Continuous monitoring of bedload flux in a mountain gravel-bed river

An automatic bedload monitoring station has been established on the perennial Tordera River, a Mediterranean mountain gravel-bed stream located in NE Spain. Bedload fluxes were obtained automatically at 1-min intervals using two Birkbeck-type pit bedload samplers aligned across the width of the channel. Flow depth and water-surface slope were both measured continuously and synchronously with bedload. Data for five flow events show that bedload flux varies considerably. Bedload flux is high when it is compared with other perennial streams. For a 15-min interval, maximum channel-average bedload flux was 0.83 kg m(-1) s(-1). Bedload records also show that there is no direct relationship between bedload flux and grain shear stress. The low correlation between bedload flux and hydraulics arises due to the different thresholds of initiation of motion for each flow event and the variable character of the bed between events. For two individual floods, there is a reasonable relationship between bedload flux and grain shear stress, in part, as a consequence of individual particle entrainment and patch movement over a stable coarse gravel-bed during moderately high discharges. These results demonstrate that the Birkbeck system functions well in a Mediterranean mountain gravel-bed stream with a longitudinal slope of 2%. (C) 2000 Elsevier Science B.V. All rights reserved.