Coastline retreat via progressive failure of rocky coastal cliffs.

Despite much research on the myriad processes that erode rocky coastal cliffs, accurately predicting the nature, location, and timing of coastline retreat remains challenging, and is confounded by the apparently episodic nature of cliff failure. The dominant drivers of coastal erosion, marine and subaerial forcing, are anticipated to increase in the future, so understanding their present and combined efficacy is fundamental to improving predictions of coastline retreat. We captured change using repeat laser scanning across 2.7 × 104 m2 of near-vertical rock cliffs on the UK North Sea coast over 7 yr to determine the controls on the rates, patterns, and mechanisms of erosion. For the first time we document that progressive upward propagation of failure dictates the mode and defines the rate at which marine erosion of the toe can accrue retreat of coastline above; this is a failure mechanism not conventionally considered in cliff stability models. Propagation of instability and failure operates at these sites at 10 yr time scales and is moderated by local rock mass strength and the time dependence of rock fracture. We suggest that once initiated, failure propagation can operate ostensibly independently to external environmental forcing, and so may not be tightly coupled to prevailing subaerial and oceanographic conditions. Our observations apply to coasts of both uniform and complex lithology, where failure geometry is defined by rock mass strength and structure, and not intact rock strength alone, and where retreat occurs via any mode other than full cliff collapse