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Substantial progress in research on the recession of coastal cliffs composed of soft materials has been made in recent years and data with higher accuracy have been accumulated. This paper provides the state of the art review in the recession studies and highlights two new findings obtained from the reanalysis of existing data. The review topics are: episodic and localized nature of cliff recession; the development of cliffline; the relationship between cliff height and recession rate; mechanisms of cliff toe erosion by waves; a fundamental equation for wave-induced toe erosion; factors controlling toe erosion; and slope instabilities and mass movements. The findings are presented on (1) the temporal change in cliffline recession mode and (2) the effect of beach sediment at the cliff base on the cliff erosion.

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A laboratory experiment using a two-dimensional wave tank was designed to investigate the mechanism of erosion at a cliff base by waves armed with rock fragments. The experiment was performed under constant wave conditions by systematically changing the amount of beach sand at the foot of steep model cliffs of the same slope and strength. Cliff erosion occurred when the beach material at the cliff/beach junction was moved by waves. Turbulence created by bores rushing up on the beach mobilized the sand and exerted a mechanical shearing force on the cliff face using the sand as an abrasive. The analysis of results indicated that the effect of the abrasive doubled when the cliff/beach junction was located above Still Water Level (SWL) as compared to when it was below SWL. The assailing force of the sediment-laden water masses was proportional to the square of the bore speed immediately in front of the cliff face. The factor of proportionality is related to the quantity of beach sand entrapped in the turbulent fluid.