Controlling of internal processes on estuarine sediment dispersal:
internal hydraulic jump and enhanced turbulence mixing
Jiaxue Wu, Huan Liu & Chaoyu Wu
Research Center of Coastal Ocean Science and Technology, Sun Yat-sen University,
Guangzhou 510275, China.
E-mail: jesse-wu@tongji.edu.cn (J. Wu)
Abstract
Two important processes associated with estuarine sediment transport will be
examined in this presentation, i.e. internal hydraulic jump and benthic boundary layer
dynamics. Field investigations were undertaken into the river plume generated by the
Herbert River, Australia, following a moderate flood event induced by Cyclone Fritz
in 2004. The forced plume experiences an abrupt transition from supercritical to
subcritical via an internal hydraulic jump, as defined by a mode-1 internal Froude
number computed using the phase speeds from the Taylor-Goldstein equation. The
hydraulic theory of a two-layer stratified flow was used to identify the plume shape
and the mechanical energy loss within the jump. The hydraulic jump energy loss is
primarily transferred to the buoyancy-driven potential energy, uplifting the river
plume. Intense stratification decreases the bottom stress, damping the resuspension.
Therefore, a separative nepheloid dispersal system occurs at the jump section. Both
the upper and lower nepheloid flows are confined to the inner shelf, but have different
dispersal behaviors and mechanisms. The upper nepheloid flow, which is primarily
controlled by advection and settling, satisfies an exponential decay law of the total
suspended sediment concentrations versus the offshore distance. The lower nepheloid
flow dominated by deposition is detached seaward near the lift-off point of the river
plume.
A bottom-mounted instrumental tripod was deployed in the tidally energetic Zhujiang
(Pearl River) Estuary to examine the contrasting properties of the bottom boundary
layer (BBL) flows between estuarine and tide-affected river systems. Three aspects of
the BBL flows were discussed to understand the mechanism of the turbulence
responses to the large-scale ambient forcing: the flow structures (profile, anisotropy,
and spectra), shearing strains and stresses, and the balance of turbulent kinetic energy
(TKE). Single log-law profiles and turbulence anisotropy predominated in the two
systems, but the non-log regime and stronger anisotropy occurred more frequently at
the slack tide in the estuary. The ADV-based turbulence intensities and shearing
strains both exceeded their low-frequency counterparts (frictional velocity and mean
shear) derived from the logarithmic law. On the contrary, the ADV-based Reynolds
stress was smaller than the bottom stress, so the hypothesis of a constant stress layer
can not be well satisfied, especially in the river. The bandwidth of the inertial
subrange in the river was of one decade larger than in the estuary. The balance
between shear production and viscous dissipation was better achieved in the straight
river. This first-order balance was significantly broken in the estuary and in the
meandering river, by non-shear production/dissipation due to wave-induced
fluctuations or sediment-induced stratification. All these disparities between two
systems in turbulence properties are essentially controlled by the anisotropy induced
by the large-scale processes such as secondary currents, stratification. In conclusion,
the intensity of acceleration of unsteady flows determines the profile structure of the
BBL flow, and the degree of turbulence anisotropy results in the invalidation of the
phenomenological relations such as the constant stress hypothesis and the balance of
TKE production and dissipation.
Keywords: river plume, sediment transport, internal hydraulic jump, bottom boundary
layer, turbulence mixing.
This material was based upon work supported by the National Natural Science
Foundation of China under Grant No 40676054 and 40306016.