Tuesday, January 19, 2010

Alki Beach

What happens when you add a foot and a half or two feet of sea level rise to a normal January on Puget Sound? You get this past week. Normally inoccuous 12' tides became 14' tides. Tide levels that we normally only see once or twice a year happen every day for a week! The bathtub starts to overflow and places get wet that normally don't.

Thursday morning I went out to Alki Beach. The water was pretty calm - save for the occasional boat wakes. The tide was predicted to be 12.0'. It arrived around 8:30 just short of 14 feet and began to spill out onto the sidewalk. The drain notches in the seawall must be around 14', although the wall isn't quite level and some sections dipped a little lower than others. The sandy berm on the beach must also be just over 14', since it took a little bit of wave action to push the water over the top and back into the volleyball area.

This week, folks on Camano's Driftwood Shores and on Bainbridge's Point Monroe were reminded of their vulnerability to the tides and to the old real estate adage of "location, location, location" (they are both sand spits). King County had problems with one of its pump stations, too.

So what's going on? Starting a little more than a week ago, the observed tides in Seattle started diverging from the predicted ones. And for the last week, the 1.5-2' difference has been pretty continuous. One obvious explanation is a series of low pressure systems that have driven the barometer down and kept it down (atmospheric pressure has a direct effect on sea level). The pressure plummeted at about the same time the tides started rising.

During the 82-82 and in 97-98 El Ninos, sea level on the west coast rose significantly during the winter, bringing many more extreme tides than in normal years. This has been attributed to atmospheric conditions, to warmer ocean water, and to things called Kelvin Waves that raise water levels right along the coast. We don't know yet what is going on this year - and low pressure is certainly the simplest explanation - but stay tuned. And if the pattern persists, and you live on a sand spit, you may want to be prepared to wring out your carpets the first week of February, when even the regular tides are supposed to reach 13'!

The tide plot is from NOAA's Tides and Currents website (Seattle) (most folks around here will want to use Local Time and Feet above MLLW, not Greenwich Time and Meters)! The photos in this post were taken on the morning of the 21st.

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