How to explain variations in sea cliff erosion rates? Insights from a literature synthesis

Rocky coast erosion (i.e. cliff retreat) is caused by a complex interaction of various forcings that could be marine, subaerial or due to rock mass property. It turns into variable erosion rates (over 4 orders of magnitude at least, from 1 mm yr−1 to 10 mm yr−1). While numerous local studies exist and explain erosion processes on specific sites, there is a lack at global scale. In order 5 to quantify and rank the various parameters influencing erosion rates, we compiled existing local studies in a global database called GlobR2C2 (for Global Recession Rates of Coastal Cliffs). This database records erosion rates, cliff setting and measurement specifications; it is filled from peer reviewed articles and national databases. In order to be homogeneous, marine and climatic forcings were recorded from global models and reanalysis. Up to now, GlobR2C2 contains 58 publications which represents 1530 cliffs studied and more than 1680 erosion rate estimates. A statistical analysis was conducted on this 10 database to explore links between erosion rate and forcings at global scale. Rock resistance, inferred through Hoek and Brown (1997) criterion, is the strongest signal explaining variation in erosion rate. Median erosion rates are of 2.9 cm yr−1 for hard rocks, 10 cm yr−1 for medium rocks and 23 cm yr−1 for weak rocks. Concerning climate, only the number of frost days (number of day per year below 0 °C) for weak rocks shows a significant, positive, trend with erosion rate. Every other relations with both climatic and marine forcings are very spread and non-significant.