The Bending Effect
As waves travel from deep to shallow water, they don't just slow down—they bend. The part of the wave closest to shore hits shallow water first and slows. The deeper part continues at full speed, causing the wave crest to pivot and align with the shore.
This phenomenon is called wave refraction, and it's critical for finding clean water.
The Physics (Snell's Law)
Wave speed in shallow water depends on depth: Speed = √(g × depth). As depth decreases, speed drops. This speed differential causes the wave to bend toward shallower water.
Same principle as light bending through a prism
How Refraction Works
The classic scenario: A wave approaches a headland at an angle.
- Deep water approach: Wave travels at full speed (√(gT/2π))
- Shallow water contact: Part of wave near headland slows first
- Speed differential: Outside portion continues faster
- Wave pivots: Crest line rotates toward the headland
- "Wrap" effect: Wave bends around the point into sheltered area
Energy Focusing vs Energy Shadow
Refraction doesn't just bend waves—it redistributes their energy:
Energy Distribution
At Headlands
Energy focuses
Waves converge, increase height, break harder
Behind Points
Energy spreads
Waves wrap, lose height, create clean glide zones
Why It Matters for Foilers
Headlands and points create sheltered zones with unique characteristics:
- Reduced wave height: Big swell becomes manageable bumps
- Aligned wave direction: Waves wrap parallel to shore
- Cleaner faces: Less chop, smoother surface
- Consistent spacing: Predictable bump rhythm
- Protected from wind swell: Only long-period groundswell wraps effectively
Finding Clean Water Spots
If the swell is large but wind is good, look for:
Refraction Zone Checklist
Find prominent headlands: Rocky points that extend into ocean
Check lee side: Ride in the sheltered zone behind the point
Look for crescent bays: Natural refraction amplifiers
Verify swell direction: Refraction strongest when swell approaches at 30-60° angle
Check bathymetry: Gradual depth changes create best refraction
When Refraction Works Best
Optimal refraction conditions:
- Long period swell: 12-18s wraps better than short chop
- Angled approach: Swell hitting point at 30-60° creates maximum bend
- Gradual bathymetry: Gentle depth changes allow smooth refraction
- Exposed headland: Point extends far enough into deep water
Summary
Look for points or curved bays when the swell is overhead. Waves will refract around the land, bending into the sheltered zone and creating cleaner, more manageable bumps. Refraction transforms big, messy swells into foilable glide windows.