Why two breaks a mile apart get different waves

Waves slow down in shallower water. When one part of a wave crest crosses a shoal while the rest is still in deep water, the shallow part slows and the crest bends toward it — the same effect that bends light through a lens.

Over a headland or reef, that bending concentrates energy onto a small area, which is why points and reefs produce a defined peak in the same place every swell. Over a bay it does the opposite, spreading energy out and taking the size down. We compute that focusing per spot; it routinely ranges from about 0.6 — a break that loses nearly half the arriving energy — to over 1.1, a break that gathers more than what was sent to it.

A break tucked behind a point may be completely shadowed from a northwest swell and perfectly exposed to a south. Two spots a mile apart can face different directions, and that alone decides whether today's swell arrives with full force, wraps in at half size, or misses entirely.

Where a swell has to bend around a headland over miles to reach a spot, the amount it turns is a real calculation, not an approximation. Get it wrong and the model has the swell arriving from the side when in reality it has straightened out toward the beach.

Reading a regional forecast tells you what is arriving. It cannot tell you what your break will do with it, because that depends on ground the forecast never looks at. That is the gap this site is built to close: every break is modelled on its own measured seafloor, so you can compare them and go to the one that is actually working.