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March 13th, 2011

gfish: (Default)
Sunday, March 13th, 2011 11:17 pm
As those of you familiar with the Seattle area know, there are two bridges crossing Lake Washington, and they're both kind of unusual. They float. Lake WA is surprisingly deep, see, so piers would be difficult and an unsupported span would be much more expensive. They've become something of a local specialty, actually. 4 of the 5 longest floating bridges in the world are in Washington.

For the most part, they're just like any other bridge. An important difference is that occasionally they sink. (I still have trouble saying Evergreen Point Floating Bridge -- the Pentium bug was only a few later, so half the time I find myself saying Evergreen Floating Point Bridge.) The most obvious difference is that they're very low to the water. This means the water choppiness is usually different on either side -- whatever it's like on the windward side, the leeward side is dead calm. Very visually striking.

Now that I'm doing the vanpool to get to work, I'm commuting across 520 regularly for the first time in a decade. This has lead me to think about this effect. I realized that one reason it is so visually striking is that the choppiness of the windward side is artificially inflated. The bridge not only acts as a wave break, after all, it also is a wave reflector. So not only should you get twice the maximum amplitude on the windward side, but the wave pattern will be much more chaotic and irregular. Which, once I thought to look for it, it is. It always looks like the mess you get when the wind direction has changed, disrupting an established wave pattern.

I had idly been thinking about posting this, but wasn't sure it was quite interesting enough. Then there was a windstorm last Thursday which demonstrated the effect so perfectly, I just couldn't resist. As you can see, it was pretty dramatic: