Flash flood sediment transport in a steep sand-bed ephemeral stream

This paper reports about flow and sediment transport measurements undertaken during the rainy season (July-August) 2008 on Gereb Oda, a steep, sand-bed ephemeral stream, draining the western margin of the Kobo structural basin in northern Ethiopia. Gereb Oda streambed is dry for most of the year since it is subjected only to sporadic flash floods in response to individual, high intensity and spatially confined rainstorms. Information about hydraulic and sediment transport processes in steep sand-bed, ephemeral stream is very poor and, though only two flash floods occurred during the field campaign, the data gathered may contribute to improve our knowledge on these kind of rivers. Froude numbers were calculated in order to verify the occurrence of supercritical flow conditions as postulated by a few authors from the analysis of sedimentary structures characteristics. Flow data are also compared to simple models to predict flow velocity and discharge since flow recording systems are seldom installed on ephemeral streams in remote areas and developing countries. Dryland rivers are known for their very high suspended sediment transport, but very little bedload data exist for sand-bed, ephemeral streams. The variation of suspended sediment concentration with discharge is analysed and simple rating curves for both suspended and bed load transport are derived. A few equations to predict bedload are tested against Gereb Oda data and the relationship between en masse bedload transport processes as thin sand sheets and the development of horizontal lamination through the migration of leaf-shaped, sheet-like bedforms is investigated. Individual, large boulders were observed to move at flow depths of the same order of magnitude of their size. A few functions to predict the threshold conditions for large particles entrainment are used to verify if and to what extent they match Gereb Oda field conditions. The increased density of the water-sediment mixture, for the very high suspended sediment concentrations ranging from 100,000 to 200,000 ppm, is considered as well.