Coastal boulder deposits attesting to large wave impacts on the Mediterranean coast of Egypt

International audienceCoastal 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