LOW-ANGLE DUNES IN BIG RIVERS: MORPHOLOGY, OCCURRENCE AND SPECULATIONS ON THEIR ORIGIN

Low-angle dunes (LADs), which have often been defined as those dunes possessing a leeside angle less than 30 degrees, have been reported from several large rivers and shown to possess a different fluid dynamics, and likely internal stratification, to those produced by high-angle dunes (HADs). Herein, we assemble and analyze available data to present a dataset concerning the leeside angle, morphology and classification of dunes in a range of large rivers. We find that low-angle dunes also co-exist with higher angle dunes in large alluvial channels, and suggest their presence is linked to several factors, including suspended sediment concentration, grain size and bedform superimposition. We propose that the classification of LADs should be based on the leeside angle at which permanent flow separation ceases to exist, or less than of 15°.