Near-bed turbulence characteristics in unsteady hydrograph flows over mobile and immobile gravel beds

Improvements to current deterministic sediment transport prediction methods requires enhanced understanding of the changes in near-bed turbulent flow characteristics occurring during the unsteady hydrograph flow conditions, under which sediment transport is most likely to occur. This paper investigates specifically at relative influence of mobile and immobile bed conditions on the near-bed turbulent structure, focusing specifically on the generation of so-called burst and sweep events, as well as the reciprocal influence of unsteady flow on the nature of these turbulent flow events over both mobile and immobile gravel beds. Quadrant analysis is used to determine the number and strength of turbulent burst and sweep events from detailed three-dimensional ADV velocity data collected over both types of bed. Results indicate that the mobile bed experiences less, but higher magnitude, near-bed burst and sweep events compared to the immobile bed. Further consideration of the results using a hole-size analysis within the quadrant plots may permit the correlation with observed higher-order turbulence events with sediment transport under unsteady hydrograph flows. In this respect, the results demonstrated that higher strength, rather than a higher volume, of turbulent burst/sweep events are found during unsteady flow conditions that are favorable for sediment transport.