1. Wave Refraction and Longshore Drift

• Due to drag on bottom, incoming waves refract or bend when they approach beach.
• Waves converge on promontories and diverge in bays; tends to straighten coastline.
• Waves striking coast at an angle cause sand to be transported parallel to beach. Zig-zag pattern of transport. The net transport is called longshore drift; the current that carries the samd is called longshore current.

2. Coastal Dynamics (delicate balance between sediment supply, wave erosion, and sea level)

• Winter/Summer Beaches
• response to seasonal differences in wave energy
• winter beach is much smaller, sand is stored in offshore sandbars
• summer beach is wider, sand is stored on beach and foreshore (shallowest water).
• Sea-Level Fluctuations
• causes landward or seaward beach migration, particularly in barrier island systems.
• Can be quite dramatic (lateral change ~1000x vertical change in sea level).
• Concept of Sand Budgets (another simple box model!)
• Inputs = rivers and seacliff erosion. Longshore currents are transport mechanism.
• Ultimate output is the deeper offshore environment (outer shelf and cont. slope).
• Any disruption of the system will have consequences. For example, damming rivers will cause loss of sand supply and thus cause beach erosion.

3. Engineering Techniques (attempt to protect property and stabilize erosion )
Two main philosophies:

• Hard Stabilization
• Seawalls: wave energy is reflected downward, removes sand producing a much narrower beach; pretty much a disaster for barrier island systems.
• Groins/Jetties: traps sand on upstream side; used to maintain beach width or keep inlet open; also reduces wave energy striking beach; severly disturbs natural longshore drift process.
• Soft Stabilization
• Beach nourhisment: artifical addition of sand onto beach to produce positive sand budget; much more desirable (in theory). Main problem is COST, sand may be eroded quickly by storms and permantly lost to deep water. Dredging in enviromentally sensitive areas of the backbarrier areas (tidal inlets; marshes) is a problem.
• Dune stabilization: fences; planting dune grasses may help, but not a long term solution.