How does an incoming or outgoing tide affect the waves?
If you've ever overheard surfers talking about the best conditions, you've probably heard conflicting opinions about tides. Some swear by incoming tides for more powerful waves, while others prefer outgoing tides for steeper rides. But here's a thought that might make you rethink your tide app: what if the direction the tide is turning isn't as crucial as we think? What if just the amount of water is what really matters?
There's a widespread belief that incoming tides somehow boost wave power, while outgoing tides undercut waves, making them steeper. It sounds logical, right? Water moving in the same direction as the waves should give them a boost, while water moving the opposite way should slow them down. But there are a few reasons that this idea is wrong in San Diego.
At most beach breaks, the tidal currents moving in and out from the beach are surprisingly small. They're often not strong enough to significantly impact wave dynamics in the way we imagine. Waves break when the water at their crest moves faster than the wave itself. This typically happens as waves enter shallow water, slowing down and steepening until they topple over. While it's true that currents can affect wave speed (think of those moving walkways at airports), the effect of tidal currents is often negligible compared to other factors.
Along the California coast, tidal currents don't follow the easy-to-imagine east-west pattern. Instead, they flow north-south, parallel to the shoreline. And although they’re not talked about in this way in the lineup, the tides are massively long waves; we're talking hundreds-of-miles-long waves driven by the sun and moon that travel in huge circles in the Pacific Basin. In San Diego, the parallel-to-shore currents caused by the tides are more than 10 times stronger than any water movement heading straight in or out from the beach.
This isn’t enough to make the waves break differently when the tide is rising or falling unless you are at the mouth of a bay, river, or estuary.
Tidal waves are so long that we as surfers can really only observe one thing about them: their height. Tide height directly affects water depth, which is crucial for how and where waves break. During high tide, there's more water over nearshore features like sandbars and reefs. This can lead to waves breaking closer to shore or not at all over certain features. More water means less interaction with the bottom, potentially resulting in mushier waves.
During low tide, we see the opposite. Shallower water causes waves to break further out. Increased interaction with the bottom can create more hollow, powerful waves. But it's not always that simple. Some spots work better at high tide, while others shine at low tide. It all depends on the specific bathymetry (underwater topography) of the break.
So, what's the takeaway for your next surf session? Pay attention to the tide height, not just whether it's “coming in” or “going out”. If you catch a great session, make a mental note of the tide level. Chances are, you'll find similar conditions when the tide reaches that same height again, regardless of whether it's on the way up or down.
And as always, the best way to understand your local break is to get out there and observe. If you play your cards right, someone will pay you to call this “field work.”
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