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My daily commute take me across the Evergreen Point Floating Bridge on Lake Washington. I have noticed on several occasions that the water on the southern side of the bridge will be quite choppy while the water on the northern side is relatively still (not completely still but there is a noticeable difference from the other side). What causes this to happen?

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Does it depend on the prevailing wind direction? – DumpsterDoofus Mar 28 '14 at 18:04
I have only noticed it when driving west after work around 5:15. And I have only noticed the south side being choppier than the north side. I am not sure about the wind direction. – prototypetolyfe Mar 28 '14 at 18:06
up vote 5 down vote accepted

This Washington State Department of Transportation page makes it clear that the choppiness is at the very least highly correlated with windstorms and high winds. This page is a good resource as well.

enter image description here

The choppiness occurs on the upwind side of the bridge; thus, a south wind (which blows north) will make the south side choppy. The reason for this is that the wind needs relatively large expanses of flat water surface to transfer energy into the waves. However, this energy is very poorly transmitted through the bridge, so that the wind must start afresh on the downwind side of the bridge.

One implication of this is that the bridge must swallow up all of the momentum in the waves, which puts it under tremendous stress. As you'll be aware, the bridge has not held up very well under that stress and is due to be replaced (partly) because of this.

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Thanks for the info. I would vote up, but I don't have enough rep here – prototypetolyfe Mar 28 '14 at 18:30
No worries. Thanks for the great question! There's some amazing pictures there. – Emilio Pisanty Mar 28 '14 at 18:31

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