Abstract
Abstract. Due to their relevance to the environment and economy, coastal areas are considered national strategic segments that should be preserved. Since erosion phenomena occur intensively in those areas, it is capital to monitor them in order to identify risk zones. In addition to national and regional studies, it is also necessary to conduct local monitoring of erosion prone areas, especially those which are often frequented by people, such as beaches limited by high cliffs. Large scale vertical mapping is necessary to model their susceptibility to mass movements, in order to provide adequate prevention, protection and assistance. Recent techniques like laser scanning or aerial photogrammetry using UAVs allow the definition of the status quo of a cliff wall and its situation a few years back. But to assess the susceptibility to rock mass movements in such cliff segments, inventories of past events are of primordial importance. These inventories allow applying several statistic models to better understand susceptibility together with a set of variables of internal and external nature regarding the cliff site. We present a case study focused on the beach of Ribeira d’Ilhas (Mafra, Portugal), where a workflow of terrestrial photogrammetry for present day situation and recovery of old analogue stereoscopic pairs (1999, 2000, 2003) was implemented. A mass movement inventory (1999-2014) was compiled by multitemporal comparison followed by a detailed characterization of the cliff using a GIS software. Finally, the application of the logistic regression method allowed the definition of a susceptibility map of the cliff wall towards the occurrence of mass movements.
Highlights
1.1 MotivationCoastal areas are vital for the environment as well as for the economy of littoral countries
Since vertical aerial photographs are, in general, not suitable to acquire data on the cliff walls, where most mass movement events occur, other methods are applied such as terrestrial photogrammetry (TP), terrestrial laser scanning (TLS) (Rosser et al, 2005; Lim et al, 2005; Collins and Stock, 2012; Hoffmeister et al, 2012; Katz and Mushkin, 2012; Salvini et al, 2013) and aerial photogrammetry using UAVs (Letortu et al, 2018)
The model presented in this study was applied to a beach, recurring to older photographs, in analogue format, and more recent photogrammetric surveys, in order to enlarge the temporal window of the observations
Summary
1.1 MotivationCoastal areas are vital for the environment as well as for the economy of littoral countries. Cliff monitoring is normally based on periodic surveys and comparison of the acquired data in the different observation epochs. Contrasting with the intensity of each erosion event, which consists of a sudden mass movement down the cliff, the frequency of such events is low, following an inverse power law (Marques, 2008) Due to this fact, a wide temporal window for the observations is needed in order to be able to classify the cliffs according to their susceptibility to rockfall. TLS and TP require access to the cliff base and a certain distance from the cliff wall for suitably acquiring the data These techniques are normally applied in beaches that are surrounded by high cliffs. The innovation of this work lies in several aspects: the study focuses on the cliff wall in 3D instead of analysing cliff top lines in 2D; the terrain units used are 3D points and not an interpolated surface area; predisposing factors of geometric
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