Alongshore variability of cross-shore bar behavior on a nontidal beach

Abstract

We report on a 6-year nearshore bathymetric dataset from the Danube Delta (Romanian Black Sea coast) that comprises 16 km of erosive, stable and accumulative low-lying micro-tidal beaches northward of Sf. Gheorghe arm mouth. Two to three two-dimensional longshore sandbars exhibit a net multi-annual cyclic (2.8–5.5 years) offshore migration (20–50 m yr1) in a similar way to other coasts worldwide. Bar morphology and behavior on the sediment-rich accretionary (dissipative) sector differ substantially from that on the erosive (intermediate) sector. Shoreface slope is the most important factor controlling sandbar number and behavior. It determines different wave-breaking patterns in the surf zone, translated into different offshore sediment transport and bar zone widths along the study site. Additionally, sediment availability, as a result of the distance from the arm mouth and of the long-term evolution of the coast, controls the sandbar volume variability. These are all ultimately reflected in the variations of sandbar migration rates and cycle periods. A non-dimensional morpho-sedimentary parameter is finally presented, which expresses the bar system change potential as offshore sediment transport potential across the bar zone.