Assessment of video imaging and particle counting techniques for calculation of bedload fluxes

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Rui Aleixo, Luís Mendes, Federica Antico, Pedro Sanches, Francisco Alegria, Rui Ferreira

Thursday 2 july 2015

11:45 - 12:00h at Africa (level 0)

Themes: (T) Special session, (ST) Acoustic monitoring of flow, turbulence and river discharge

Parallel session: 11F. Special session: Acoustic monitoring of flow, turbulence and river discharge

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Sediment transport is a fundamental process of Nature. It is studied by different areas of knowledge such as geology and civil engineering. In particular it is an important process in fluvial hydraulics as it is associated to morphological changes in the fluvial basin and thus impacting many human activities such as river management and dam operation. It is a process that has different spatial and time scales associated with it, from the small scales of the sediment to the larger scales of the river basin. When studying sediment transport one normally separates suspended and bedload. In the present paper only bedload will be analyzed. Even in laboratorial environment, bed load fluxes measurements are usually hard to perform since one needs to calculate the number of particles that cross a given section per time unit. Previous studies such as Heyman et al. (2013) used a devices based on a metallic plate and accelerometers to count the particles impact on the plate and Roseberry et al. (2012) used high-speed video recording to count individual particles. In this paper a technique is presented to measure the bedload flux based on the particles impact on a sensitive surface. This technique allows for counting the number of particles’ impacts in a given time interval, which is directly comparable with the number of particles that cross a certain cross-section in the same interval, quantified by digital image acquisition and analysis. The thus estimated bedload fluxes are also compared with the estimate based on the product of the particle area with the particle velocity, measured with image analysis. The advantages of the impact surface technique, like high-frequency acquisition (virtually continuous measurements), no need for optical access and low amount of digital data (allowing real-time analyses) are discussed and presented alongside with its limitations. References Heyman, J., F. Mettra, H.B. Ma and C. Ancey (2013), Statistics of bedload transport over steep slopes: Separation of time scales and collective motion, Geophys. Res. Letters, 40, 128–133, doi:10.1029/2012GL054280. Roseberry, J. C., M. W. Schmeeckle, and D. J. Furbish (2012), A probabilistic description of the bed load sediment flux: 2. Particle activity and motions, J. Geophys. Res., 117, F03032, doi:10.1029/2012JF002353.