Prediction of Soft-cliff Retreat with Accelerating Sea-level Rise

Reliable estimates of future cliff recession are needed to assess coastal vulnerability and evaluate management policies with regard to the widespread sea-level rise thought likely to result from global warming. A research gap is identified in providing appropriate predictive methods. This paper reviews the possible effects of sea-Ievel rise upon soft-rock cliffs over a 50-100 year planning timescale. It evaluates different methods of analysing historical recession and highlights the main assumptions and rules governing future extrapolation of retreat rates. Simple predictive models including a modification of the Bruun Rule are developed and applied to estimate cliff sensitivity to sea-level rise In southern England. The complexity of factors interacting over variable spatial and temporal scales is identified as a major problem. Irrespective of sea-level rise, recession assessments need to accommodate episodic cliff failures occurring within regular erosion cycles and differentiate instances of runaway systems change. Predictions must rely upon methods of extrapolating historical retreat. Different methods are applicable according to the presence or absence of shoreface sediments. The modified Bruun Rule appears the most appropriate for situations where cliff sediments accumulate on the shore profile. Results obtained using this model indicated that recession could increase by between 22% and 133% by 2050 according to site. Cliffs on exposed coasts and those containing high proportions of clay appear the most sensitive to change. Attention is drawn to some of the inherent uncertainties including those caused by different landslide types, lags in response and the effect of protective beaches.