Chapitre D'ouvrage Année : 2024

Fate of two cities built on sinking ground: slow and fast submergence at Thônis -Heracleion and Canopus, Nile River Delta, Egypt

Résumé

Deltas are, by nature, places affected by widespread ground subsidence. From the day they are settled, the cities that are built on deltas must cope with a number of issues arising from subsidence. These issues become more and more concerning as centuries go by, and may eventually leave no other choice than the abandonment of the cities. The most dramatic, if not the most common outcome, is the submergence of a city in the sea. The rediscovery of the sunken cities of Thônis-Heracleion and Canopus in 2,000 AD in Abu Qir Bay, for this reason caught the world's attention. Even more dramatically, it was soon proposed that their submergence did not take the form of a slow, ineluctable flooding, but a rapid process that forced the inhabitants to hastily abandon the cities. If such a rapid submergence event occurred today, it would affect a much more densely populated delta, and would have catastrophic consequences. Other studies have since questioned the occurrence of fast submergence in the bay of Abu Qir. Some archaeological data from the submerged sites are difficult to explain, however, without resorting to fast submergence. Besides, rapid ground subsidence has also been documented at Alexandria, at the time proposed for the submergence of Thônis-Heracleion and Canopus. Considering the considerable hazard that fast submergence represents, more work is needed to establish their existence and quantify their amplitude. The present paper aims at clarifying the geological background in which these events would take place, because a variety of different geological phenomena have been invoked to account for fast or slow subsidence. The paper then reviews and analyses the strength and witnesses of the evidence put forward in support for either slow or fast subsidence. Although no definitive conclusion is reached, the most likely scenario that arises from the available data is that of rapid, localised ground collapse restricted to the cities, possibly by liquefaction as the result of ground shaking, superposed to long-term, slow delta subsidence.<

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hal-04745214 , version 1 (20-10-2024)

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  • HAL Id : hal-04745214 , version 1

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Gilles Brocard, Jean-Philippe Goiran, Damian Robinson. Fate of two cities built on sinking ground: slow and fast submergence at Thônis -Heracleion and Canopus, Nile River Delta, Egypt. CNRS Editions. Urban Geoarcheology, pp.381-400, 2024. ⟨hal-04745214⟩
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