double-up
Two separate waves merged into one; distinguished, as it breaks, by a midface step or terrace. A double-up wave is stronger and more dangerous than a regular wave, and can be totally unridable. But under the right circumstances, it might also provide the biggest, widest, most spit-filled tube of a surfer's life.
British surfer and PhD-holding oceanographer Tony Butt believes that double-up waves are linked, if not caused directly, to the existence of an outer sandbar or reef, which can divid...