Wave and Current Effects on Beaches: Erosion, Accretion, and Morphology

Beaches are dynamic equilibria shaped by the interplay of waves and currents. Understanding these processes explains beach form, change, and management.

Wave Processes

Erosion

• Mechanism: Breaking waves suspend sediment; undertow/rip currents transport offshore
• Storm waves: High energy, steep → cut scarps, move sand to offshore bars
• Chronic: Sea-level rise, sediment deficit, armoring (seawalls)

Accretion

• Swash deposition: Low-energy waves push sand up beach face
• Onshore bar migration: Fair-weather waves weld bars to shore
• Aeolian: Wind moves dry sand to dunes (supply-limited)

Longshore Transport (Littoral Drift)

• Driver: Oblique wave breaking → alongshore current
• Formula: Q = K × H_b² × C_gb × sin(2θ_b) (CERC)
• Direction: Net drift determines "updrift" (source) vs "downdrift" (sink)
• Interruptions: Jetties, groins, harbors trap sand → downdrift erosion

Rip Currents

• Narrow, offshore-directed jets (0.5–2.5 m/s) through surf zone
• Hazard: #1 beach rescue cause; escape: swim parallel to shore
• Morphologic role: sediment exchange, bar breaching

Beach Cusps

• Rhythmic horn/bay patterns (λ = 10–100m)
• Self-organizing: swash-sediment feedback
• Common on mixed sand/gravel, reflective beaches

Current Processes (Non-Wave)

Tidal Currents

• Reversing, predictable; strong in inlets, estuaries
• Ebb-delta / flood-delta formation at inlets
• Can dominate sediment transport in macrotidal coasts (Bay of Fundy, Mont-Saint-Michel)

Wind-Driven / Ocean Currents

• Longshore current augmentation/reversal
• Upwelling/downwelling affects nearshore temp, nutrients
• Loop Current eddies (Gulf) → cross-shelf transport

Resulting Beach Morphologies

Human Impacts

• Dams: reduce sediment supply → coastal recession (Mississippi, Nile, Mekong)
• Sand mining: direct volume loss
• Hard structures: disrupt longshore transport
• Nourishment: artificial sediment addition (temporary, recurring)