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

An automatic bedload monitoring station has been established on the perenni al 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 chan nel. Flow depth and water-surface slope were both measured continuously and synchronously with bedload. Data for five flow events show that bedload fl ux 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 di rect relationship between bedload flux and grain shear stress. The low corr elation between bedload flux and hydraulics arises due to the different thr esholds of initiation of motion for each flow event and the variable charac ter of the bed between events. For two individual floods, there is a reason able 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 d emonstrate that the Birkbeck system functions well in a Mediterranean mount ain gravel-bed stream with a longitudinal slope of 2%. (C) 2000 Elsevier Sc ience B.V. All rights reserved.