Groundswell Decay Curves – How 12s Period Becomes 8s Near the Beach
You check the buoy reading. It says "4 feet @ 12 seconds." You grab your light wind gear, expecting long, powerful glides. You arrive at the beach, and the waves are short, punchy 8-second chop.
What happened? The period didn't change (physics says period is conserved), but the energy did. The long-period power was filtered out by the seabed.
The Bottom Friction Filter
Long period waves (swells) have deep roots. A wave "feels" the bottom at a depth equal to half its wavelength.
- 12s Swell: Wavelength ~225m. Feels bottom at 112m deep.
- 8s Chop: Wavelength ~100m. Feels bottom at 50m deep.
As the swell travels across the continental shelf, the 12-second energy drags on the sea floor for miles. The 8-second energy floats on top, untouched.
By the time the set reaches the beach, the long-period energy has been attenuated (weakened) by friction. The shorter period waves arrive with their full energy intact. The "Peak Period" of the spectrum shifts from 12s down to 8s.
Refraction and Shadowing
Long period waves bend (refract) more than short waves. If your spot is tucked behind a headland or island, the 12s swell might wrap 180 degrees and lose all its power in the turn.
Short period wind swell (the 6-8s chop) travels in straight lines. It ignores the refraction. It hits the beach directly.
This explains why a "protected" spot often has terrible wave quality. The protection filters out the clean swell but lets the messy wind gusts chop through.
Implications for Wingfoiling
When you see a forecast for long period swell (14s+), you need to know your bathymetry (seabed depth).
- Deep Water Spots: You will feel the full surge of the swell. You need a wing with good flag-out stability to ride the speed.
- Shallow Shelf Spots: The swell will be slower and smaller than predicted. The 15–25 knots of wingfoil wind will generate local chop that overpowers the decaying groundswell.
Summary
Don't trust the offshore buoy blindly. If you have a wide continental shelf, the Groundswell Decay Curve will eat your period. Expect the waves to be smaller and shorter interval than the deep-water forecast claims. Rig for the wind speed, not the wave height.