Author(s): Andrea Marion; Andrea Bottacin Busolin; Simon J. Tait; Matteo Tregnaghi
Linked Author(s):
Keywords: Particle image velocimetry; Incipient motion; Turbulence; Solid transport
Abstract: Several experimental tests were carried out using a gravel bed tilting flume and an advanced Particle Image Velocimetry (PIV) system. Sediment movement at the threshold was surveyed at the grain scale under uniform flow conditions. A natural river gravel with well sorted sediment was used. Flow field measurements were made over an area of approximately 400 cm2, on a plane located at 5 mm above the original sediment bed. Image processing and cross-correlation techniques were used to obtain flow velocities. Sequences of images of the bed surface were taken simultaneously with the measurements of the flow field. Statistics of the movements of the grains were collected through a semi-automatic procedure. Significant changes in the local flow field were observed in the proximity of the entrained or deposited particles. Such variations of velocity, which can be either in terms of the longitudinal and lateral components or in terms of the relevant fluctuations from the time average, appear as a direct effect of the removal or deposition of individual grains. A strong correlation is shown between the changes in the streamwise velocity and the movement of the particles: local velocity increases were observed to correspond with the entrainment of grains, while local velocity reduction occurred as a grain was deposited. The theory of Grass is revisited and further developed. The new formulation provides a way to estimate the probability distribution of individual grain resistance from statistics of the near bed velocity field and of the entrainment risk.
Year: 2007