Morpho-textural implications to bedload flux and texture in the sand-gravel ephemeral Poveda Gully

We report on channel morpho-texture and bedload transport in a natural, steep, sand-gravel ephemeral channel draining the small Poveda Gully watershed in the mining area of the Alto Tajo Natural Park, Spain. First-ever continuous bedload flux and texture monitoring in a transitional sand-gravel environment was undertaken by two independent Reid-type slot samplers. Morphological changes in the feeder reach have been quantified by TLS (terrestrial laser scanning) and SfM (structure from motion) technologies. We identified a pattern in channel-bed morphology and texture (morpho-texture): when the channel is incised its texture is coarser, otherwise sand-filling occurs. These changes determine bedload flux and texture: sand fill brings rise to high fluxes and fine-grained bedload, whereas incision caused by evacuation of sand leaves a sandy-gravel surface with lower bedload fluxes and coarser texture. A video camera recording during events allowed identification of dramatic changes in bedload flux and texture owing to the appearance and erosion of bars, supplying field evidence to explain the difficulty in the prediction of bedload flux at short time intervals.