A comparative approach of monitoring techniques to assess erosion processes on soft cliffs

Coastal cliffs are subject to erosion processes of continental and marine nature, whose assessment is particularly challenging given the inaccessibility of these landforms and the episodic and irregular nature of their evolution. For this reason, diverse techniques have been developed over the last decades to monitor coastal cliff changes. In this work, three monitoring methods are applied to the Torre Bermeja coastal cliff (South Spain), a conglomeratic Plio-Pleistocene cliff whose instability poses a threat to people walking along the fronting beach. Conventional aerial photogrammetry (AP), structure-from-motion terrestrial photogrammetry (SfM-TP) and terrestrial laser scanning (TLS) are applied on two sites and compared in terms of erosion rates obtained, advantages and disadvantages, uncertainties involved and key aspects to be considered in planning surveys. Results obtained by aerial photographs indicate an average recession rate of the cliff foot around 0.15 m/year over the last 40 years, with a high spatial variability and an associated uncertainty of 0.1 m/year. The use of SfM-TP along an 8-month period yielded an average erosion rate of the cliff face around 2.8 mm/day in one of the sites, mainly related to a large rockfall, and around 0.5 mm/day in the other site, implying an annual recession rate of 0.18 m/year. Results of TLS are very similar to those of SfM-TP. The data show that AP is only convenient for a general evaluation of medium-term cliff evolution along large coastal stretches, while both SfM-TP and TLS can be used for a detailed investigation of erosion processes and for the quantification of local-scale morphological changes. SfM-TP and TLS allowed obtaining high-resolution 3D models of the cliff, with negligible differences between the surfaces generated by SfM-TP point clouds and by TLS meshes. The main advantages observed in SfM-TP are the low cost of the equipment and the easier and faster data collection in the field when compared with TLS. This allows the monitoring of cliff changes with a high temporal resolution, in order to assess cliff response to external forcing. The main advantages found in TLS are the better representation of shadow areas and shorter post-processing times.