The evolution of Sf. Gheorghe (Danube) asymmetric deltaic lobe

Abstract

The wave asymmetric Sf. Gheorghe lobe is the only active in the Danube delta where river mouth bar (and the associated barrier islands and spits) continuously deployed a cyclic development for almost two millennia. During first stage, the Sf. Gheorghe distributary had a small discharge (with an order of magnitude lower than present) which after that experienced a rapid increase in consequence of the successive avulsions of Împuţita (southern distributary of Sulina arm) and Dunavăţ distributaries. Our morphological analyses together with the newly obtained chronology, revealed the multiple ridgesets structure of Sf. Gheorghe deltaic lobe. In fact, all ridgesets (10) follow a common morphodynamic pattern characterized by the cyclic succession of three stages: i) subaqueous mouth bar development, ii) barrier island emergence, iii) barrier spit phase with several secondary spits derived from an updrift trunk ridge. The size of each ridgeset increased exponentially with every new cycle due to the constant lengthening of the coastline as the downdrift side of the lobe advances seaward through a series of progressively larger similar quadrilaterals, yielding to a constant enlargment of the delta front size.