Bedload Movement in Desert Floods / הסעת גרופת בנחלי מדבר

This paper describes the results of two new bedload studies conducted in the Negev. The first study, in the Northern Negev (Nahel Hebron), dealt with fluvial processes in stream channels; the second, in the Southern Negev (Nahel Yael), dealt with sedimentation in reservoirs and bedload movement. In Nahel Hebron an attempt was made to evaluate the pattern of bed load movement and the three-dimensional dispersion of labelled cobble size bed material. The tracing technique was based on a pre and post-flood search, with a magnetic locater, of transported cobbles into which a small ceramic magnet had been inserted. Of 282 such particles dispersed by one flow event, 93% were recovered,of which 53% were found buried up to depths of 42 cm. No significant relationship was found between transport distance and particle weight. Furthermore, no significant relationship was found between burial depth and particle weight. The burial depth varied greatly. Pre and post-flow event particle location indicates the existence of vertical interaction (mixing) between the bed surface material and the underlying material. The variation of particle burial depth and transport distance indicates the existence of three different channel reaches as well as indicating that they behave and react in different ways in the flow event; (i) stable reaches, small variation in particle burial depth, with most particles remaining on the bed, (ii) dynamic/active reaches, deep burial depth with great variation and most particles moving long distances, and (iii) intermediate reaches between the stable and active reaches. In 1978 an earth-dam was constructed in Nahel Yael at the apex of the fluvial fan, in an attempt to create a closed sediment and water system. Since 1978 two flow events have occurred. The resulting reservoir bed surface was covered by a thin silty-clay layer. This layer will serve as a border between the previous flow and any forthcoming event. The sediment survey under this layer was done by point sampling (pits) and by spatial sampling (trench). The coarse material found in the reservoir comprised a large proportion of the transported material. The trench and pit stratigraphic sections support the hypothesis of bed load movement in "waves".