Flash floods, sediment transport and bedforms in the ephemeral streams of Kobo basin, northern Ethiopia.

Ephemeral streams are typical of arid and semi-arid regions. Their bed is dry for most of the time allowing direct inspection of bedforms and channel deposits architecture. However, such favourable condition is not paired by sufficient data on flow hydraulic parameters and sediment transport processes associated with them. These issues were studied in the ephemeral streams of the structural basin of Kobo characterised by a semi-arid, monsoon affected climate in northern Ethiopia. Since no flow data is available, bankfull discharge for a few of the study reaches has been obtained throught the field survey of representative cross-sections, streambed gradient and bed material grain size, with average flow velocity assessed by a few resistance equations, derived for sand bed rivers. 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 that is typically associated with upper plane bed and transition flow regime. Small bedforms, not prevously reported in the literature, are found and described as produced mainly by scouring rather than accretionary processes. Such bedforms are supplemental tools for the interpretation of ancient ephemeral streams deposits. A few equations to predict particle entrainment proved that large, individual boulders standing scattered on the stream bed can be transported at bankfull flow also for long distances. These outsized particles protruding on the streambed are rooted in a coarse-grained, reversely graded layer and commonly are immersed in fine sediments that do not show any flow perturbation in their vicinity. Direct inspection of the old alluvium and the modern channel deposits allow to identify a typical arrangement within a single bed consisting of a coarse core division, supporting the large boulders, underlain by a massive or reversely graded, fine gravel and sandy division and overlain by planar, horizontal laminated sand and grains. This model of division association is explained in terms of vertical distribution of shear stress and compared with hyperconcentrated flow and traction carpet models reported in the literature. This study model allows to interprete the characteristics and generating process of horizontal laminae as thin bedload sheets, in contrast with recent theories of flat asymmetrical bed waves progradation at Froude numbers close to unity.