Science Response 2012/045

Transport of Japan tsunami marine debris to the coast of British Columbia: An updated review

Context

On December 9, 2011, Fisheries and Oceans Canada’s (DFO) Ocean Sciences Division (OSD) in Pacific Region requested that DFO Science, Pacific Region, provide information and advice regarding the transport of debris to the west coast of Canada from the March 2011 earthquake and tsunami in Japan. This request arose because OSD has received multiple requests from other federal government departments and agencies, the Province of British Columbia, and the media for information on the timing, location and quantity of debris generated by the earthquake and tsunami that might reach Canadian waters and shorelines.

In response, a Science Special Response Process (SSRP) was conducted on March 6 2012 based on existing information on the debris, including results from two independent ocean circulation models simulating debris transport in the North Pacific Ocean. The results of this SSRP are available online. It included responses to the following questions that were put to OSD:

1. When and where is debris from the 2011 Tōhoku tsunami expected to reach Canadian waters and shorelines?
2. What types of material are expected in the debris and what is the estimated quantity of material likely to enter Canadian waters and/or reach shorelines?
3. What monitoring of the debris is occurring while it drifts at sea from a Canadian/international perspective?
4. What risks, if any, does this debris pose for species, habitats, and ecosystems in Canadian waters? and,
5. What are the potential navigational impacts in Canadian waters?

The present report represents an update on the original SSRP which it now supersedes. This update is necessary due to several developments that have taken place since the original document was prepared. It was recognized in the original report that an update to the advice provided would be likely required as new information became available. Recent developments include observations of tsunami debris at sea, and the arrival of significant pieces of debris along the west coast of North America. Complementing these events, new model simulations have been made public. Whereas the simulations that were available previously were for objects with essentially zero windage, the new simulations examine the influence of windage on the transport of debris. In addition, rather than climatological statistics, the new model runs make use of the actual wind forcing over the Pacific from March 2011 through August 2012.

The questions considered in this update are as given above, and unchanged from those of the original SSRP. The following are the updated responses/conclusions given in response to these questions:

1. When and where is debris from the 2011 Tōhoku tsunami expected to reach Canadian waters and shorelines? Here it is necessary here to make a distinction based on the windage of the debris. Large objects that float well above the water and are subject to direct forcing by the wind have already arrived in Canadian waters and washed up on the shores of British Columbia. Model simulations show that such high windage debris started to arrive along the BC coast during the winter of 2011-12. Most of the debris, however, is thought to consist of objects with low to moderate windage. Based on model simulations, such debris from the Japan tsunami is transported more slowly and expected to approach the coast of North America during the second half of 2012, through the first half of 2013. Debris will likely continue to arrive at the coast over a large spatial area from Alaska to northern California for several years. Most of this debris will consist of small pieces rather than large objects owing to the effects of surface currents, winds, and waves. It is important to note that the debris generated by the tsunami will be in addition to the existing debris load floating into Canadian waters and washing ashore in British Columbia every day.Existing patterns of debris deposition on shorelines are not expected to change when debris from the tsunami begins arriving. Since the origin of the most debris washing ashore is not identifiable, the only indicator of tsunami debris may be an increase in the quantity of debris (by weight) washing ashore relative to the long-term average. It is unlikely that debris from the tsunami will enter the Strait of Georgia due to seaward flowing near-surface currents at the mouth of Juan de Fuca Strait and the barrier presented by the Gulf and San Juan Islands.
2. What types of material are expected in the debris and what is the estimated quantity of material likely to enter Canadian waters and/or reach shorelines? Both the quantity and composition of tsunami debris expected to reach North America are highly uncertain. Initial estimates of the mass of debris swept into the ocean ranged between 20 and 25 million tonnes. However, an updated estimate from the Government of Japan is that about 1.5 million tonnes of tsunami-generated debris remains afloat as of March 2012. While the composition of the debris is not known in detail, the Government of Japan estimates that the preponderance of the debris is construction material from houses that were swept to sea. As a result, it is thought that lumber, which is used extensively in Japanese buildings, represents an important component of the debris. Individual pieces of debris will vary enormously in size and weight, with the largest weighing hundreds of tonnes or more. The vast majority of objects are likely to be very much smaller. Based on existing knowledge of oceanographic processes and marine debris transport, only the most buoyant and durable objects will survive the trans-Pacific crossing and reach North America. Models used to forecast debris movements show that most of the low windage tsunami debris will remain in the ocean for many years and collect within an area of the Sub-tropical Convergence Zone referred to colloquially as the “North Pacific Garbage Patch”. It is unlikely that debris caught in the Garbage Patch will subsequently reach the coast of British Columbia.
3. What monitoring of the debris is occurring while it drifts at sea from a Canadian/international perspective? Debris swept into the Pacific Ocean was tracked by an environmental monitoring satellite for about one month after the tsunami. Attempts to locate debris with higher resolution satellite imagery are ongoing, but are limited to small patches of the ocean. In the absence of systematic monitoring of debris by satellites, opportunistic sightings by passing vessels were compiled and catalogued by the Government of Japan up to November 2011. Presently, the National Atmospheric and Oceanic Administration (NOAA) of the U.S. Dept. of Commerce is engaged in an ongoing effort to compile sightings of tsunami debris.
4. What risks, if any, does this debris pose for species, habitats, and ecosystems in Canadian waters? It is impossible to quantify the risk to marine species, habitats or ecosystems in British Columbia associated with tsunami debris and whether this risk surpasses any thresholds for effects. The baseline risks to marine habitats, species and ecosystems in Canadian waters from the existing marine debris load are poorly understood and documented and as a result the expected incremental increase in risks associated with the arrival of tsunami debris cannot be estimated at present. The potential does exist for the tsunami debris to act as a vector for the transfer of invasive species to the coastal waters of British Columbia. On the other hand, the risks from radioactivity on the debris associated with 131I and 137Cs originating from the Fukushima nuclear plant are believed to be low. Limited testing of tsunami debris have found that radioactivity levels were below detection limits.
5. What are the potential navigational impacts in Canadian waters? Navigational impacts in Canadian waters associated with marine debris are poorly known.The highest risk to navigation is likely related to large objects arriving in coastal waters or in shipping lanes, particular if they are floating mostly below the water line and thus largely invisible to ship’s radar and crews. This may include objects such as partially submerged vessels, shipping containers, docks, etc. The probability of houses surviving a trans-Pacific crossing intact is believed to be very low. Although drifting nets, ropes and other entangling debris from the tsunami pose a risk, this risk and the resulting impacts are likely incremental increases on the current navigational risks associated with entangling debris. Small objects (e.g., logs or small pieces of wood) are not believed to pose any additional risk to vessel traffic off the west coast of Vancouver Island.

This Science Response report is from the Fisheries and Oceans Canada, Canadian Science Advisory Secretariat, Regional Science Special Response Process (SSRP) of November 5, 2012 on the Transport of Marine Debris from the 2011 Tōhoku Tsunami to the west coast of Canada.

Complete document (PDF, 1083 KB)

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