Contrasts between estuarine and river systems in near-bed turbulent flows in the Zhujiang (Pearl River) Estuary, China

Liu, Huan; Wu, Chaoyu; Xü, Wei; Wu, Jiaxue

doi:10.1016/j.ecss.2009.05.005

Cited by 15 publications

(11 citation statements)

References 26 publications

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“…The profiling of currents in coastal oceans is mainly conducted using upward-looking, bottom-mounted ADCPs, whereas highfrequency temporal variations of velocity components at a specific height above the bottom can be obtained by acoustic Doppler velocimeters (ADVs). Field studies on BBL dynamics in shallow waters (Lueck and Lu 1997;Foster et al 2000;Elliott 2002;Howarth and Souza 2005;Lozovatsky et al 2008a, b;Liu et al 2009a) have confirmed the general applicability of log-layer approximation to the velocity profiles U z ð Þ ¼ u 2 z ð Þ þ v 2 z ð Þ ½ 1=2 in a well-mixed BBL and therefore the possibility of law-of-the-wall dissipation estimation " wl z ð Þ ¼ cu 3 » =kz for near-bottom turbulence. Here u and v are horizontal components of the velocity vector, u * the friction velocity, κ≈0.4 the von Karman constant, c a nondimensional constant of order to unity, and ζ the height above the seafloor.…”

Section: Introductionmentioning

confidence: 73%

“…Time in hours from the beginning of ADCP measurements (0, 3, 6, 9, 12, 15, 18) is depicted for ζ=0.4 mab (magenta) and for the OTIS ellipses A tidal current, being a non-steady flow, cannot formally satisfy many of these requirements. In spite of such restrictions, logarithmic-like velocity profiles are often observed in a variety of natural currents (e.g., Schauer 1987;Elliott 2002;Moum et al 2002Moum et al , 2004Liu et al 2009a), although, as pointed out earlier, u * obtained via fitting velocity profiles to Eq. 1 and that based on eddy correlations (Friedrichs and Wright 1997;Howarth and Souza 2005;Lozovatsky et al 2008b) do not agree for thick BBL (h B ∼10 m).…”

Section: Background Tidal Dynamics and Stratificationmentioning

confidence: 96%

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Shallow water tidal currents in close proximity to the seafloor and boundary-induced turbulence

et al. 2011

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Velocity measurements with vertical resolution 0.02 m were conducted in the lowest 0.5 m of the water column using acoustic Doppler current profiler (ADCP) at a test site in the western part of the East China Sea. The friction velocity u * and the turbulent kinetic energy dissipation rate ε wl (ζ) profiles were calculated using loglayer fits; ζ is the height above the bottom. During a semidiurnal tidal cycle, u * was found to vary in the range (1-7)Â10 −3 m/s. The law-of-the-wall dissipation profiles ε wl (ζ) were consistent with the dissipation profiles ε mc (ζ) evaluated using independent microstructure measurements of small-scale shear, except in the presence of westward currents. It was hypothesized that an isolated bathymetric rise (25 m height at a 50-m seafloor) located to the east of the measurement site is responsible for the latter. Calculation of the depth integrated internal tide generating body force in the region showed that the flanks of the rise are hotspots of internal wave energy that may locally produce a significant turbulent zone while emitting tidal and shorter nonlinear internal waves. This distant topographic source of turbulence may enhance the microstructure-based dissipation levels ε mc (ζ) in the bottom boundary layer (BBL) beyond the dissipation ε wl (ζ) associated with purely locally generated turbulence by skin currents. Semi-empirical estimates for dissipation at a distance from the bathymetric rise agree well with the BBL values of ε mc measured 15 km upslope.

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Section: Introductionmentioning

confidence: 73%

Section: Background Tidal Dynamics and Stratificationmentioning

confidence: 96%

Shallow water tidal currents in close proximity to the seafloor and boundary-induced turbulence

et al. 2011

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“…[27], which is used to describe DBL thickness in systems where turbulence controls this thickness [18]. It is interesting that the variation of BBL thickness follows the speed [23,24,28], contrary to the DBL finding. The variation of diffusion flux may be interpreted by two mechanisms, which are responses to the change of water overlying the sediment and the turbulent intensity.…”

Section: Discussionmentioning

confidence: 67%

“…Nevertheless, the relationship between the turbulent energy dissipation rate and mean velocity is not strong. This indicates that the turbulence is caused not only by shear of the mean velocity, but also probably by surface waves, local topography, sediment transport and buoyant flux [23][24][25]. Estimating the production of turbulence is limited by the data, and thus will not be discussed further.…”

Section: Numerical Model Profilementioning

confidence: 99%

Variable diffusion boundary layer and diffusion flux at sediment-water interface in response to dynamic forcing over an intertidal mudflat

