Learn · The four levers · 5 min
How the tide moves the break
Why is my spot good at one tide and useless three hours later?
Example break
Mon Oct 19 · 8amAt Pleasure Point right now, the tide is 4.1 ft and falling, high for this fortnight, and the wave breaks in about 5.4 ft of water. This spot is listed as best on low to mid (first peak best low) tide.
- Tide fallingHigh for this spot this fortnight · NAVD884.1 ft
- Next low3.3 ft12:28pm
- Where it breaksWater depth at the break, at this tide5.4 ft
Forecast built Today 6:47am. Tide, its turns and the break depth are the forecast’s own, from this spot’s tide station.
Depth is what makes a wave break. A swell travelling through deep water barely notices the seafloor; as the water shallows the wave slows, steepens, and eventually trips over itself. The depth at which that happens is roughly a fixed fraction of the wave's own height, so if you change the depth, you change where it breaks.
The tide changes the depth of everything, everywhere, twice a day. That is why a spot can be excellent at one hour and pointless three hours later without the swell changing at all.
Why the break moves shoreward as the tide fills in
At low water the wave meets the shallow part of the reef further out, so it breaks earlier and further from the beach. Add five feet of water and the same wave carries further in before it finds depth shallow enough to trip it, and the whole breaking zone slides toward the sand.
This is also why the shape changes. A wave breaking over a steeper part of the bottom pitches harder; the same wave breaking over a gentler section spills. Move the break, and you have moved it onto a different slope.
When the tide drowns a spot entirely
Over a shallow sandbar, enough water can stop the wave breaking at all. The swell passes over the bank with too much depth beneath it, reforms closer in, and what was a punchy peak becomes a soft mush burger against the beach.
This is the single most common reason a forecast reading "3-4 ft" turns out to be worthless. The size was right. The water was too deep for that size to do anything at that bank.
A finding we retracted
We once reported that Ocean Beach breaks hollower at high tide. It did not survive checking. The result was manufactured by a slope estimator that measured the beach face at high water while the waves were actually breaking two hundred metres further out on a flat terrace — and the estimator's bias was itself a function of tide, so it invented a difference out of nothing. Measured properly at the breakpoint, the two tides are statistically indistinguishable across about 51,700 breaking events each. The one result that did survive: low tide gives longer makeable walls, on the median, the 90th percentile and the maximum, in seven of eight sections of the beach independently.
What to actually do with this
Every spot has a tide window where its particular bathymetry does its best work, and the window is different for a steep reef than for a shallow outer bar. Our spot pages list the tide each break prefers, and the forecast carries the tide alongside the size so you can see the two together rather than checking them separately.
The next piece is about the tide itself — why there are two highs a day of different heights, why it all shifts fifty minutes later tomorrow, and how to pick which high tide you actually want.
Solved at real breaks: Pleasure Point, Ocean Beach, The Hook. The physics behind these figures, its sources and its limits are written up in the methodology.