The rapid detachment and downslope movement of material from a coastal cliff face due to marine undercutting, weathering, or slope instability.

Principle

Marine processes (wave attack, tidal scour) and subaerial processes (weathering, groundwater seepage) remove support at the cliff toe or weaken the cliff mass; when driving forces exceed resisting strength, episodic cliff failure detaches blocks or masses that move downslope.

Demonstration

Wave-driven notch formation at the cliff base removes toe support; over time the upper slope becomes unsupported and an overhanging section collapses in a rockfall or slump that rapidly retreats the cliff line and deposits debris at the shore.

Misapplication

Describing any coastal retreat as cliff collapse, including slow shoreface erosion by longshore transport or seasonal beach lowering; confusing cliff failure with submarine landsliding not involving the cliff face.

Consequence

Sudden loss of land, destruction of coastal infrastructure and habitats, increased coastal retreat rates and hazardous falling debris onto beaches and nearshore waters.

Reversal

Progradation of the shoreline by beach nourishment, sediment deposition, cliff stabilization works, or decreased wave energy can slow or locally reverse cliff retreat and reduce collapse likelihood.

Boundary

Concerns near‑vertical to steep coastal slopes where marine and subaerial processes interact to produce episodic detachment; excludes gradual shoreline recession dominated solely by sediment budget imbalance or deep offshore slope failures unrelated to the cliff face.

Semantic Tension

Overlaps with terms like rockfall, landslide and coastal erosion — collapse emphasizes rapid mass detachment from the cliff face, whereas coastal erosion includes slower, distributed processes along the shoreline.

Synthesis

Coastal cliff collapse is an episodic, often rapid failure of steep coastal slopes driven by marine undermining and weakening processes that detach blocks or masses, causing abrupt shoreline retreat and localized hazards distinct from slower erosional retreat.