Modelling river-sea continuum: the case of the Danube Delta

Abstract. Understanding water transport and circulation in coastal seas and transitional environments is a key focus of oceanographic and climate research, particularly in recognizing the role of the land-sea interface. The Danube Delta serves as a natural laboratory for river-sea hydrodynamic modelling due to its complex morphology, composed of multiple river branches, channels, and lagoons. Moreover, this coastal environment is subjected to various natural and anthropogenic stressors, and numerical modelling can provide a scientific basis for assessing the impact of human activities. In this work, the SHYFEM finite element hydrodynamic model was applied to the entire river-sea continuum of the Danube Delta region to describe the transport and mixing processes within and between the interconnected water bodies forming the delta. The model was run for the period 2015-2019 and enabled the characterization of: (1) water discharge distribution among the river branches; (2) general hydrodynamic characteristics of the coastal region of freshwater influence; (3) transport time scale of the Razelm Sinoie Lagoon System. Finally, the Danube Delta modelling tool was used to evaluate the potential effects of hydrological reconnection (restoration) measures in the Razelm Sinoie Lagoon System aimed at improving connectivity and water renewal.