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During the 19th century, remains of an ancient harbour were found underwater at a depth of 5 to 6 meters in the eastern port of Alexandria. A research program was undertaken to determine when the harbour of Alexandria submerged underwater. Data were collected through underwater surveys by scuba diving and by campaigns of corings on land. Geomorphological (i.e. notches and pebble beaches), archaeological (harbour structures), and biological (i.e. marine macrofauna, bioconstructions, and biodepositions) sea level indicators were correlated to understand changes in relative sea level during the last 6 millennia. For each proxy, the altitudinal (vertical) and chronological ranges of imprecision were discussed. The results indicate that the rate of the relative sea level rise is ~80 mm per century between the middle of the 6th millennium and the 5th‒6th c. AD. An abrupt relative sea level rise (3.5 m + 1.5 m) occurred during the mid 8th c. to the end of 9th c. A D. I n t he 8 th c. A D, a s imilar p henomenon was observed for Heracleion (25 km east of Alexandria). Thus, a wide movement of sinking affected in a synchronous manner the western coastal margin of the Nile delta. Since this 8th- 9th c. AD event, the subsidence has increased around 2 m. The role of abrupt sinking events and subsidence remain determining in the deltaic context to anticipate future coastal adaptations and the risk of submersion.

International audience; During the 19th century, remains of an ancient harbour were found underwater at a depth of 5 to 6 meters in the eastern port of Alexandria. A research program was undertaken to determine when the harbour of Alexandria submerged underwater. Data were collected through underwater surveys by scuba diving and by campaigns of corings on land. Geomorphological (i.e. notches and pebble beaches), archaeological (harbour structures), and biological (i.e. marine macrofauna, bioconstructions, and biodepositions) sea level indicators were correlated to understand changes in relative sea level during the last 6 millennia. For each proxy, the altitudinal (vertical) and chronological ranges of imprecision were discussed. The results indicate that the rate of the relative sea level rise is ~80 mm per century between the middle of the 6th millennium and the 5th‒6th c. AD. An abrupt relative sea level rise (3.5 m + 1.5 m) occurred during the mid 8th c. to the end of 9th c. A D. I n t he 8 th c. A D, a s imilar p henomenon was observed for Heracleion (25 km east of Alexandria). Thus, a wide movement of sinking affected in a synchronous manner the western coastal margin of the Nile delta. Since this 8th- 9th c. AD event, the subsidence has increased around 2 m. The role of abrupt sinking events and subsidence remain determining in the deltaic context to anticipate future coastal adaptations and the risk of submersion.