Formation of repeating bar-flat bedforms in ephemeral gravel bed channels: 1. Field observations

Repeating fluvial macroforms are ubiquitous. The streamwise alternation of steeper, coarse-grained, cobble/pebble bars and near-horizontal, fine-grained, sand/granule flats is characteristic of upland, single-thread, dryland channels. We have monitored the rate of formation of a bar-flat sequence by flash floods, noting the disposition and extent of each macroform as they evolve. To accomplish this, the bed material of a straight reach of the Nahal Yatir in the northern Negev Desert, Israel, was thoroughly mixed to a depth of 0.5 m with the aid of a mini excavator, obliterating a well-formed sequence of bars and flats. The ten flow events of two succeeding rain seasons were recorded, as were the changing topography and textural roughness of the bed following each complete post-flood dewatering. Embryonic flats, texturally like those present before disturbance, formed under the first flow event, occupying a fifth of the total length of the flats that had existed in the reach before experimental disturbance. Their length increased and their inclination decreased with each flow event, returning to the natural, pre-disturbed, aggregated length within two rain seasons. Restoration of the pre-disturbance bar-flat sequence was almost complete, the location of bars differing only marginally in places. Resemblance with the natural original was high, reflecting the sedimentary dynamism of desert flash floods. A mechanism that models and explains the formation and stability of these macroform sequences is developed in a companion paper.