How old is that litter on the beach?

Christoph Gruetzner

How old is that litter on the beach?

One person’s trash is another person’s science. That’s the idea behind new research that uses litter left behind by extreme floods to help plan for similar hazards in the future. Once dumped into the environment, litter is subject to natural transportation processes and typically ends up in the sea, forming gyres of marine debris. While most of it ultimately sinks to the sea floor, a significant portion is washed up onshore, adding to the “wrack lines” of driftwood, kelp, and other natural flotsam deposited at high-tide marks. Extreme flooding events—such as those caused by tsunamis or storm surges—can also leave behind wrack lines. Now, researchers have proposed that these wrack lines could be used to identify past floods and map out their impact, thereby providing information on flooding in remote regions for which significant data are not usually available. Litter has an advantage over natural flotsam for this purpose, because trash often tells you how old it is: Just look for the production or expiration dates on discarded packaging. Because wrack lines containing trash must have formed after the most recent date found on the litter, scientists realized they could use that information to roughly date the flood that caused a particular wrack line. To demonstrate their idea, the researchers used GPS to map out litter deposits along the coastline near the city of Ras al Hadd in eastern Oman—a region vulnerable to both tsunamis and cyclone-induced storm surges. They identified two extensive wrack lines of litter (one of which is pictured) located as far as 900 meters inland. The youngest trash in each of the wrack lines was stamped with production dates of 9 January 2007 and 31 March 2010, respectively. That means the wrack lines likely originated in the floods caused by cyclones Gonu and Phet, which occurred slightly later in those same years, the researchers say. By knowing the maximum water level of past flooding episodes within a particular region—and how often floods occur—scientists can better predict the extent and impact of future floods, informing the development of worst-case scenarios, evacuation plans, and possible relief efforts.

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