Characterizing and identifying bedforms in the wandering reach of the lower Yellow River

The lower Yellow River (LYR) is a fully alluvial system with a fine-grained bed that has a high proportion of silt. Bathymetric survey data collected with a Multi-Beam Echo Sounder (MBES) from the wandering reach of the LYR indicates that the bedforms are characterized by large aspect ratios (wavelength/height) and low lee-side angles. Since the Xiaolangdi Reservoir (XLD) has been operational in the middle reach of the Yellow River, bedforms have been dominated by two-scales of dunes, that is, a frame of compound dunes with first-order dunes superimposed on the stoss and lee sides, which disappear during high sediment concentration floods that occur during the annual sediment and water regulation. The bedform formation regime has changed with dune beds having more chances, and upper regime beds having less chances, to form. Longitudinal profile data measured using a sounding-rod before the XLD began filling demonstrate that the bedforms were mainly dunes during low flows (less than 1200 m³/s) or upper regime beds during high-flows (larger than 2000 m³/s). A discriminator is proposed to describe the conditions for bedform formation and to predict the alluvial resistance which gives better results than those from Manning approach. When a bedform system is almost at equilibrium, the changes in the sediment concentrations versus the discriminator can be defined into three segments in the LYR.