Abstract: Turbulence, bursting and sediment re-suspension

 

 

Authors: Charitha Pattiaratchi

School of Environmental Systems Engineering,
The University of Western Australia, Nedlands, WA 6009, Australia

In classical sediment transport theory, it is postulated that a critical velocity (the ‘Shields’ criterion) should be exceeded before sediment re-suspension and transport occurs.  To sediment transport rates, either using field measurements or numerical models, the mean near bed velocities are converted into a mean shear stress and predictive formulae are used to estimate sediment transport.  This may be a correct approach in high-energy tidal and wave dominated systems where shear generated by higher near-bed mean velocities are responsible for sediment re-suspension.   However, field measurements have shown that sediment re-suspension and transport occurs in low energy environments where the critical velocity is rarely exceeded.  This discrepancy may be related to near-bed turbulence. Field and laboratory measurements have shown that instantaneous Reynolds stresses much greater that the mean stress value, with events lasting several seconds, and relatively quiescent  periods in between events.  These coherent turbulent events within the bottom boundary layer is associated with a sequence of motions (burst, sweep, outward and inward interactions), known collectively as “bursting”. Even though intermittent events of strong turbulence (‘bursting phenomenon’) occur over short periods of time, they can dominate the turbulent stress causing a burst-like suspension of the bottom sediments.

High temporal resolution near bed hydrodynamic and suspended sediment concentration (SSC) collected from three contrasting environments are presented and related to bursting events to SSC: (1) micro-tidal upper Swan River estuary (low energy); (2) a site in 385 m of water off the north-west shelf of Australia (low energy); and (3) nearshore beach environment (high energy).  Data from all three locations indicated that the sediment re-suspension was intermittent and was associated with events corresponding to ‘bursts’.   In the low energy environments the sediments were resuspended at velocities below that of the threshold velocity predicted by the Shields criterion.   In all three environments it was identified that the acceleration/deceleration associated with reversal of the mean currents genereated high Reynold stresses and was an important mechanism for sediment resuspension.