Controlling of internal processes on estuarine sediment dispersal: internal hydraulic jump and enhanced turbulence mixing Jesse Wu （吳加學）, Huan Liu & Chaoyu Wu Center of Coastal Ocean Science and Technology, Sun Yat-sen University, Guangzhou , China International Sediment Transport Symposium, Kaohsiung, Taiwan, 23th-27th March, 2009

Background Estuarine sediment dispersal are closely associated with Estuary and delta evolution Shelf deposition Environmental and ecological issues (e.g. HAB) But the processes and mechanism of river-borne sediment transport are not yet completely unclear (see Ocean Science at the New Millennium, NSF ). Two case studies in tropical estuaries: 1. The Herbert Estuary, AU: a small mountainous river, an internal jump; 2. The Pearl Estuary, CHN: a larger river, the BBL dynamics.

Part 2: The BBL flows in the Pearl Estuary

Motivation and Questions BBL flows and sediment transport are important processes in estuarine dynamics; BBL structures and parameters (e.g. bottom stress, frictional velocity) are usually inferred from mean flow profiles under some hypotheses; But these hypotheses can not be well achieved in the estuarine BBL. Accordingly, we need (a) to rigorously verify/validate these hypotheses by turbulence investigation, (b) to quantitatively determine the difference between the theoretical and actual state, and (c) to identify the possible mechanism for this difference.

Three aspects of the BBL flows will be examined to understand the mechanism of turbulence responses to the large-scale estuarine forcings: a.the flow structures (profile, anisotropy, and spectra), b.shearing strains and stresses, and c.the balance of turbulent kinetic energy (TKE)

(Liu et al., submitted to ECSS) Four tripod mooring sites: YM01:estuarine, tide + current + wave; YM02: rock-bound gorge, the tripod overturned; YM03: straight tide-affected river; YM04: sinuous tide-affected river. Survey time: July hr each site The Huangmaohai Estuary, the Pearl Estuary Complex

Schematic figure of a bottom-mounted tripod used. ADV: 64 Hz; PC-ADP: 1Hz, a bin size 1.6 cm; XR-420 CTD: 1 Hz; XR-620 OBS: 6 Hz

Two regimes in the BBL flow: Log regime (self-similar, inertial); Non-log regime (non-similar, accel/deceleration) Log-law velocity profiles

Breakdown of the Log Law at slack tides at Site YM01 non-log velocity profiles (Fig. 4e, f), two-layer exchange flow (Fig. 4g) Mid-depth halocline (Fig. 4b, c), Benthic halocline (Fig. 4d)

Log profile: constant stress layer Isotropic turbulence: Equivalent conditions for the isotropic log BBL flow Mean shear Turbulence shear

Turbulence anisotropy at three sites; (a-c) Site YM01: (d-f) Site YM03: (g-i) Site YM04: Straining cascade from mean shear to vertical turbulence shear!

Breakdown of the hypothesis of constant stress

The Kolmogorov’s first universality assumption (for isotropic turbulence) TKE Spectra of the vertical fluctuation in the log regime Bandwidth of the inertial subrange

Breakdown of the -3/5 Power Law at slack tides (non-log regime)

A first-order balance between TKE production and dissipation for log profiles (isotropic turbulence) where BUT the effect of sediment stratification may be very important in BBL! where

Conclusions The unsteadiness of the tidal current determines the BBL flow structures, and the degree of turbulence anisotropy results in the invalidation of the similarity or phenomenological relations, e.g. the constant stress hypothesis and the first-order TKE balance. The logarithmic law, constant stress hypothesis, isotropy and an inertial subrange all seem to be the states or conditions of being equivalent in the BBL flow. The non-log regime near the slack tide exhibits a stronger acceleration effect and more complicated flow regimes. How the external forcing influences the BBL flow at the transient slack needs to be further explored. Meanwhile, the turbulence anisotropy induced by large-scale estuarine forcing is for certain a key problem to further investigate into.

Part 1: The internal hydraulic jump in a small mountainous estuary

General patterns of riverine outflows

(b) Wu et al. (2006), JGR Part 1: Internal hydraulic jump in the Herbert Estuary (a)

A highly-stratified river plume

A sandwiched pattern of sediment dispersal offshore

Site J2 supercritical in the near field Site J7 subcritical in the far field

Internal Hydraulic Jump

The specific mechanical energy of each layer in terms of the Bernoulli equations

Normal interface geometry in the two-layer stratified flow

A critical interface in the two-layer stratified flow If, i.e. at the lift-off point, then, is always required

Mechanical energy loss within the internal hydraulic jump where

Sediment Mass Balance Equation ( may be defined as a dispersal coeff) ( indicates SSC, the outflow velocity, the river plume thickness, bulk settling velocity )

Bulk Effective Settling Velocity mm/s 0.1 mm/s in the Eel River flood plume estimated by Hill et al. [2000] Stokes terminal settling velocity

Conclusions An internal hydraulic jump in a supercritical outflow was investigated both experimentally and theoretically in a mountainous estuary. A sandwiched dispersal system occurred at the jump section. The two nepheloid flows (upper and lower) have different dispersal behaviors and mechanisms. The upper flow primarily controlled by advection and settling, satisfies an exponential decay law of the SSC versus the offshore distance.

Thanks for your attention!

 Mean flow measurements have demonstrated non-log velocity profiles induced primarily by acceleration (or deceleration) at slack tides and salinity stratification. The hypothesis of constant stress can not be strictly satisfied under larger bottom stresses.  In a word, the combined effects of internal hydraulic jump, salinity intrusion, and tidal straining have significant influences on sediment dispersal through the change of the flow structures and turbulent mixing near the head of salt intrusion.  the BBL flow is controlled primarily by the acceleration of unsteady currents, and turbulence anisotropy is an essential response of various turbulence properties or relations to large-scale ambient processes.

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 balance between shear production and viscous dissipation was better achieved in the straight river. This first-order balances were 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.