Overview
Beaches undergo dramatic seasonal changes due to varying wave activity. Wave energy determines sediment transport direction, creating wider summer beaches and narrower winter ones through swash and backwash processes.
Seasonal Beach Changes
- Summer beaches feature wide berms from light wave activity favoring swash over backwash.
- Light waves move less water onto beaches, allowing more infiltration than runoff.
- Sediment carried landward drops on shore, building up wide berms for recreation.
- Winter brings heavy wave activity with energetic waves breaking fast in succession.
- Excess water overwhelms beach infiltration capacity, increasing backwash and removing sand.
- Removed sand forms longshore bars just offshore rather than disappearing completely.
- Rocks visible in winter were present year-round but buried under summer sand.
- Summer beach formation takes months; storms destroy them in hours.
- Dredging longshore bars eliminates sand supply, creating permanent winter-like conditions.
Wave Activity and Beach Formation
| Wave Type | Berm Size | Formation Time | Beach Width | Destruction Time |
|---|
| Light Wave Activity | Large berm | Weeks to months | Wide beach | N/A |
| Heavy Wave Activity | Reduced/destroyed berm | Hours to days | Narrow beach | Hours to days |
- Light waves gradually build berms through slow sediment accumulation processes.
- Heavy waves rapidly erode berms through powerful backwash exceeding infiltration rates.
- Same beach experiences both conditions depending on seasonal wave patterns.
Longshore Drift and Currents
- Longshore drift moves sediment parallel to shore in zigzag patterns.
- Beachgoers experience longshore currents by drifting from original entry points.
- Currents transport humans and sediment efficiently along coastlines.
- Wave refraction aligns crests nearly parallel but waves break at angles.
- Angled swash pushes sediment toward berms; gravity-driven backwash flows perpendicular.
- Sediment pulled perpendicular away, pushed back at angle creates zigzag motion.
- Down coast indicates current destination; up coast marks sediment source.
- Current direction varies based on incoming wave approach angles.
- Longshore currents transport massive sediment volumes at speeds up to 4 km/h.
- Transport only occurs in surf zone where waves feel bottom.
- Deep water prevents wave refraction and sediment movement.
- Longshore drift depends on erosional-depositional balance and water depth.
Rip Currents
- Breaking waves create swash running toward berms; some returns as backwash.
- Backwash typically flows along ocean bottom as undertow or sheet wash.
- Rip currents occur when backwash flows back at water surface.
- Currents range 15-45 meters wide, flowing 7-8 km/h (2 m/s).
- Olympic record speed for 50m averages 2.4 m/s, barely faster than rip currents.
- Currents extend hundreds of meters offshore before dissipating.
- Visible from elevated positions as regions disrupting incoming wave patterns.
Safety Guidelines
- Never swim directly against rip current toward shore; exhaustion causes drownings.
- Swim parallel to shore, perpendicular to current flow direction.
- Swimming against current requires near-Olympic speed just to stay stationary.
- Observe water from vantage points to identify rip current locations.
- Look for regions breaking up incoming waves as rip current indicators.
Key Terms & Definitions
- Swash: Water movement up beach toward berm after wave breaks.
- Backwash: Return flow of water from beach back toward ocean.
- Berm: Elevated ridge of sand above normal water level.
- Longshore bar: Underwater sand ridge parallel to shore formed by backwash.
- Longshore drift: Zigzag sediment movement parallel to coastline.
- Longshore current: Water current flowing parallel to shore causing sediment drift.
- Rip current: Surface flow of backwash moving away from shore.
- Undertow/Sheet wash: Backwash flowing along ocean bottom.
- Surf zone: Shallow area where waves break and refract.