Why the water pulls you sideways

When a wave breaks at an angle to the beach, it delivers a shove that has a component running along the shore. Do that thousands of times an hour and you get a river — the longshore current — running parallel to the beach inside the surf zone. This force, from the waves themselves, is typically an order of magnitude stronger than the wind blowing directly on the surf zone.

The strength of that shove is not proportional to the angle. It follows the sine of twice the angle, which means it peaks when waves arrive at forty-five degrees to the beach, and falls to nothing when they arrive straight in. Most people assume more angle means more drift right up to ninety degrees. It does not — a very oblique swell pushes less than a moderately oblique one.

That gap is the single most useful thing on this page. Refraction bends swell toward the shallows, which means toward facing the beach square-on, and by the time a groundswell reaches the break its angle is usually a fraction of what it was in deep water. Long-period swells, which start refracting much further out, straighten out the most.

So the days with vicious longshore drift are not necessarily the days with the most oblique forecast. They are the days when the swell reaches the break still carrying an angle — short-period windswell that never had time to bend, or a beach whose contours run at an angle to the coast.

Water piling into the surf zone has to return offshore, and it does not do so evenly. It finds the path of least resistance — usually a deeper channel where the waves are not breaking, because breaking is what piles the water up in the first place. That concentrated seaward flow is a rip current.

The research literature sorts them into four families, and knowing which kind you are looking at tells you how it will behave. Channel rips sit in gaps between sandbars and stay in the same place for as long as the bars do — these are the ones locals know by name. Focus rips form where offshore bathymetry concentrates wave energy onto one stretch. Flash rips appear and vanish on beaches with no channels at all, driven by instabilities in the current itself, and they are the ones that catch people out because there was nothing to see beforehand. Deflection rips run alongside a headland, jetty or pier, where the longshore current hits an obstacle and turns seaward.

The visual signature of the first two is a gap: a lane of darker, calmer water where the waves are not breaking, often with a rippled or churned surface, sometimes carrying foam and sand outward. That calm is not a safe channel. It is calm because it is deep, and it is deep because a river of water is running out through it.

Ocean Beach in San Francisco has a reputation for current that is entirely earned, and the reason is not the beach — it is what sits offshore of it. The beach lies in the shadow of the ebb-tidal delta at the mouth of San Francisco Bay, an enormous submerged sand body built by centuries of water flushing in and out of the Golden Gate.

The USGS ran a multi-year coastal processes study there starting in 2004, and its central finding is that wave refraction over that offshore delta — not the beach's own sandbars — sets the alongshore pattern of wave height. That is why Ocean Beach is famously different block to block on the same day: the delta is focusing energy onto some stretches and starving others before the swell ever reaches the sand.

The study also mapped something surfers feel without having a name for it: a persistent alongshore transport divergence, a point on the beach where the drift runs north on one side and south on the other. It is not fixed. Between 1956 and 2005 that divergence point migrated about 500 metres south while the transverse bar offshore moved roughly a kilometre north, as reduced sediment supply from the Bay contracted the ebb-tidal delta and changed the whole focusing pattern. The researchers even found that an exposed sewage outfall pipe scours a depression that measurably lowers wave height and reorganises the circulation around it.

The practical reading: at Ocean Beach the current is doing something structurally different at different stretches of the same beach on the same day, and which stretch you paddle out at genuinely matters.

Everything above is about currents moving surfers around. At one spot on this roster the current does something else — it changes the wave before it arrives.

Fort Point breaks under the Golden Gate Bridge, in the throat of the strongest tidal jet on the US West Coast, where ebb flows can exceed six knots. When that ebb pours out of the Gate against an incoming groundswell, it does not just oppose it. Wave action is conserved rather than energy, so as the current slows the wave's energy transport, the height climbs. At the same time the opposing flow shortens the wavelength. Both effects push in the same direction: the swell gets steeper.

The numbers are large. Against a 14-second swell, a 4.4-knot ebb shortens the wavelength from about 306 metres to 250 and raises the height from 2.5 metres to 3.1 — a 51% increase in steepness. Short-period swell fares far worse: the same ebb takes a 7-second swell to nearly three times its steepness, and a 4.5-second windswell is blocked outright, unable to make headway against the current at all. The rule of thumb our own probe settled on is roughly 10 to 15 percent more steepness per knot of opposing ebb.

There is a subtlety that makes the spot surfable at all. The current the wave is fighting is the throat jet, offshore of the break. The water at the takeoff itself sits in a weak back-eddy running under a knot. The wave is steepened by a current the surfer is not sitting in — which is why Fort Point can be jacking and heavy on an ebb while the lineup itself is manageable, and why it is one of very few places where the tide changes the wave's shape for reasons that have nothing to do with depth.