Bedforms and sediment transport processes in the ephemeral streams of Kobo basin, Northern Ethiopia

Bedforms and channel deposits architecture were studied in the ephemeral streams of the structural basin of Kobo in northern Ethiopia.The Froude numbers, calculated at bankfull flow for a few of the study reaches, are in agreement with the extensive occurrence of horizontal, planar lamination. Small bedforms, not previously reported in the literature, are found and described as produced mainly by scouring rather than accretion processes. Equations to predict particle entrainment proved that large, individual boulders standing scattered on the stream bed can be transported at bankfull flow for long distances. Direct inspection of the old alluvium and the modern channel deposits allow to identify a typical arrangement within a single bed consisting of divisions with specific characteristics. This model of division association is explained in terms of vertical distribution of shear stress and compared with hyperconcentrated flow and traction carpet models in contrast with recent theories of flat asymmetrical bed waves progradation at Froude numbers close to unity.