Morphotextural characterization of dryland braided channels

Braided streams have been extensively studied in humid environments or in flumes that simulate them. Dryland rivers have bare catchments, rapid recession hydrographs, and unarmored beds, resulting in high bed-load discharges. Our objective was to characterize the morphotexture of the morphological elements-bar, anabranches, chutes, and confluences-in a dryland braided system. Morphological elements measured in Wadis Ze'elim, Rahaf, and Roded, located in southeast Israel, were scanned using terrestrial laser scanning, from which digital elevation models were generated. Roughness was evaluated from the standard deviation of detrended surfaces or from Wolman-sampled surface texture. Armoring of the bar and the adjacent anabranch was also evaluated. Scour chains were deployed in confluences to measure scour and fill during flow events. Dryland anabranches were finer grained and less armored than the adjacent bars. Bed load, deposited as midchannel bars or on top of existing bars and thus associated with the depositional braiding process, was enriched by the coarser fractions, while the remaining bed-load was deposited in the anabranches during the rapid flow recession. Chutes associated with erosional braiding processes were formed as topographic features with high local gradients, enabling winnowing of fines, deposited immediately downstream or transported further away. Scour chains inserted in the confluence zones demonstrated the formation and obliteration of a scour hole during a flow event. The terrestrial laser scanning-based digital elevation models and roughness analyses suggest that the hydrological regime of ephemeral channels with short-lived recessions is responsible for the morphotextural setting of braid systems in arid environments.