Impact of the October 2018 Storm Vaia on Coastal Boulders in the Northern Adriatic Sea

Abstract

Boulder detachment from the seafloor and subsequent transport and accumulation along rocky coasts is a complex geomorphological process that requires a deep understanding of submarine and onshore environments. This process is especially interesting in semi-enclosed shallow basins characterized by extreme storms, but without a significant tsunami record. Moreover, the response of boulder deposits located close to the coast to severe storms remains, in terms of accurate displacement measurement, limited due to the need to acquire long-term data such as ongoing monitoring datasets and repeated field surveys. We present a multidisciplinary study that includes inland and submarine surveys carried out to monitor and accurately quantify the recent displacement of coastal boulders accumulated on the southernmost coast of the Premantura (Kamenjak) Promontory (Croatia, northern Adriatic Sea). We identified recent boulder movements using unmanned aerial vehicle digital photogrammetry (UAV-DP). Fourteen boulders were moved by the waves generated by a severe storm, named Vaia, which occurred on 29 October 2018. This storm struck Northeast Italy and the Istrian coasts with its full force. We have reproduced the storm-generated waves using unstructured wave model Simulating WAves Nearshore (SWAN), with a significant wave height of 6.2 m in front of the boulder deposit area. These simulated waves are considered to have a return period of 20 to 30 years. In addition to the aerial survey, an underwater photogrammetric survey was carried out in order to create a three-dimensional (3D) model of the seabed and identify the submarine landforms associated with boulder detachment. The survey highlighted that most of the holes can be considered potholes, while only one detachment shape was identified. The latter is not related to storm Vaia, but to a previous storm. Two boulders are lying on the seabed and the underwater surveys highlighted that these boulders may be beached during future storms. Thus, this is an interesting example of active erosion of the rocky coast in a geologically, geomorphologically, and oceanologically predisposed locality.

Holocene Hurricane Deposits Eroded as Coastal Barriers from Andesite Sea Cliffs at Puerto Escondido (Baja California Sur, Mexico)

“…A related but somewhat different phenomenon concerns the detachment of large boulders from the seafloor and onshore transferal to low rocky shores. The recent study by Biolchi et al (2019) fits this category in relation to movement of limestone boulders in the northern Adriatic Sea onto the Premantura (Kamenjak) Promontory in Croatia [33]. In this case, however, the coastline is formed by a low-angle rocky shore that is more like a ramp in configuration than a sea cliff.…”

Finding Coastal Megaclast Deposits: A Virtual Perspective

“…There, Cenozoic volcanic, volcaniclastic, and reefal carbonate deposits crop out [69]. Presumably, their destruction by wave abrasion and weathering leads to megaclast creation; it cannot be excluded that some megaclasts were formed as a result of slope collapse and subsequent downslope The Mediterranean is a 'classical' region for studies of coastal megaclasts and boulder deposits [9][10][11][12][13]. Particularly, megaclast accumulations have been reported from the Western Mediterranean continental and island coasts [66][67][68].…”

“…The modern studies of megaclast focus much (even over-emphasize) on their origin and, particularly, their relevance to extreme events such as storms and tsunamis [4][5][6][7][8][9][10][11][12][13][19][20][21][22][23][24][25][26][27]. Evidently, these studies deal with the dynamics of the coastline and megaclasts themselves.…”

“…These studies have been conducted in so different places of the world as Baja California in Mexico [4,5], North Eleuthera of the Bahamas [6,7], and Rabat in Morocco [8]. Although the works have tended to focus on only some regions like the Mediterranean (e.g., [9][10][11][12][13]), the global evidence of large clast accumulations has become huge already, and it continues growing. This evidence requires extension and generalization for further conceptual treatment.…”

“…Additionally, Cox et al [10] offered a novel approach for measurements of roundness of megaclasts, and, thus, the noted nomenclature development should emphasize not only on grain-size categories, but also on various morphological parameters. The second aspect of coastal megaclast deposits is linked to genetic investigations and, particularly, interpretations of the evidence of past storms and tsunamis (storm-versus-tsunami origin of megaclasts is a popular and hotly-debated topic in the modern sedimentology) [4][5][6][7][8][9][10][11][12][13][19][20][21][22][23][24][25][26][27][28]. Irrespective of which of these aspects and particular opinions to follow, finding new localities coastal megaclast deposits is of utmost importance.…”