Image-based Measurement of Bed-load Transport in Closed Channel Flows

Wei Miao, Danxun Li, Huai Chen, Qigang Chen

Thursday 2 july 2015

14:20 - 14:35h at Mississippi (level 1)

Themes: (T) Sediment management and morphodynamics, (ST) Sediment transport mechanisms and modelling

Parallel session: 12A. Sediment - Transport

We introduce a novel instrument for measuring bed-load transport in closed channel flows. The instrument is based on modern innovations in high-frequency acquisition and processing of digital images, and its novelty allows for quantifying sediment transport rate in an intuitive way. With this instrument, experiments were conducted in a closed rectangular channel under various water pressures. For each experiment, uniform sediment particles were paved on the flat bed, and their motions were recorded with a high-speed camera. Velocities of active particles (particle entrained to motion) were calculated by identifying and tracking particles in motion. The effects of various parameters were analyzed, including sampling frequency, sampling area, flow intensity, and water pressure. Preliminary analysis of the experimental results yields the following findings: • The instantaneous particle entrainment from the channel bed displays a non-uniform pattern, i.e, sediment particles tend to get into motion in patches rather than individually. This non-uniformity indicates that large-scale coherent structures near channel bed may play an important role in “picking up” sediment particles. • The number of active particles increases logarithmically with the sampling frequency; the particle velocities, however, decrease with the decreasing of sampling frequency. • The number of active particles increases exponentially with the flow intensity; the particle velocities seem to be irrelevant to flow intensity. Therefore, the number of active particles dictates transport rate of bed load. • Under relative strong flow intensity, the streamwise and transverse particle velocities exhibit exponential probability density functions. • The absolute pressure shows negligible influence on the entrainment of non-cohesive particle (one millimeter in diameter) in the experiments. The experiment provides more data to the study of bed-load transport. Moreover, it clarifies that the change in instrumentation may substantially alter the final result.