Comment on esurf-2022-18

Abstract

Long-term coastal erosion is not yet well studied given that it is difficult to quantify. The quantification of long-term coastal erosion requires a proper reconstruction of the coast’s initial geometry. It is also important to determine the timing of the start-up. Volcanic islands are good candidates fulfilling these two conditions: their initial shape is roughly conical and the age of the lavas that generated this geometry is easily measured. We have developed a method to reconstruct the initial shape of simple volcanic edifices from aerial and submarine topographic data. The reconstructed initial shape and associated uncertainties allow us to spatially quantify the coastal erosion since the building of the island. This method is applied to Corvo Island in the Azores archipelago. We calculated the initial radius and peak elevation of the island to be approximately 3.8 km and 1 km, respectively. We calculated that, due to coastal erosion, the island has lost a volume of 6.5 ± 2.7 km3 corresponding to a reduction of roughly 80 % of its surface area since it was first built. Taking the large uncertainty in the age of the topmost lava flows (90 to 770 ka) into account, our reconstruction yields a conservative range of long-term coastal erosion rates between 7 and 370 mm/yr. These values are consistent with the orders of magnitude of short-term coastal erosion rates measured on similar lithologies. Lastly, we show a strong correlation between long-term coastal erosion and the spatial distribution of the waves. Specifically, we highlight a stronger erosion control by moderate and usual waves than by storm waves. The next step will be to apply this method to other volcanic islands in order to: (i) strengthen and consolidate the method, and (ii) verify the correlations observed in the present study.