The impact of sequential bursting events of coherent flow structures on sediment entrainment at thereshold conditions in meandering channel- Time Fraction Approaches

Abstract In this study, the effect of bed shear stress variation on quadrant time fractions and bed topography change in a meandering river was analyzed in an experimental model. Also this research tests the hypothesis that particle entrainment is associated with bursting quadrants and sequencing time fractions. Also, the variation of this parameter was examined with shear stress variation. Experimental tests were carried out in four different threshold conditions in an equilibrium state. The quadrant time fractions are probed with the Acoustic Doppler Velocimetry (ADV) at a near-bed in a Comprehensive point grid throughout the model in 120 points for each test. The results indicate that although the classical view about the dominant processes of sweep and ejection is accurate in straight channels and the central line, the maximum time fractions of these two phenomena have yet to occur in the eroded areas. After that, we describe the quadrant's time series using the sequences of quadrants. Examining three sequences of groups (GE), singles (SE), and the vortex-related sequences (VRS) and comparing their time fractions with eroded areas showed a good correlation between some sequences and erosion. The results point to the group sequences, especially sweep groups with a time fraction of more than 70% at the very lower threshold, and vortex-related sequences with a time fraction of 34% at the upper limit of the threshold were entirely under the eroded areas.