I was surfing small waves on an out going tide this morning. It was good but as the tide ran out the rip spread over the bank and pretty much screwed it. Hard to find the line up, shifting peaks, general weirdness.
Great post. Where I surf I've heard other surfers say an incoming current helps increase the quality of the swell (including size) whereas an outgoing current "knocks the swell" down". Others have told me an outgoing current helps the waves stand up, which seems to follow the logic you've presented. So are incoming currents in no way beneficial for surf quality?
Thanks Mat, great question. This was actually one of the first ideas I had for an article. The short answer is that at most open coast breaks, tidal currents aren't strong enough to change wave shape much.
Tides along the coast propagate parallel to the beach, not in and out. On the West Coast, the tide moves as a wave from south to north with wavelengths of 100s to 1000s of kilometers. What we see as rising and falling water is the peak and trough of that wave passing by. The cross-shore component of tidal velocity is much weaker than the alongshore current, and it has to go to zero at the shoreline anyway. Otherwise we'd have water piling into our cities every tidal cycle.
This weak cross-shore current gets overwhelmed by wind-driven currents, wave-driven currents, and internal wave effects. On top of that, water depth itself, which changes with the tide, is usually a bigger factor in wave shape than any current.
The physics in the article applies where water gets squeezed through a constriction. Fort Point in San Francisco Bay, Ralph's in San Diego Bay, breaks near river mouths and lagoon entrances. At those spots, tidal currents absolutely change the waves. At your average beach break, they don't.
What surfers often perceive as "incoming tide makes it better" is probably the depth change affecting how waves interact with the bottom, not current-wave interaction.
As always, each spot has its own mix of wind, tidal motion, bathymetry, orientation relative to swell, wave period, etc. that make the waves respond more or less strongly to each of its factors.
I was surfing small waves on an out going tide this morning. It was good but as the tide ran out the rip spread over the bank and pretty much screwed it. Hard to find the line up, shifting peaks, general weirdness.
Great post. Where I surf I've heard other surfers say an incoming current helps increase the quality of the swell (including size) whereas an outgoing current "knocks the swell" down". Others have told me an outgoing current helps the waves stand up, which seems to follow the logic you've presented. So are incoming currents in no way beneficial for surf quality?
Thanks Mat, great question. This was actually one of the first ideas I had for an article. The short answer is that at most open coast breaks, tidal currents aren't strong enough to change wave shape much.
Tides along the coast propagate parallel to the beach, not in and out. On the West Coast, the tide moves as a wave from south to north with wavelengths of 100s to 1000s of kilometers. What we see as rising and falling water is the peak and trough of that wave passing by. The cross-shore component of tidal velocity is much weaker than the alongshore current, and it has to go to zero at the shoreline anyway. Otherwise we'd have water piling into our cities every tidal cycle.
This weak cross-shore current gets overwhelmed by wind-driven currents, wave-driven currents, and internal wave effects. On top of that, water depth itself, which changes with the tide, is usually a bigger factor in wave shape than any current.
The physics in the article applies where water gets squeezed through a constriction. Fort Point in San Francisco Bay, Ralph's in San Diego Bay, breaks near river mouths and lagoon entrances. At those spots, tidal currents absolutely change the waves. At your average beach break, they don't.
What surfers often perceive as "incoming tide makes it better" is probably the depth change affecting how waves interact with the bottom, not current-wave interaction.
As always, each spot has its own mix of wind, tidal motion, bathymetry, orientation relative to swell, wave period, etc. that make the waves respond more or less strongly to each of its factors.