Monday, November 26, 2012

The Vexing problem of "tsunami debris"

The debris associated with the March 2011 Tohoku tsunami in Japan continues to turn our attention to the dynamics of the great ocean to our west. It must be the first time in history that the world's attention has been riveted by reports of single debris items (like last month's report of Hawaii's first confirmed item from the tsunami) washing up around an ocean basin. Its striking to watch this issue unfold and in particular how the uncertain science and modelling of wind- and current-driven transport across the ocean interacts with the concerns of communities. Above all it seems that communities, already struggling to wrap their arms around a host of crises, are wrestling with how much attention to pay this issue. Some sense of certainty would be nice...that is clear.

But for all of the oceanographic advances of the last few decades, the dynamics of the ocean still hold some mystery. As a result, most of the planning at the community and state level that I've been exposed to has adopted a very reasonable adaptive response capacity...the response will depend on what happens and given the huge number of scenarios that one can imagine its hard to outline one specific response plan. This approach may start to look like the wrong one IF we suddenly get a massive influx of debris that strains our capacity to mobilize resources quickly and effectively...but in the face of uncertainty it is difficult to justify doing otherwise.

At this point, a year and a half after an estimated 1.5 million tons of debris floated away from the coast of Japan, the uncertainty remains as large as ever. Debris will make landfall - that is a certainty if only because it already has. But the real questions are, "How Much?", "Where?" and "Will it cause damage, especially immediate damage?". This last question is not at all clear - the ecological effects of debris accumulation on beaches are not well researched or established. This leads to the fourth question, "Will the debris be of a kind (i.e. toxic or large) or volume that will overwhelm the capacity of existing systems to deal with it?".

So where is it all? What prompted me to sit down and write this blog was Dr. Curtis Ebbesmeyer's recent statements in the Peninsula Daily News. Dr. Ebbesmeyer is a noted debris-tracker who tracks and analyzes interesting flotsam from around the world, and he correctly surmised the approximate timing of landfall of the first items from the Japanese tsunami to make landfall in Washington State. Last week Dr. Ebbesmeyer was quoted in the Peninsula Daily News as saying, "the first big field of tsunami debris is about 400 miles off the Washington coast and probably will land on beaches next month". Dr. Ebbesmeyer suggested that this statement was based on reports from off-shore fishermen combined with results from an OSCURS ocean model. The Pensinsula Daily News then added that "Ebbesmeyer speculated that the main debris field, which is 2,000 miles long and 500 miles wide, could include items as large as buses, cars and parts of houses."

I think that what struck me about this is what an ominous picture it painted - a field of debris 2000 miles long by 500 miles wide is only 400 miles off-shore and headed for our coast. The implication, at least to me, was that we should expect all of this to come ashore shortly. The image conjured is of an overwhelming volume of debris pasted to the beach up and down the coast. However, other oceanographic modelling efforts suggest that we may need to paint a more nuanced picture to help us prepare. More and more I've been turning to the SCUD model results produced by the International Pacific Research Center (IPRC) at the University of Hawaii, and particularly some of their more recent analyses that integrate what was learned from some of the early landfalls of wind-blown debris. The video at the top of this post, for example, is a model hindcast of debris dispersion for debris items with various exposure to the wind (the sizing is odd - just click on the part you see and a full screen version will pop up).

I could stare at this animation for a long time, but I found that looking at model results for the various levels of windage in isolation was interesting and instructive. For example, the animation below shows only the modeled trajectories of particles with lots of wind exposure, and is consistent with the first arrival of the lightest items (floats, etc.) that were found in November of 2011 on Washington's coast. This animation also suggests, however, that most of this high-windage material had already moved away from the Washington coast by last summer:

Looking at items with slightly less windage we find a similar pattern, but see that we could have expected that most of this material might have come ashore in the late spring or early summer of 2012...exactly when a small bump in the quantity of debris on Washington's beaches was anecdotally reported by a variety of observers.

The no-windage category suggests that items that are not directly exposed to wind are moving much more slowly across the Pacific and are likely still well away from the coast:

But it is the mid-range windage items that may be the next big thing on our coast, and it is this debris that I assume Dr. Ebbemeyer was referring to. The model results for the 2% windage category (what exactly this means isn't entirely clear to me - but I will assume that it is items that are somewhat less buoyant than a plastic float, and somewhat more buoyant than something that is floating at or below the water's surface) suggest that some portion of this material is still making its way westward, even as most of it has already entrained in the broad southward moving California Current and is heading southward on its way to eventual entrainment in the North Pacific Sub-tropical Convergence Zone:

It seems quite plausible to expect, based on these results, that some of this debris may come ashore...but again we are really interested in how much, and there is no way to really predict that with much certainty based on any of the data sources that I am familiar with. For starters, its not at all clear what fraction of the total debris out there falls within this windage category (and therefore might be lingering just off-shore). Its also not clear exactly how windage affects how efficiently debris can be moved on to the beach from locations just offshore - it is thought, for example, that local coastal winds may play a big role in driving material on shore. If debris windage is reduced, therefore, it may not be blown on to the beach as easily. Couple all of this uncertainty with inherent model uncertainty and its easy to throw your hands up and walk away from the whole thing. However, if we make the assumption that the model tells us SOMETHING, than what is suggested by these model results is a small fraction of the total debris in this category should be expected to near the Pacific Northwest coast, and that some fraction of it could make landfall on Washington beaches associated with our typical winter "downwelling" winds.

But what these model results do not seem to support is that the entire field will come ashore, much less in Washington. Will we see more "tsunami" debris items...almost certainly. Will we see individual large items (i.e. cars or buses)? I have no idea, but it seems like it is still within the realm of possibility. Will we see huge quantities of debris? Well, that depends in large part on your definition of "huge", but these IPRC models, as well an analysis that I published a few months back, both suggest that the lions share of this debris will still end up in the Sub-tropical convergence zone and not on Washington's shores.

So now its time, as winter ramps up and the southwest winds blow, to test all of these model results against observations. Since I am spending far too much time inside these days, please, PLEASE, let me know what you are seeing on the beach...

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