Using UAV and Structure-From-Motion Photogrammetry for the Detection of Boulder Movement by Storms on a Rocky Shore Platform in Laghdira, Northwest Morocco

Abstract

The detachment and mobilization of boulders from rocky shore platforms by waves involves complex geomorphic and hydrodynamic processes. Understanding these processes requires precise information on the rates and patterns of movement of these megaclasts scaled against the wave conditions that generate boulder mobility. Repeat photogrammetry and structure-from-motion (SfM) models commonly used in geomorphic analyses are an interesting option for monitoring boulder dynamics. In this study, we used unmanned aerial vehicle (UAV)-based digital photogrammetry and SfM differential models to identify recent boulder movements over a rocky shore platform in Laghdira, Morocco. Combining these results with data on storm occurrence in the study area allowed us to identify storm waves as the unique driver of the dislodged and mobilized boulders. The identified storm event had a significant wave height of 5.2 m. The UAV models were built from imagery captured in September and December 2019 using a DJI MAVIC PRO PLATINUM, and we used QGIS to produce 2D and 3D model outputs. The exploitation of the 2D model differentials allowed us to appreciate the response of the boulders to the storm waves and to determine platform volumetric changes and, therefore, boulder mobility. The 3D models were valuable in determining the mode of transport of the boulders. Mobility patterns included sliding, overturning with no further mobility, and rotation and saltation, as well as boulder breakup. Storm waves did not have a preferential impact on any particular boulder shape, size category, or position at the outer edge of the platform. These results highlight the utility of combining UAV surveys with identified storm events, which are much more frequent than tsunamis, in determining observed boulder initiation and mobility.

The Safi boulders (South of Morocco): another evidence of the 1755 Lisbon tsunami?

“…In rocky shores, the detachment and landward displacement of mega-clasts is a known indicator of the occurrence of these extreme events. This type of deposit has been reported in multiple coastal areas around the world: Spain (Lario et al, 2023;Roig-Munar et al, 2019), Portugal (Costa et al, 2011;Oliveira et al, 2020), Morocco (Medina et al, 2011;Mhammdi et al, 2008;Sedrati et al, 2022), Japan (Namegaya et al, 2022;Watanabe et al, 2019), Chile (Abad et al, 2020;Carballeira et al, 2022), Scotland (Hall et al, 2010;Scheffers et al, 2010), Hawaii (Noormets et al, 2002;Richmond et al, 2011) and Taiwan (Ota et al, 2015;Terry et al, 2021). Unlike sandy deposits, boulders have a high probability to be preserved.…”

“…For the case of boulder deposits, they have been particularly described along the Larache and the Rabat-Skhirat rocky shores. The boulders in these two sites have been studied using various proxies, including a combination of geomorphological analysis and numerical models (Chiguer and Medina, 2019;Medina et al, 2011;Mhammdi et al, 2008), aerial-photo analysis (Sedrati et al, 2022) and optically stimulated luminescence rock surface exposure dating measurements (Brill et al, 2021). In addition to the Larache and Rabat-Skhirat boulders, Theilen-willige et al (2012) briefly described the presence of other blocs about 12 km south of Safi.…”

Storm surge and tsunami deposits along the Moroccan coasts: state of the art and future perspectives

“…In the rocky shore platform of Laghdira (south of Larache city), Sedrati et al (2022) used unmanned aerial vehicle (UAV)-based digital photogrammetry and SfM differential models to investigate the potential boulder transport and detachment triggered by a strong storm in 2019, and to identify possible boulder transport modes (7-9 in Fig. 6c).…”

A Machine-Learning Approach to Intertidal Mudflat Mapping Combining Multispectral Reflectance and Geomorphology from UAV-Based Monitoring

“…It also hinders accurate evaluation of their carbon uptake [5]. Recent developments in unmanned aerial vehicle (UAV) technology are reducing the constraints of spectral mixing by capturing very high spatial resolution images within a centimetre accuracy range, as well as enabling the production of detailed geomorphic maps [17][18][19][20][21]. Moreover, UAV can collect images with a higher survey frequency within a semi-diurnal tidal cycle, or under cloudy conditions, when no image can be acquired by satellite, or when satellite overpass does not coincide with the lowest tides.…”