2012

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The diffusion boundary layer (DBL) significantly limits the exchange between sediment and overlying water and therefore becomes a bottleneck of diffusive vertical flux at the sediment-water interface (SWI). Variable DBL thickness and diffusion flux in response to dynamic forcing may influence replenishment of nutrients and secondary pollution in coastal waters. In situ measurements of velocity in the bottom boundary layer (BBL) and oxygen concentration in the DBL were made over an intertidal mudflat, using an acoustic Doppler current and mini profiler. A linear distributed zone in the oxygen profile, the profile slope discontinuity and variance of concentration can be used to derive accurate DBL thickness. Diffusion fluxes calculated from the water column and sediment are identical, and their bias is less than 6%. A numerical model PROFILE is used to simulate the in situ dissolved oxygen profile, and layered dissolved oxygen consumption rates in the sediment are calculated. The DBL thickness (0.10-0.35 mm) and diffusion flux (15.4-53.6 mmol m) vary with a factor of 3.5 during a tidal period. Over an intertidal mudflat, DBL thickness is controlled by flow speed U in the BBL, according to δ DBL =1686.1DU −1 +0.1 (D is the molecular diffusion coefficient).That is, the DBL thickness δ DBL increases with decreasing flow speed U. Changes of diffusion flux at the SWI are caused by variations in the water above the sediment and the turbulent mixing intensity. The diffusion flux is positively related to the turbulent dissipation rate, friction velocity and turbulent energy. Under the influence of dynamics in the BBL, DBL thickness and flux vary significantly. diffusion boundary layer, diffusion flux, dynamic forcing, mini profiler, dissolved oxygen, intertidal mudflat Citation:Wang J N, Zhao L, Wei H. Variable diffusion boundary layer and diffusion flux at sediment-water interface in response to dynamic forcing over an intertidal mudflat. Chin Sci Bull, 2012Bull, , 57: 15681577, doi: 10.1007 The diffusion boundary layer (DBL) is a thin (less than 1 mm) film of water covering the sediment. When solutes and particles are transported from the bottom boundary layer (BBL) to the DBL, molecular diffusion instead of turbulent eddy diffusion dominates the vertical transport [1][2][3]. The DBL limits exchange between the sediment and overlying water, and therefore becomes a bottleneck of diffusive vertical flux at the sediment-water interface (SWI). theoretically revealed that dynamic forcing in the BBL has a direct effect on DBL thickness and diffusion flux at the SWI. Under the influence of dynamics in the BBL, DBL thickness and flux vary significantly. Understanding how the DBL and diffusion transport responds to dynamic forcing is therefore crucial for accurately quantifying diffusion flux, and for estimating the replenishment of nutrients and secondary pollution in coastal waters. Modeling also requires numerical parameterization of diffusion flux as a function of BBL dynamics. The diffusive vertical flux of dissolved oxy...

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“…In a program of research on the estuarine BBL flows, Liu et al (2009) concentrated on the canonical (quasi steady) BBL flows in the high velocity flow regime, whereas this paper focuses on the noncanonical (threedimensional and unsteady) BBL flows near the slack tide. So far, there is no information on the spiral of the oscillatory BBL flow or a useful explanation for its origin, life history, and persistence.…”

Section: Introductionmentioning

confidence: 99%

Cyclonic Spirals in Tidally Accelerating Bottom Boundary Layers in the Zhujiang (Pearl River) Estuary

Liu

Ren

et al. 2011

Journal of Physical Oceanography

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A velocity spiral in the tidally accelerating bottom boundary layer (BBL) was defined as a directional shear of the prevailing flow with the elevation and the tidal phase. However, so far there is no information on the spiral for the oscillatory BBL flows or a valid explanation for its origin, life history, and persistence. To investigate this rotating current in the tidally accelerating BBL flow, the authors performed instrumented tripod observations in the tidally energetic Zhujiang (Pearl River) estuary. The tidal BBL flows may fall into three distinct regimes: (i) the quasi-steady phase in the peak tide; (ii) the accelerating-decelerating phase at the slack tide; and (iii) the transition between (i) and (ii), when a cyclonic spiral occurs only in the early-late ebb. The subcritical spiral, defined by a Froude number of the oscillatory BBL flow, may be analytically examined by unsteady linearized turbulent BBL equations. The spiral is formed under the momentum balance between local acceleration and bottom friction, independent of stratification conditions. The spiral consists of the ''diffusive'' and oscillatory boundary layers in the streamwise and spanwise direction, respectively. The streamwise spiral presents an exponential degradation (growth) in the decelerating (accelerating) ebb, indicating its limited life history over a tidal cycle. The transient in bottom stresses induced by the growth or the degradation of the spiral may be the mechanism for sediment trapping in the very little bed friction in the tidal estuary.

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Contrasts between estuarine and river systems in near-bed turbulent flows in the Zhujiang (Pearl River) Estuary, China

Cited by 15 publications

References 26 publications

Shallow water tidal currents in close proximity to the seafloor and boundary-induced turbulence

Shallow water tidal currents in close proximity to the seafloor and boundary-induced turbulence

Variable diffusion boundary layer and diffusion flux at sediment-water interface in response to dynamic forcing over an intertidal mudflat

Cyclonic Spirals in Tidally Accelerating Bottom Boundary Layers in the Zhujiang (Pearl River) Estuary

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