On a windy spring day at San Francisco’s Ocean Beach, Jet Ski rider Tim Elfers powered through the surf in a straight line, keeping his knees loose to bounce with the waves. Elfers was carrying sonar devices that ping the seafloor, in search of all the sand that was sucked off the beach by enormous waves over the winter.  “It certainly looked as bad as the last El Niño winter,” said Dan Hoover, research scientist for the U.S. Geological Survey. “To see something this year that looked comparable is surprising.”  Hoover was comparing the dramatic aftermath of a January storm to the winter of 2015-16, when huge waves caused the worst erosion on California beaches in 150 years, a USGS study found. That season, Ocean Beach narrowed by as much as 180 feet between the shoreline and the Great Highway that runs along it.