Coastal boulder accumulations are documented along the Mediterranean coast of Egypt between Alexandria and Marsa Matrouh at four distinct sites. The spatial distribution and dimensions of 116 medium to large boulders were documented along four representative coastal profiles. Boulders weigh up to 23 metric tons and are located up to 40 m from the shoreline. Geomorphologic features, morphometric properties and the presence of attached marine organisms attest that the boulders are detached and transported from original subtidal or intertidal settings by the impact of unusually large waves. Adapted hydrodynamic models were applied to evaluate the height of the transporting waves. Our result shows that largest boulders could be transported by tsunami waves of 2.6 m or storm wave of about 10 m in height. Radiocarbon dating was performed on fixed marine gastropod (Vermetidae and Dendropoma) shells found on four representative large boulders. A calibrated age from the easternmost site is roughly coincident to the well-known tsunami of 1303 AD in the eastern Mediterranean. Three other calibrated ages correspond to a period ranging from the eighteenth century AD to present. A large tsunami like the event of 1303 AD would have been able to transport all of the studied boulders. However, radiocarbon ages and morphological properties such as freshly broken edges and surfaces suggest younger ages for the majority of boulders. Since there have been no large, post-1303 AD tsunamis reported, we suggest that the majority, if not all, of the boulders were most likely deposited by multiple intense storms. According to the wave height model, storms with wave heights exceeding 9 m at their breaking point probably occur about once every 100 years. A relationship between the boulder deposits and the high storm frequency that characterized the little ice age in the Mediterranean Sea is plausible. This study emphasizes the potential hazard of large waves on this part of the Mediterranean coast of Egypt.
Bibliographical noteFunding Information:
This work is a joint Franco-Egyptian contribution to the research project IMHOTEP. It has been partially funded by the Egyptian Academy of Scientific Research and Technology, the University of Aix-Marseille (France), Faculty of Education of Ain-Shams University (Cairo) and ANR-GEOMAR funded by French National Center for Scientific Research (CNRS). This work has been carried out thanks to the support of the Labex OT-Med (ANR-11-LABX-0061) and of the AMIDEX project (n° ANR-11-IDEX-0001-02), funded by the “Investissements d’Avenir” French Government program, managed by the French National Research Agency (ANR). Authors wish to thank three reviewers for their constructive comments and Pr. James Goff from the University of New South Wales, Australia, for very useful comments and English corrections.
© 2016, Springer Science+Business Media Dordrecht.
- Boulder deposits
- Coastal hazard
- Mediterranean coast of Egypt
ASJC Scopus subject areas
- Water Science and Technology
- Atmospheric Science
- Earth and Planetary Sciences (miscellaneous)