Monday, January 23, 2012

The Convergence

We had a really interesting and rare set of processes that combined yesterday to bring some potentially destructive sea conditions to the North Olympic Peninsula. I haven't heard any confirmation of serious damage, though initial reports from La Push suggested the possibility of some damage to their harbor. In Port Angeles where I was three factors came together. First, we were at the tail end of the the last of a series of "king tides" for this winter. These exceptionally high tides occur when the sun, the moon and the earth are in alignment AND the moon is relatively close to the earth in it elliptical orbit. The gravitational forces combine to give us what are generally the highest tides of the year.

Additionally, a low pressure system passed over that coincided exactly with the predicted high tide, which elevates sea levels a bit due to the "inverse baramoter" effect (nicely explained by Cliff Mass here). And, because low pressure is often associated with wind, the strong southerly winds on the outer coast probably also acted to elevate water levels in the Strait of Juan de Fuca. In Port Angeles, local water levels were elevated by about 2 feet ABOVE the predicted high tide:

and the part of it that made yesterday's event interesting was that the peak in that water level residual corresponded almost exactly with the predicted high tide. In sum, the measured water level reached 9.3 feet above MLLW, only about a foot shy of the highest water level on record (from Jan 2 2003). Part of that is due to the fact that yesterdays convergence was at the tail end of the king tide series, and the predicted high was about 6 inches less than the high predicted from two days before. As I've said before, coastal flooding is a game of probabilities.

The final piece of yesterday's puzzle that made it potentially dangerous was the waves. Waves almost always approach from the west here, which typically turns out okay for Port Angeles since the western approach is protected by Ediz Hook. But every once in a while we get strong east winds that can generate waves in the eastern Strait of Juan de Fuca and push them straight into Port Angeles harbor. I am jealous that some even had the chance to surf in the harbor (and it wasn't from the Sequim Gazette)

My visual estimates suggested wave heights of about 4 feet, with a 5 second period. Our closest real-time wave measurements are made just south of the San Juan Island, where significant wave heights were about 2.5 feet with periods of about 5 seconds. Waves, of course, deliver the energy that often does the most damage during high water events. In this case the combination of very high water levels and large waves send water up and over the Olympic Discovery Trail (photos below also courtesy of the Sequim Gazette) and on to the public wharf.

Again, coastal flooding is a game of probability. When the processes converge is when we can see the ocean starting to influence us in ways that don't necessarily work to our advantage. The final part of the equation not considered here is sea level rise, and how the probability of damaging events changes with time as the mean level of the sea rises. More on that in future posts...

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