Effects of varied lithology on soft-cliff recession rates

• Presents modifications to the Soft Cliff and Platform Erosion (SCAPE) numerical model
• Effects of varied lithology on cliff geomorphology and retreat are quantified.
• An asymmetric increase in cliff toe retreat rates for weaker layers is reported.
• Attributed to varying magnitudes of feedback governing profile reshaping
• Implications of interactions between variable layers and sea-level are discussed.

Geomorphic modelling is a key method to understand the soft cliff recession process to predict future rates of retreat and responses to climate change. A range of process-based models have been used; however the influence of varied vertical lithology has yet to be quantified. This paper describes modifications to the 2D SCAPE (Soft Cliff and Platform Erosion) model, carried out to explore such interactions between vertical changes in cliff resistive strength and prevailing coastal conditions. As expected, weaker (/more resistant) layers lead to more (/less) rapid retreat. However, this effect is strongly influenced by the position of such layers relative to mean sea level, where the erosive potential is greatest. Moreover, model simulations reveal that layers of variable resistance give an asymmetric response in terms of both rates of retreat and the timeframe for the effect to be realised. For example, a reduction of material strength of 1/5 (in comparison to the remainder of the cliff) about mean sea level results in a rapid 130% increase in the rate of retreat in comparison to the introduction of a five times more resistant layer of the same characteristics. This variation in response can be attributed to the different magnitudes of feedback governing profile reshaping associated with the change in lithology. For example, the introduction of a weaker layer amplifies erosion through its greater erosive potential combined with steepening of the overlying section. The results have important implications for the management of coastal cliffs exhibiting variable stratigraphy, combined with the potential for future interactions with sea-level rise.