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

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, mea- sured 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. Con- versely, few tsunamis have been recorded in historical docu- ments to have reached the Maltese archipelago.

We present a multi-disciplinary study, which aims to de- fine the characteristics of these boulder accumulations, in or- der 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 boul- ders were calculated using various hydrodynamic equations.

Particular attention was given to the quantification of the in- put parameters required in the workings of these equations,

such as size, density and distance from the coast. In addition,

accelerator mass spectrometry (AMS) 14C ages were deter- mined from selected samples of marine organisms encrusted

on some of the coastal boulders. The combination of the re- sults obtained both by the hydrodynamic equations, which

provided values comparable with those observed and mea- sured 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 exis- tence 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 m3 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 dat- ing 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.