. However, if the wave level stayed high enough long enough, then eventually enough would flow over to equalize the water level on the land side. Plus as the water builds up from 0 to 8m you have a bit more warning to run away.
(IIRC, they have buoys floating out to sea. WHen these see elevated sea levels, they set off the alarms. So you have maybe - i’m guessing - 3 or 4 extra minutes of alarm before it overflows the sea wall.)
You can watch the water overflowing the seawalls on several of the videos they keep running. Most are taken form high points, so I assume the camera operators had decent time to head for the hills before the water overflowed the seawalls. You can also see cars driving along the roads just before the water reaches them - what were those people thinking?
Don’t quote me on this, but I’m pretty sure that 2 metres of water pouring over the wall would end up doing about as much damage - bear in mind that in a tsunami the sea level rises for a considerable time, not just “one wave”, so the water will have time to pour over the wall and probably rise up almost as high as it would have done anyway.
Also, waves overtopping dams or walls are pretty good at destroying them. I’m not sure how the concrete sea walls stood up to the force of water, but once you have a rush of water full of debris, ships, etc going over the top of the wall I’d have thought it would destroy it fairly effectively.
There was an article about the sea walls in the Times on Sunday, and one expert was quoted as saying that even if the sea walls did not prevent damage, they did slow the waves and let more people escape, and so saved a lot of lives.