Sunday, March 13, 2011

A cage full of dirt...

Yesterday we dove to set up a pressure sensor for recovery. This is the fourth time that this instrument has been recovered and each of the previous three times the instrument has been recovered by divers, leaving the cage on the bottom. This time was the last, and se we had a boat to pull up the entire thing: cage, instrument and weights. And thank goodness for that because we found the cage chock full of sand and gravel. I've had to dig sand and gravel out of there before, but typically just a little. This time it was filled to the brim and probably would have taken me a full tank of air just to dig the instrument out of there. Interesting, since there was not appreciable accretion on the sea-floor around the cage. Sign of a wave of sediment passing this location?

Friday, March 11, 2011

Time Lapse of PA Harbor during Tsunami

Using CHDK on my bottom of the line Canon PowerShot I shot about 60 photos on a time lapse setting (one every 30 seconds) of Port Angeles Harbor between about 2:30 and 3:00 PM local time. I got lucky - the periods of the tsunami waves propagating through the Strait of Juan de Fuca are about 30 minutes - so I captured a full wave before my POS camera failed. The currents generated within the harbor associated with these waves are fairly minimal, though there is a noticeable current at the entrance - maybe 2-3 knots. NOAA CO-OPS data suggests a wave amplitude at this time of about 50-60 cm. You sort of have to look close - focus first on the dock piling in the near field.

Tsunami on the North Olympic Peninsula

My son is 13 months old. Just after he was born the Chilean tsunami generated a wave that was visible on the shores of Santa Cruz, where we lived at the time. I dragged the family down to a building above the beach and we watched as the waves hit. It was visible, but damage was minimal, and it provided more of an ooh-ahh moment. I posted to this blog the day of that event.

Today's tsunami was much more significant for the west coast. Santa Cruz harbor has experienced damage in its harbor. Initial reports from Crescent City suggest significant damage. Here in Port Angeles I am getting reports of well over a meter (54 inches) of water level fluctuation, though thankfully without enough current to do any damage yet.

For those of us on this side of the Pacific this event will do some damage, but, on the scale of what we may experience in the future, this is a small event. Our thoughts must be first for those killed or missing in Japan, Russian and other parts of Asia most impacted by this earthquake and tsunami. Some of the video coming from Japan and Russia is astounding. Unlike most of the video that emerged from the Indonesian tsunami, much of what we are seeing is hi-def and taken from the air. It provides some insight regarding what we could experience from a Cascadia Subduction zone earthquake generated off of the shores of Oregon and Washington.

NOAA has generated a propagation model for this tsunami, the results of which suggest that Chile will see some of the most damaging wave heights from this event on this side of the Pacific:

The scale that they provide here, I assume (I haven't yet looked into the details of this model) tries to predict the wave height in deeper water. Tsunami waves are hugely long, many kilometers, but in the open ocean their heights can be just tens of centimeters. However, as they approach the continental shelf interactions between the energy propagated in the wave and the bottom causes the wave to slow and the wavelength to shorten. The wave responds by growing in height. My point in providing this brief explanation is to suggest that the scale on this model output shouldn't be taken as suggestive of the wave height at the shoreline.

What comes next are a series of plots taken from the NOAA tides and currents web site, generated by NOAA Center for Operational Oceanographic Products and Services. Each plot has three tracers: In blue is the predicted tidal water level, in red is the observed water level, and in green is the difference between the two. The green one is the one to pay attention to.

On this side of the Pacific it appears as of right now that Crescent City, CA is seeing the largest waves associated with the tsunami

Data from the North Olympic Peninsula illustrates how complex the propagation of these waves are. It appears that Port Angeles is seeing the biggest wave heights (though not by much), suggesting that the Strait of Juan de Fuca may be somehow funneling energy propagating from the Pacific. In Westport amplitudes are smallest:

Moving up the coast La Push saw some slightly larger waves:

but in Neah Bay they initial waves were a bit smaller:

The hydrograph from Port Angeles suggests that we are in the middle of the wave train:

and finally the most recent data from Port Townsend suggests that the wave is just reaching there, and has attenuated since reaching Port Angeles: