Application of Structure-from-Motion Terrestrial Photogrammetry to the Assessment of Coastal Cliff Erosion Processes in SW Spain

Abstract

Del Río, L.; Posanski, D.; Gracia, F.J. and Pérez-Romero, A.M., 2020. Application of structure-from-motion terrestrial photogrammetry to the assessment of coastal cliff erosion processes in SW Spain. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 1057–1061. Coconut Creek (Florida), ISSN 0749-0208.The episodic and irregular nature of cliff changes makes their assessment a challenging task, essential in order to cope with risks related to cliff erosion. In this work, structure-from-motion terrestrial photogrammetry was used to measure erosion processes in the Torre Bermeja cliff in SW Spain, where cliff instability poses a threat to people walking along the fronting beach. Two sites were investigated on 14 photogrammetric surveys conducted along a 9-month period in order to detect morphological changes in relation to external forces. The erosive events at both sites showed a strong episodic character, with some short periods reflecting intensive erosion and long periods of stability. Average figures for the observation period indicate a mean retreat of 2.8 mm/day at one of the sites and 0.5 mm/day at the other site. The differences were attributed to variations in the lithology and a higher wave exposure of the first site, due to a smaller fronting beach. A comparison of erosion and meteorological data of the survey period showed a positive correlation between mean retreat and maximum rainfall intensity. However, the storms with highest wave heights did not significantly affect the cliff foot, as most wave-related erosion occurred when a modal storm coincided with a high spring tidal range. Therefore, rainfall can be considered as the trigger for most of the erosion events at the study site, although the unstable conditions required for large-scale collapses are generated by wave attack in the first place. In this respect, it is assumed that climate change will lead to higher retreat rates in the investigated cliff, due to a rising mean sea level and a higher frequency of heavy rainfall events.