Monday, December 15, 2008

More blogging on Venice flooding

As I said before, Venice was flooded (again) in the morning, so we took a train out about 4 hours earlier than we had planned. I'm not terribly surprised, but CNN did a pretty bad job of explaining what was going on - this website gives context to the information I linked to before. I'm writing this on the train to Venice, because I actually downloaded this in the hotel room but didn't realize what I've downloaded:

Centro Maree forecasts the level day by day, summarized by tables and graphs. Here is how to read them:

* if the water level in St.Mark's basin is 65-68 cms., expect to find some water on the ground in front of St.Marks' church entrance. Important: do not think you'll see water 65 cms. high in St.Mark's square: this measure refers to the sea level.
* if the level is up to 85, expect some big puddles in St.Mark's square and in some "calli" (streets) in Venice. The middle area of St.Mark's square will be dry, as it is not perfectly flat. Wear water-proof boots!
* at 90 cms., St.Mark's square will be a lake of salty water. Some vaporettos (public boats) routes will be cancelled and alternative routes will be used. This is because they won't be able to pass under some bridges. Quite a confusion at boat stops. Lower areas in Venice can be accessed via special "walkways"
* at 110 cms, you'll hear sirens: this informs population to get ready. Most shops and warehouse at ground floors are likely to be flooded.
* from 120 cms to 140 cms, Venetians will get worried. Important: again, do not think you'll see water 120 cms. high all around in the streets: this measure refers to the sea level. Acqua alta can result in a 50 cms. water in the streets: which is not little anyway.

The point is that there is a "tide" in Venice that is about a third "regular old lunar" tide, half atmospheric effects, and some grab bag other issues like how much Antarctica's been melting this week. While we were there, the composition of the various sine waves meant that the tide was lowest at dinnertime.

If we read that chart backwards into the past (I couldn't find a "recent history of water levels" when I looked) then we get this story: when Rachel arrived at about 10:30, the water was near it's "second peak," about 100 cm - the high air pressure probably kept it lower the first day than it might otherwise have been. While she waited for me to show up in the train station, it peaked at 110/120 cm and then started going down. This, practically, meant that the plaza in front of the hotel and train station had one spot about a foot or so underwater, and that St. Mark's square was probably a foot underwater, maybe a bit more. The water went down about a foot while Rachel napped off some jet lag and by the time we took the "slow bus" (vaporetto) to St. Mark's square the square was above water, barely, and it kept getting lower while we were inside the chapel up until dinnertime. The air pressure then dropped in the evening and overnight, and that goofy siren about 8 AM was when the water passed 110cm again. When we went to get a train at 11, the water in front of the train station/hotel was back where it had been when I arrived the day before, or maybe an inch higher.

Now we're in Florence!

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