Boulder accumulations related to extreme wave events on the eastern coast of Malta

Abstract

Abstract. The accumulation of large boulders related to waves generated by either tsunamis or extreme storm events have been observed in different areas of the Mediterranean Sea. Along the eastern low-lying rocky coasts of Malta, five sites with large boulder deposits have been investigated, measured and mapped. These boulders have been detached and moved from the nearshore and the lowest parts of the coast by sea wave action. In the Sicily-Malta channel, heavy storms are common and originate from the NE and NW winds. Conversely, few tsunamis have been recorded in historical documents to have reached the Maltese archipelago.We present a multi-disciplinary study, which aims to define the characteristics of these boulder accumulations, in order to assess the coastal geo-hazard implications triggered by the sheer ability of extreme waves to detach and move large rocky blocks inland.The wave heights required to transport 77 coastal boulders were calculated using various hydrodynamic equations. Particular attention was given to the quantification of the input parameters required in the workings of these equations, such as size, density and distance from the coast. In addition, accelerator mass spectrometry (AMS) 14 C ages were determined from selected samples of marine organisms encrusted on some of the coastal boulders. The combination of the results obtained both by the hydrodynamic equations, which provided values comparable with those observed and measured during the storms, and radiocarbon dating suggests that the majority of the boulders have been detached and moved by intense storm waves. These boulders testify to the existence of a real hazard for the coasts of Malta, i.e. that of very high storm waves, which, during exceptional storms, are able to detach large blocks of volumes exceeding 10 m 3 from the coastal edge and the nearshore bottom, and also to transport them inland. Nevertheless, the occurrence of one or more tsunami events cannot be ruled out, since radiocarbon dating of some marine organisms did reveal ages which may be related to historically known tsunamis in the Mediterranean region, such as the ones in AD 963, 1329, 1693 and 1743.

Intertidal boulder transport: A proposed methodology adopting Radio Frequency Identification (RFID) technology to quantify storm induced boulder mobility

“…The methodology can also be utilised with emerging technologies, such as UAV monitoring (Pérez-Alberti and Trenhaile, 2015aTrenhaile, , 2015bBiolchi et al, 2016) to provide greater spatial resolution on the mechanisms that facilitate the transport of intertidal boulders. Furthermore, deployment in coastal revetment blocks could help mitigate against damage or loss to coastal defence engineering works.…”

“…It is well documented that storm waves have the ability to detach, transport and deposit boulders on intertidal shore platforms (Barbano et al, 2010;Etienne and Paris, 2010;Paris et al, 2011;Shah-hosseini et al, 2011;Stephenson and Naylor, 2011;Cox et al, 2012;Biolchi et al, 2016). Boulder accumulations on shore platforms frequently develop distinctive geomorphic features such as clusters, ridges, fields and cliff top deposits (Nott, 2003a;Paris et al, 2011).…”

Geomorphological mapping of terrestrial and marine areas, northern Malta and Comino (central Mediterranean Sea)

“…Other coastal features, such as tidal notches, sea stacks, arches and caves occur mainly along the western coast (Biolchi et al, 2016b). Along the eastern coast of Malta and Comino, large boulder deposits ascribed to extreme wave events have been described by Biolchi et al (2016a) and Farres (2014, 2015). Furlani et al (2013) and Lambeck et al (2011) reported that sea level was 130 m lower than the present day during the LGM and that the Maltese archipelago has been tectonically stable since the Last Interglacial.…”

Coastal boulder deposit as evidence of an ocean‐wide prehistoric tsunami originated on the Atacama Desert coast (northern Chile)

“…In the last decades numerous works have focused on the deposition by extreme marine events of very large boulders in coastal areas Costa et al, 2008Costa et al, , 2011Barbano et al, 2010;Richmond et al, 2011;Biolchi et al, 2016;Dewey & Ryan, 2017;among others). Most of these studies emphasize the great difficulty in distinguishing boulder fields deposited by storms from those originated by tsunamis (Noormets et al, 2004;Dawson et al, 2007;Imamura et al, 2008;Goto et al, 2009Goto et al, , 2010, although until recently it was believed that storm waves were neither capable of moving large boulders nor to achieve this at high altitudes above sea level.…”