Deep Water Power
Long period swell forms in deep water, far offshore. The energy sits deep below the surface—sometimes 100 meters down. The waves travel fast, stay organized, and maintain clean lines across thousands of kilometers.
This is why groundswell from distant storms arrives at your beach days later, still carrying power.
The Decay Zone
As the waves reach the continental shelf, everything changes. The bottom of the swell touches the sea floor. Friction slows the wave. The period shortens. The wave energy becomes compressed vertically, creating steeper faces and tighter spacing.
This is called shoaling—the process where deep water waves transform into shallow water waves.
The Physics
Wave speed in deep water depends only on wavelength. But in shallow water, speed depends on depth. As depth decreases, the wave slows, the wavelength shortens, and the period drops.
Formula: Shallow water speed = √(g × depth) where g = gravity
What Foilers Feel
Period decay increases bump size and reduces spacing. The water becomes more vertical. You need tighter footwork and faster reactions. The glide window between waves shrinks.
On a long-period swell (14s+) offshore, you can cruise between bumps. Near shore where decay happens, that same swell becomes steep, punchy, and unpredictable.
Decay Stages
Deep Water
14s period
Wide gaps, smooth faces
Easy gliding
Mid Shelf
10s period
Closer spacing, building faces
Moderate challenge
Shallow Water
7s period
Tight spacing, steep faces
Advanced riding
Riding Strategy
If you want smooth, long-period bumps:
- Ride further out: Stay in deeper water before decay starts.
- Check bathymetry: Gradual shelves create gentle decay. Sudden ledges create violent shoaling.
- Choose your spot: Points and headlands often have deeper water closer to shore.
Practical Tips
Use smaller foils in shallow decay zones (more maneuverability)
Larger foils work better offshore where bumps are spaced wide
Check tide charts: Low tide creates more shallow-water effects
Summary
Long period swell becomes messy and compressed near shore. Ride further out to find longer, cleaner bumps with wider spacing. Understanding decay helps you choose the right position and equipment.