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...

Tuesday, January 10, 2012

What Sea Level Rise Looks Like

I've been doing a lot of reading of late, dredging up all of the peer-reviewed scientific literature on coastal climate change relevant to the outer coast of the Olympic Peninsula. There's not a lot, but there is some. People that spend there time thinking about and planning for climate change locally make much of tectonic uplift trends around here, which thus far have out-paced sea level rise on parts of the Olympic Peninsula (Neah Bay in particular). As a results, Neah Bay has a relative sea level rise pattern that looks like this:

Local sea level is effectively falling in Neah Bay, largely because the current sea level rise rate, estimated to be about 2mm/yr, is easily outpaced by uplift, estimated at about 4mm/year at Neah Bay. By contrast check out Seattle's relative sea level trend:

Uplift in Seattle is thought to be minimal, and the trend in local sea level measured there has been tentatively attributed almost entirely to rise in the true sea level due primarily to warming ocean water (the "thermosteric" component) and the melting of land-based ice.

The outer coast of the Olympic Peninsula is not entirely off the hook though, for a variety of reasons. First, many models of sea level rise suggest an acceleration in the true sea level rise rate during the coming century. My analysis of the available literature suggests to me that true regional sea level rise on the order of 1 m is quite likely by 2100, whereas extrapolation of the current estimated sea level rise rate to 2100 would suggest a rise of only about 20cm. Also, there is some evidence (though the literature isn't fully in agreement on this point) that waves, and particularly the storm-generated extreme waves, in the North Pacific are getting larger. The larger the wave, the higher the "run-up" pushed up on the beach, and the greater the likelihood of flooding. Finally, there is general agreement that storm tracks in the North Pacific will change over time (and there is some evidence that this is already happening), which could potentially alter our seasonal pattern of sea level driven by local winds and sea level pressure. All three of these processes could conspire to bring the influence of sea level rise to us much quicker than is suggested just by extrapolating the current relative sea level curves into the future. And the key point? The evidence suggests that we can't rely on relatively rapid tectonic uplift to save us, even in Neah Bay where the uplift rate is greatest.

Today's Peninsula Daily News ran a picture of beach at La Push today, and said that the school in La Push was closed yesterday and other actions were taken due to the threat of coastal flooding. This is what sea level rise looks like here. Its not inundation - its an increased risk of coastal flooding. Fortunately in this case there was no reported flooding.

The recipe in this case was two-part. First, a dose of parigean high tides ("king" tides) generated by the combination of the monthly high "spring" tides (when the moon is closest to the earth in its orbit) and the full moon (when the moon, sun and Earth are all in alignment). This special combination happens two times a year - once in the winter and again in summer. The second component was large waves. Here are the wave measurements made by a buoy off-shore of Aberdeen, WA:

Yesterday's 3 to 3.5 m significant wave heights are big. Here is the web cam shot from right now, with offshore wave heights on the order of 3m:

In this image waves are breaking a good distance offshore, but they still aren't really that big for La Push in the winter. You can see that on Friday significant wave heights crested 6m offshore- about 20 feet. Probably the biggest fortunate turn of events is that the "non-tidal residual" (the difference between the actual water level and the water level predicted from astronomical forcing alone) has been effectively zero at La Push over the last few days. Note the green line in these water level data collected in La Push:

You can make a case that this is unusual for the winter on the coast. Usually water levels are elevated above what would be predicted due to astronomical tidal forcing due to winds that pile water up against the coast and low sea level pressure. Together they can raise the local water level by 1 meter or more - and that may have been plenty to send water over the beach berm in La Push yesterday. Its a game of probability - bringing all of these different processes together. Sea level rise just slowly raises the probability that everything will line up and send water over the berm.

Monday, January 2, 2012

Ediz Hook getting a new dose of gravel

Just wanted to add a few views of the most recent gravel nourishment on Ediz Hook. The Army Corps contracted with Bruch & Bruch construction (a local P.A. based construction firm), agreeing to pay them $626,000 to dump and spread 50,000 tons (about 40,000 cubic meters - probably somewhere, in a sorta kinda way- around what the Elwha spits out during a big flood) of mostly gravel (with some sand mixed in) in the intertidal zone. For the two short (~500') hook segments over which the gravel is spread the hook gets about 20' wider. But its obvious that this material is quickly transported along shore (and, I am guessing a lot of it is transported off-shore), spreading out along the hook, due to the oblique wave angle. It would be pretty cool to tag some of these...

Its amazing to see the slopes maintained by the seaward edge of this gravel pile. Unlike most of our beaches, which are mixed, the "beach" formed by this temporary sediment pile is a true gravel beach, of the sort studied in the UK. Waves interacting with this beach crash impressively on the steep beach face, and standing on top of the pile you feel like you are going to get washed away by the run-up. But then, the whole wave just dies as the energy is dissipated into the void in the rocks. As a result, even though the beach is so steep, there is very little apparent reflection.