This thread got me surfing about tsunamis again (no pun intended). When I wrote that if you're far out at sea, you just go up for a while and then down for a while, it wasn't entirely accurate, as it sounded like you're going a long way up and then a long way down. In deep water, even major tsunamis actually have a small surface amplitude -- just a few feet.
So that raises the question -- if they're only a few feet high when they start, and they don't build (unlike wind-caused waves), why are they so devastating on shore? It's because of a crucial difference. Wind-caused waves, the kind we are used to, are caused by a surface influence, and so they don't really affect the water much deeper than the trough. If you are swimming in a two-foot chop and you dive ten feet down, you'll notice the water is perfectly still.
Tsunamis, on the other hand, are caused by something either at the bottom or reaching the bottom (in the case of a landslide) and therefore extend all the way from surface to bottom. That's a huge amount of water -- I read somewhere yesterday that the sea-bottom earthquake of Dec 26, 2004 displaced seven cubic
miles of water. The waves also travel very fast in deep water -- several hundred kilometers per hour, which was how they made it to Banda Aceh in 15 minutes, India in two hours and Africa in seven. In shallower water, this tremendous energy is forced upwards, so they lose speed but gain height.
Surfing around, I find that the earliest recorded tsunami was recounted by Herodotus as taking place in 479 BCE at Potidea -- saving the citizens from Persian invaders. The Persians had taken advantage of a suddenly-dry sea bottom (receding waters are often the first effect of a tsunami) to storm the city's sea wall, unaware -- like many intrigued tourists in Indonesia and Thailand exploring an exposed sea bottom -- that the sea would come crashing back with a vengeance.
Author Benjamin Franklin Howell cites another author who catalogued 141 tsunamis in the Eastern Mediterranean starting in the second millennium BCE, which suggests to me that the Greeks might have been familiar with them; Mary Renault seemed to think so when she wrote
The King Must Die, in which she portrays Theseus, who considers himself a son of Poseidon "Earth-shaker," as having a psychic ability to sense impending earthquakes. I haven't read it for a while but as I recall, he has a clue about tsunamis. The great eruption that blew apart Santorini, aka Thera, and destroyed the Minoan civilization (or so it is theorized) has a starring role in the book also. It's hard to imagine that the Greeks and thus Alexander didn't have knowledge, at the very least folklore, about tsunamis.
Athena's Owl wrote:The Indus is at this time (i am aware that it had a different mouth at the time) is tidal for nearly 60 miles up river, so the idea that Alexander was not aware of tides before the groundings occurred seems illogical to me. But then tides are part of my daily existence. I can not imagine that Alexander and his captains did not notice the daily ebb and surge of the river as they traveled down it. Increasing in strength the closer they got to the mouth.
Couple of theories: the delta wasn't as big then, so the tidal effect didn't extend 60 miles upriver; and, the fleet travelled fast enough downriver that they didn't beach within the tidal zone until they were in the ocean proper. A trireme apparently could move at 6 knots, which is slightly more than six miles per hour, so that at full speed, and especially going downstream, the fleet could have made even 60 miles in ten hours. I doubt Alexander, eager to see Ocean for the first time in his life, would have had the fleet travelling at anything less than max.
Even though I was way ahead of myself, I wrote this scene for my Alexander novel, and had fun with it. ("This [local] fellow wasn’t used to schooling his expressions; clear as day on his face was the thought, ‘You are a mighty king, all in gold and silver with so many ships and so huge an army, and you don’t know about
this?’ ") Just as people long mistook tsunamis for "tidal waves," I have Alexander first assume, when he finds out the sea has dropped... that it's a tsunami
A fond wave to all,
Karen