Monitoring the sedimentary budget and dislocated boulders in western Greece – results since 2008

Abstract

Dislocated boulders are one sign of high‐energy wave impacts on coasts. These high‐energy impacts, caused by severe storms or tsunamis, can trigger initial cracking and transport of boulders. Monitoring of these boulders, as well as the associated coastal sites is important in distinguishing between gradual coastal processes and high‐energy events. Western Greece is a seismically active area, where tsunamis and high‐energetic storms might occur and such past events are documented by historic and geoscientific research, making it an ideal location for monitoring dislocated boulders. Since 2008, monitoring of eight different coastal sites in this region was conducted by terrestrial laser scanning and photogrammetric approaches, with low‐cost unmanned aerial vehicles. The re‐use of similar surveying points in following years, allowed highly accurate monitoring. Point clouds derived from these methods were evaluated for change detection by point cloud comparisons. The data were also used to establish accurate three‐dimensional models of dislocated boulders (n = 70). The determined boulder volumes of these accurate three‐dimensional models were incorporated in wave transport equations and wave decay curves, and compared with monitoring results. A comprehensive overview of dislocated boulders in western Greece is presented. Three‐dimensional boulder reconstruction is compared to an approach which uses a tape‐based measuring of boulder axes, with the tape‐based measurement showing a mean overestimation of mass by 32%. Accurate monitoring over time by both methods, is achieved by using fixed networks of reference points. Changes for each site over time, detected by direct point cloud comparisons, are fit to the possible inundation calculated by wave decay curves based on computed minimum wave heights for boulder transport. Both storm and tsunami waves may have initiated movement from the cliff edge and further transport is also possible. However, boulders showed no further movement from their current position in the area for the time period of this study.

The Imprint of Recent Meteorological Events on Boulder Deposits along the Mediterranean Rocky Coasts

“…Location Meteorological Event(s) Mastronuzzi and Sansò (2004) [16] South Adriatic identified Barbano et al (2010) [47] West Ionian identified Barbano et al (2011) [48] West Ionian roughly identified Hoffmeister et al ( 2013) [49] East Ionian roughly identified Shah-Hosseini et al ( 2013) [50] Gulf of Lion not identified Torab and Dalal (2015) [51] Eastern Mediterranean not identified Biolchi et al ( 2016) [52] South Central Mediterranean not identified Causon Deguara and Gauci (2017) [53] South Central Mediterranean not identified Piscitelli et al ( 2017) [54] Gulf of Lion not identified Roig-Munar et al ( 2017) [55] Balearic Sea identified Galea et al ( 2018) [46] South Central Mediterranean identified Pepe et al ( 2018) [56] South Tyrrhenian not identified Roig-Munar et al ( 2018) [57] Balearic Sea not identified Biolchi et al (2019a) [17] North Adriatic identified Biolchi et al (2019b) [58] North Adriatic identified Delle Rose et al (2020) [59] North Ionian identified Hoffmeister et al (2020) [60] East Ionian not identified Mottershead et al (2020) [61] South Central Mediterranean not identified Scicchitano et al (2020) [62] West Ionian identified Delle Rose et al ( 2021) [45] North Ionian identified…”

“…Only one repetition was found among the reviewed articles: both Shah-Hosseini et al [50] and Piscitelli et al [54] treat the same notable case of polyphasic boulder activation that happened at Martigues coast (Gulf of Lion) before December 2003. Some articles address two or more stretches of Mediterranean coasts [45,48,51,52,56,57,60], thus the screening process has allowed the identification of 44 sites showing evidence or clues of CBD production/activation as a result of recent (i.e., during about the last halfcentury) storms. Many articles concern cases belonging to the central Mediterranean [16,17,[45][46][47][48][49]52,53,56,[58][59][60][61][62], (see Figure 3).…”

“…Some articles address two or more stretches of Mediterranean coasts [45,48,51,52,56,57,60], thus the screening process has allowed the identification of 44 sites showing evidence or clues of CBD production/activation as a result of recent (i.e., during about the last halfcentury) storms. Many articles concern cases belonging to the central Mediterranean [16,17,[45][46][47][48][49]52,53,56,[58][59][60][61][62], (see Figure 3). Some studies are relative to the western Mediterranean [50,54,55,57], while only one study to the eastern Mediterranean [51].…”

Tsunami deposits: Present knowledge and future challenges

“…These authors generated accurate 3D models of dislocated boulders after almost a decade of monitoring. Finally, 3D models of dislocated boulders are considered to offer a reliable base to detect the effects of future changes in the sedimentary budget, to determine the dimensions of high-energy impacts and to serve as a base for further hydraulic modelling approaches (Hoffmeister et al, 2020).…”

“…Hoffmeister et al . () monitored boulder movement at eight different coastal sites in Greece using terrestrial laser scanning and low‐cost unmanned aerial vehicle‐based photogrammetric approaches with low‐cost unmanned aerial vehicles. These authors generated accurate 3D models of dislocated boulders after almost a decade of monitoring.…”