PARIS • Last year's unusually potent El Nino produced monster waves that carved away record-breaking swathes of West Coast beaches in the United States, a study has said, warning that recovery may take years.
Erosion at 29 beaches from Washington to southern California during the winter of 2015/2016 was 76 per cent more than usual, by far the highest rate recorded, said the study published on Tuesday in the journal Nature Communications.
Most Californian beaches lost more sand compared with historical extremes, researchers said, warning that such events would become more regular as climate change exacerbates El Nino's effects.
"If severe El Nino events such as this one become more common in future... this coastal region, home to more than 25 million people, will become increasingly vulnerable to coastal hazards," said a statement from the US Geological Survey (USGS), which contributed to the study.
The research team found that West Coast beaches retreated 35m on average - and up to 55m at one beach in San Francisco. Typically, the average retreat is about 20m in winter.
Sediment lost is usually replenished by mild waves moving sand onshore from the continental shelf, and more is deposited by rivers, and through rock and cliff erosion.
The team found that West Coast beaches retreated 35m on average - and up to 55m at one beach in San Francisco. Typically, the average retreat is about 20m in winter.
"This El Nino produced waves that were, on average, about 50 per cent larger than during a typical winter," said USGS coastal geologist Patrick Barnard.
"Our largest peak waves are typically about 6m - this winter the largest waves were around 9m," he told AFP via e-mail.
Last year saw one of the three strongest El Nino events since record-keeping began in 1871. The others were in 1982/1983 and 1997/1998.
El Nino is a climate phenomenon that occurs every few years, alternating with La Nina, to respectively warm or cool parts of the Pacific Ocean and influence global rainfall patterns.
With El Nino causing "warmer sea surface temperatures in the eastern tropical Pacific, more evaporation and therefore more warm, moist air is transported to the mid-latitudes," said Dr Barnard.
"Essentially this means more fuel for more frequent and powerful storms to develop in the eastern north Pacific." Measured in terms of extreme wave energy, the 2015/2016 El Nino was the strongest ever recorded on the US West Coast, said Dr Barnard.
During such highly energetic events, sand can be transported so far offshore that it takes much longer than a season to migrate back, and can even be lost forever.
Most of the 29 beaches surveyed for the study have recovered poorly so far, Dr Barnard added.
Most shorelines are still 10m to 20m more eroded than before El Nino, leaving beaches vulnerable to wave erosion and flooding.
"It may take years for some beaches to build back," said Professor Peter Ruggiero of Oregon State University, a co-author of the study.
The situation was worsened by a long-running California drought, with little rain to bring sand to replenish the beach. After the 1997/1998 El Nino, it took some beaches a decade to recover, he said.
Said Dr Bernard: "With more extensive droughts forecast for the US southwest due to climate change, these kinds of impacts will probably become more frequent."
Climate change is predicted to cause more droughts and more powerful El Ninos, as well as higher sea levels - putting coastal communities doubly at risk.