Intra-site differences in nearshore bar behavior on a nontidal beach (Sulina – Sf. Gheorghe, Danube Delta coast)

Long-term, cyclic offshore migration is a common occurrence on many multiple-barred beaches. The inter-site variability of this behavior was documented to be related to differences in certain key geometric parameters. Besides the inter-site variability, there also appears to be considerable intra-site variability in sandbar behavior, factors controlling these differences not being properly understood. A 6-year bathymetric dataset collected along 16 km of low-lying nontidal beach, located within the central and southern part of the interdistributary Sulina – Sf. Gheorghe beach (Danube Delta coast – Black Sea) was analyzed with complex empirical orthogonal functions to quantify intra-site differences and similarities in cyclic offshore progressive bar behavior. The study area can be split into 3 longshore sectors with different cross-shore bar behavior characteristics: i) Km 0 – 1: accretional sector - the lowest slopes, likely to be influenced by the mouth bar dynamics and presenting the smallest offshore migration rates and the largest cycle return periods; ii) Km 1 – 6: stable sector - the largest values for bar height and volume and median values for migration rates and cycle return periods; iii) Km 6 -16: erosive sector - the steepest slopes, with a narrow bar zone and the smallest values in the bar morphometric parameters, highest migration rates and smallest cycle return periods, including a transition area (Km 6 – 9) characterized by bar switching. Our analysis clearly states the intra-site differences in spatial and temporal sandbar behavior on a nontidal deltaic coast as a result of the medium and long-term evolution of the coast, with substantially different bar behavior on the sediment-rich accretional sector than on the erosive sector.