Tides, currents and the timing of a session

The forecast tells you swell height, period and direction, and wind. It does not usually tell you the thing that decides whether a spot works: what the tide is doing while that swell arrives.

Learning the tidal behaviour of a few local spots is the highest-value local knowledge in surfing, and it is free.

What the tide actually changes

Water depth over the bank or reef, which determines whether and how a wave breaks. Too shallow and it closes out or breaks violently on a shelf. Too deep and the wave never feels the bottom enough to stand up, going fat and unsurfable.

Every spot has a depth window it likes, and the tide moves you through that window over a few hours.

The strength and direction of currents. Water moving in and out of bays, estuaries and channels can run hard, and the strongest flow is typically in the middle of the tidal cycle rather than at high or low. A channel that is a benign paddle-out at slack water can be genuinely difficult two hours later.

The exposure of hazards. Rocks, reef shelves, and rips appear and disappear. The safe exit at high tide can be a rock garden at low.

The wave's shape. Many reefs are hollower on a lower tide and softer on a higher one, because the depth gradient the wave crosses changes.

The rough physics, briefly

Tides are driven by the gravitational interaction of the moon and sun with the ocean. Most coastlines see two highs and two lows a day, roughly, with the cycle shifting later each day.

Around full and new moon, sun and moon align and the tidal range is largest — spring tides. Around the quarter moons the range is smallest — neap tides. Spring tides mean more water movement, stronger currents, and a bigger swing between the shallow and deep states of a break.

Local geography modifies all of this enormously. Some coastlines have a range of a few tens of centimetres; others have many metres. Bays, estuaries and narrow channels amplify flow. This is why tide tables are published for specific locations and why a table for a port thirty kilometres away can be misleading.

Building local knowledge

Keep a note. Date, tide state, swell size and period, wind, and whether it was good. Twenty entries and the pattern for your local spot will be obvious.

Watch what regulars do. If the car park fills two hours before low tide on a certain swell direction, that is decades of accumulated observation being handed to you for free.

And ask. Most surfers will happily tell you when a spot works. Where they get cagey is exactly which sandbar, which is a different question.

The safety side, which matters more

Tidal currents catch people out far more often than waves do, particularly in estuaries, around headlands, and in narrow channels between islands or between an island and the shore. A swimmer or paddler who can comfortably make headway at slack water may not be able to make any progress at all at peak flow.

Two rules that cover most of it. Know which way the water will be moving when you want to come back — a current that helps you out will hinder you on the return, and being carried out easily is a well-known prelude to trouble. And plan open-water crossings around slack water, not around convenience.

Watch for a rising tide cutting off a beach or a shelf you walked along. Coastal access routes that are dry at low water are underwater at high, and people get stranded on this every year at every popular coastline.

Rip currents and tide

Rips are often strongest on a dropping tide, when the water that came in over a bank drains back out through the channels. If you are surfing a beach break with obvious rips, a falling tide is when they will be moving fastest — useful for getting out, more dangerous for a swimmer.

The practical habit

Look at the tide table before you check the swell forecast. That order sounds backwards and it is not: swell tells you whether there will be waves anywhere, tide tells you where and when to be.

Then pick your window and arrive early. The good hour at most spots is a genuinely narrow window, and the people who consistently get good waves are mostly people who understood the clock.