Seasonal Variation of Catenary-Bead Dunes in the Yangtze River Estuary: Causes and Implications

Low-angle lee-side slopes of dunes are commonly developed on the world's riverbeds, and dune migration associated with sediment transport exert a major influence on riverine processes. However, the catenary-bead dune has been identified in the Yangtze River (YR) Estuary, featuring a higher lee-side angle. To date, the morphological variation and formation reasons of catenary-bead dunes in the YR Estuary remain uncharacterized. In this study, we used a multibeam echo system (MBES) to investigate the bedforms of the YR estuary during 2014-2015, as well as to discuss the seasonal variation of catenary-bead dunes. The results indicate that the catenary-bead dunes of the YR Estuary are characterized by growth during the flood season and extinction during the dry season. The lee-side angle is typically similar to 16.7 degrees, which is larger than that of other dune types (3.7-8 degrees) in the estuary; moreover, the catenary-bead dunes are higher than other dune types of the same length in the YR Estuary. The relationship between the dune height (H) and length (L) was found to be H = 0.1667L(0.603) (R-2 = 0.38), while the other dune types yielded the relationship of H = 0.0845L(0.758) (R-2 = 0.52). Strong runoff superimposing the ebb tide led to the development of catenary-bead dunes. Furthermore, the higher coarse sediment content (69.9-72%) and lower clay content (6.3-6.7%) of the riverbed sediment are favorable for their formation, while the higher curved crest-lines are favorable for the formation of the associated elliptical pits.