Turbulence Structure and Burst Events Observed in a Tidally Induced Bottom Boundary Layer

Abstract

AbstractIn estuarine environments tidal variation affects turbulence and associated sediment transport processes near the bed. In situ turbulence and sediment data from the benthic boundary layer of the subaqueous Yangtze delta were collected over several tidal cycles and are used to gain new insights into turbulence structure and sediment mobility in the benthic boundary layer. Our analysis reveals that turbulent kinetic energy production was balanced by dissipation during flood, while during ebb an imbalance was found (dissipation > production) attributed to turbulence advection. Quadrant analysis showed a tidal variability of the four quadrants with ejections (E) and sweeps (S) being the dominant and equal contributors to Reynolds stress and sediment resuspension. Increased dissipation due to turbulent kinetic energy advection during ebb coincides with an increase in the timeshare of inward‐ and outward‐interaction events, resulting in a higher Reynolds stress contribution from ejections and sweeps. The contribution of outward (O) and inward (I) ‐interactions to sediment resuspension was found to be negligible. Turbulence intermittency is characterized using the frequency, duration, strength, and intensity of the quadrant events. The frequency and duration of outward (O) and inward (I) ‐interactions controlled bursts were 1/3∼1/2 that of ejections (E) and sweeps (S), which lead to them having limited effects on Reynolds stress. A comparison with settling velocity suggests that vertical sediment transport is mainly controlled by ejection and sweep events. Inward‐ and outward‐interactions appear to be more important for the resuspension of fine material with settling velocities of the order of 1 mm/s.